Dhcp2003

Dynamic Host Configuration Protocol for
Windows Server 2003

Microsoft Corporation
Published: October 1, 2003

Abstract

The Microsoft® Windows Server™ 2003 network operating system includes an enhanced implementation of the
Dynamic Host Configuration Protocol (DHCP). This paper outlines the features of DHCP, the Windows
Server 2003 implementation of the technology, basic deployment information, and best practices for use. Based
on industry standards, DHCP for Windows Server 2003 supports RFCs 2131 and 2132.

Microsoft® Windows Server™ 2003 White Paper

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Contents
Contents........................................................................................................................................3

Introduction...................................................................................................................................1

DHCP Features in Windows Server 2003....................................................................................2

Integration of DHCP with DNS....................................................................................................2

Enhanced Monitoring and Statistical Reporting...........................................................................3

DHCP Vendor-Specific and Class ID Option Support.................................................................4

Multicast Address Allocation........................................................................................................5

Unauthorized DHCP Server Detection........................................................................................5
Protecting Against Unauthorized DHCP Servers.........................................................................5

Protecting Against Improper Use of Workgroup DHCP Servers..............................................6

Windows Clustering for High Availability.....................................................................................7

Automatic Client Configuration....................................................................................................7

Alternate Client Configuration......................................................................................................8

Configuration Options..............................................................................................................8

DHCP Server Discover Attempts.............................................................................................8

Command-Line Management......................................................................................................9

DHCP Overview...........................................................................................................................10

Server, Clients, and Relay Agents.............................................................................................10

DHCP Servers.......................................................................................................................11

DHCP Clients ........................................................................................................................11

DHCP BOOTP Relay Agent...................................................................................................12

Managing DHCP.......................................................................................................................12

DHCP Deployment......................................................................................................................14

Determining the Number of DHCP Servers to Use....................................................................14

Defining and Configuring Scopes..............................................................................................14

Using Superscopes...................................................................................................................15

Reserving IP Addresses............................................................................................................16

Using BOOTP Tables................................................................................................................17

Best Practices.............................................................................................................................18

Set Appropriate Lease Durations..............................................................................................18


Lengthening Lease Duration for Large, Fixed Networks........................................................18

Shortening Lease Duration for Variable Networks with Fewer IP Addresses.........................18

Reserve Addresses with Reservations......................................................................................18

Integrate DHCP with Other Services.........................................................................................18

Use Default Client Preference Settings for DNS Dynamic Updates..........................................19

Keep Audit Logging Enabled for Use in Troubleshooting..........................................................19

Use Manual Backup and Restore..............................................................................................19

Prevent Undesired Forwarding and Relay of BOOTP and DHCP Message Traffic...................19

Use the Appropriate Ratio of Servers to Clients........................................................................20

Use 80/20 Scope Distribution Balance......................................................................................20
Avoid Deactivating Scopes........................................................................................................21

Use Proper Superscope Implementation...................................................................................21

Use Multiple DHCP Servers for the Same Superscope ............................................................21

BOOTP Relay Configuration.....................................................................................................21

Summary......................................................................................................................................24

Related Links...............................................................................................................................25

Appendix A:
Predefined Options for DHCP Clients.......................................................................................26


Introduction
The Microsoft® Windows Server™ 2003 network operating system builds on the longstanding Microsoft
support for the Dynamic Host Configuration Protocol (DHCP)—an open, industry standard that reduces
the complexity of administering TCP/IP-based networks. Each host computer connected to a TCP/IP
network must have a unique IP address. DHCP frees network administrators from manually configuring
IP addresses for individual desktop computers.

Internet Engineering Task Force (IETF) Requests for Comments (RFCs) 2131 and 2132 define DHCP
as open and standards-based. A DHCP server stores all available IP addresses in a central database
along with associated configuration information, including the subnet mask, gateways, and the
addresses of DNS servers. This database enables automatic IP address configuration for hosts as they
start up.

DHCP saves network administrative time—and the larger the network, the greater the savings. Without
dynamic address assignment, network administrators must manage IP addresses to avoid duplicate
use and apply configuration changes to workstations manually. The resulting lack of centralized
configuration information makes it difficult for the administrator to ensure consistent client
configurations.

By contrast, DHCP in Windows Server 2003 provides benefits and features that make it a compelling
solution to the networking needs of organizations today.

Dynamic Host Configuration Protocol for Windows Server 2003 1


DHCP Features in Windows Server 2003
DHCP in Windows Server 2003 includes enhancements to RFCs 2131 and 2132, which make DHCP
easier to deploy and manage than other implementations. These enhancements include:
• Integration of DHCP with Domain Name System (DNS).

• Enhanced monitoring and statistical reporting.

• DHCP vendor-specific and class ID option support.

• Multicast address allocation.

• Unauthorized DHCP server detection.

• Windows clustering for high availability.
• Automatic and alternate client configuration.

• Command-line management

Integration of DHCP with DNS
DNS servers provide domain name resolution for network resources. They associate the TCP/IP
address assigned by DHCP to a client with its fully qualified domain name (FQDN). This association, or
mapping, of an IP address to a domain name requires that a change in either the address or the name
necessitates an update of the information in DNS. The DHCP protocol does not automatically update
DNS in the event that the DHCP server changes the IP address of a client. To facilitate this interaction,
servers running Windows Server 2003 and DHCP and clients running DHCP can register with DNS,
allowing cooperation between the two. When DHCP changes IP address information, corresponding
DNS updates synchronize name-to-address associations for the computer.

When a DHCP server registers and updates DNS pointer (PTR) and address (A) resource records on
behalf of its DHCP-enabled clients, it uses the information contained within an additional DHCP option:
the Client FQDN option (option 81), which permits a client to provide its FQDN and any instructions to
the DHCP server that is used to process DNS dynamic updates on its behalf.

The following reasons or events can trigger a dynamic update:
• Added, removed, or modified IP addresses in the TCP/IP properties configuration for any of the
installed network connections.

• An IP address lease changes or renews any of the installed network connections with the DHCP server.
For example, when a computer starts or after use of the ipconfig /renew command.

• Upon use of the ipconfig /registerdns command, which manually forces a refresh of the client name
registration in DNS.

When one of these events triggers a dynamic update, the DHCP Client service (not the DNS Client
service) sends updates. The DHCP Client service performs this function for all network connections on
the client, including any that are not configured to use DHCP.

When a qualified DHCP client issues an update, such as a DHCP-enabled computer running Microsoft
Windows® 2000 or Windows XP Professional, servers running Windows Server 2003 and DHCP



process the update to determine in which of three ways the server will initiate updates on behalf of the
client:
• The DHCP server always registers the DHCP client for both the forward (A resource records) and
reverse lookup or pointer (PTR resource records) with DNS.

• The DHCP server never registers the name-to-address (A resource records) for DHCP clients.

• The DHCP server registers the DHCP client for both forward (A resource records) and reverse lookup
or pointer (PTR resource records) when requested to do so by the client.

The ability to register both A and PTR resource records enables a DHCP server to act as a proxy for
clients running other operating systems, such as Microsoft Windows Millennium Edition, Windows 98,
and Windows NT® Workstation 4.0, for the purpose of DNS dynamic update registration. The DHCP
server can automatically differentiate between Windows 2000 and Windows XP Professional, and other
clients.

DHCP requires the use of DNS dynamic update to keep name-to-address mapping information
synchronized. Using DHCP and DNS together on a network might cause problems when using older,
static DNS servers, which cannot interact dynamically when DHCP client configurations change. You
can avoid failed DNS lookups for DHCP-registered clients when using static DNS service by doing the
following:
• If you are using Windows Internet Name Service (WINS) servers on a network, enable WINS lookup for
DHCP clients that use NetBIOS.

• Assign IP address reservations with an infinite lease duration for DHCP clients that use DNS only and
do not support NetBIOS.

• Wherever possible, upgrade or replace older static DNS servers with DNS servers that support DNS
dynamic updates, Windows 2000 and Windows Server 2003 DNS dynamic updates.

Enhanced Monitoring and Statistical Reporting
Enhanced monitoring and statistical reporting provides notification when the number of available IP
addresses falls below a 90 percent threshold. To alert network managers, icon color, which is
associated with the remaining addresses falling below the defined level, changes to yellow. The icon
color changes to red whenever the addresses become completely depleted.

The DHCP snap-in, which supports Simple Network Management Protocol (SNMP) and Management
Information Bases (MIBs) for DHCP statistics, provides the graphical display of statistical data. This
interface helps administrators monitor system status, such as the number of available addresses versus
depleted addresses, or the number of leases processed per second. Additional statistical information
includes the number of messages and offers processed, as well as the number of requests,
acknowledgements (DHCPACKS), declines, negative status acknowledgment messages
(DHCPNACKS), and releases received.

The DHCP snap-in can display the total number of scopes and addresses on a server, the number
used, and the number available for a particular scope.



DHCP Vendor-Specific and Class ID Option Support
In Windows Server 2003, DHCP supports vendor-specific defining of options. When an option is
identified as vendor-specific, DHCP looks up the configuration as specified for the specific vendor. This
feature enables the quick introduction of compelling custom applications for enterprise networks.
Equipment from multiple vendors on a network can also use different option numbers for different
functions. RFC 2132 describes the vendor class and vendor options.

Most implementations of DHCP treat clients equally, rendering the server unaware of the specific types
of clients with which it interacts. This behavior necessitates a common configuration issued by the
server to all DHCP clients. It also restricts the server to assigning only an address from a scope, along
with the options available within that scope.

In Windows Server 2003, the implementation of user classes allows DHCP clients to distinguish
themselves by specifying a client type, such as a desktop or laptop. An administrator can then configure
a DHCP server to assign different options, depending on the type of client receiving them. For example,
the server could assign short leases to laptop clients, while desktop clients on the same network might
require other settings. User class support gives administrators greater flexibility in configuring clients.

Vendor and user classes are made available for use by DHCP clients and servers in a variety of ways.
The following table compares and contrasts the features of these classes.

Table 1. Vendor and User Classes Available for Use by DHCP Clients and Servers

User Classes Vendor Classes
Part of proposed DHCP standard drafts. For updated Part of accepted DHCP standard drafts (RFC 2131,
information, obtain the latest draft from the Request 2132).
for Comments Web site at http://ietf.org/.
Clients identify user class membership with user class Clients identify vendor class membership by using the
information. vendor class identifier option.
User class IDs can be set and viewed at DHCP clients Vendor class IDs are configured internally by DHCP
running Windows 2000, Windows XP, or Windows client system vendors, such as Microsoft and other
Server 2003 operating systems by using the ipconfig vendors.
command. When adding a vendor class at the server, you need to
specify class data at the server that exactly matches
the class ID used by same-vendor clients. For more
information, contact the applicable vendor.
With user classes, you can take advantage of Vendor classes provide classification of DHCP clients
predefined classes that can be used to support by Windows 98, Windows 2000, or Microsoft DHCP
specialized groups of clients, such as Bootstrap clients, which share the use of the Microsoft-reserved
Protocol (BOOTP) or Routing and Remote Access vendor prefix (MSFT) in any class-related data.
clients.
User classes are typically created for administrative Vendor classes are typically created for the
reasons, such as to identify all DHCP clients in a enhancement of vendor-specific DHCP functionality.
specific site or location. For example, all computers For example, Microsoft provides several additional
and printers located on the same floor in an office vendor-specific options, such as the ability to disable
building might be assigned or configured as part of the NetBIOS over TCP/IP for DHCP clients running
same user class. Windows 2000 or Windows XP.
User classes are most helpful to users for managing Vendor classes are most helpful to vendors for
DHCP option assignments based on their needs. If managing DHCP option assignments based on vendor-
your DHCP network is large enough to benefit from specific needs without disturbing other non-vendor



user classes, you can freely customize your DHCP DHCP clients. With a vendor class, options are passed
environments with them. In most cases, the user class to clients using the vendor specific information option,
provides a way to assign and override standard DHCP which encapsulates the options it sends. This behavior
option values set at either the server, scope, or helps non-vendor clients to recognize the information
reserved client level. as vendor-specific so they can ignore and discard it as
needed.

Multicast Address Allocation
DHCP in Windows Server 2003 allows the assignment of multicast addresses in addition to unicast
addresses. This feature benefits network administrators by enabling assignment of multicast addresses
in the same fashion as unicast addresses, allowing complete utilization of the existing infrastructure.

Conferencing and audio applications typically use multicast address allocation, which requires users to
specially configure multicast addresses. Unlike IP broadcasts, to which all computers on the network
have access, broadcast multicast addresses are used to send traffic to a group of computers using the
concept of group membership to identify the message recipients.

The multicast address allocation feature has two parts: The server implementation provides multicast
addresses and the client has APIs that applications can use to request, renew, and release multicast
addresses. To use this feature, the administrator first configures the multicast scopes and the
corresponding multicast IP ranges on the server using the DHCP snap-in, allowing multicast address
management that is similar to the typical management of IP addresses. The client can use APIs to
request a multicast address from a scope. The underlying implementation uses DHCP-compatible
packets between client and the server.

Unauthorized DHCP Server Detection
DHCP in Windows Server 2003 prevents unauthorized DHCP servers from creating address
assignment conflicts. This feature solves problems that could otherwise occur if users accidentally
created unauthorized DHCP servers that could unintentionally assign IP addresses to clients elsewhere
on the network. For example, a user could create a local DHCP server by using nonunique Net 10
addresses from the private address space, unintentionally leasing the addresses to clients requesting
addresses.

DHCP in Windows Server 2003 includes management features that both prevent unauthorized
deployments and detect existing unauthorized DHCP servers by requiring authentication by an
authorized administrator to make a DHCP server active on the network.

Protecting Against Unauthorized DHCP Servers
When a member of a Microsoft Active Directory® domain DHCP server comes up, it can query against
the list stored in Active Directory and determine if it is authorized. If not, it does not respond to DHCP
requests. Only a domain or enterprise administrator has write access to the folder location in Active
Directory that contains the authorized list.

Administrators create the list of authorized servers in Active Directory with the DHCP console. When a
DHCP server first starts on a network, it tries to establish contact with Active Directory to determine its
membership in the list of authorized servers. If it fails to connect, it cannot respond to client requests.



Figure 1 below illustrates the sequence of checks by which a DHCP server gains authorization on a
network.

Figure 1. DHCP Server Authorization Sequence

Protecting Against Improper Use of Workgroup DHCP Servers

When DHCP servers that do not belong to a domain (such as a member of a workgroup) start, the
following occurs:
• The DHCP server broadcasts a DHCPINFORM message on the network. Any other DHCP server that
receives this message responds with a DHCPACK message and provides the name of its domain.

• If a workgroup DHCP server detects another member DHCP server of a domain on the network, the
workgroup DHCP server does not service requests.

• If the workgroup DHCP server detects the presence of another workgroup server, it ignores it.



Even when a workgroup server starts and is able to run—for example, because of the absence of a
domain member server or workgroup server on the network—it continues to send DHCPINFORM
messages every 60 minutes. If an authorized domain member DHCP server starts later, the workgroup
server becomes unauthorized and stops servicing DHCP requests from clients.

Windows Clustering for High Availability
Windows Clustering allows management of two or more servers as a single system. It also enables
servers running DHCP to provide higher availability, easier manageability, and greater scalability.

Windows Clustering can automatically detect the failure of an application or server and quickly restart it
on an alternate server. As a result, users experience only a momentary pause in service. With Windows
Clustering, administrators can quickly inspect the status of all cluster resources and easily move
workloads to different servers within the cluster, allowing manual load balancing and updates without
taking important data and applications offline.

Windows Clustering enables the virtualization of DHCP servers so that if one of the clustered nodes
becomes unavailable, the namespace and all of the services provided can reconstitute transparently to
the alternate node. This behavior means no changes for the client, which sees the same IP address for
the clustered DHCP server.

Without clustering, network administrators might split scopes between servers. If one server becomes
unavailable, at least half of the available addresses remain available. Clustering makes more efficient
use of IP addresses by removing the need to split scopes. A database stored on a remote disk tracks
address assignment and other activity so that if the active cluster becomes unavailable, the second
node becomes the DHCP server, with complete knowledge of previous assignments and access to the
full scope of addresses. Only one node runs at any given time as a DHCP server; the Windows
Clustering remotely stored database provides transparent transition when needed.

Because Windows Clustering works with all clustering-enabled Windows services, the same cluster
servers used for DHCP can also support high availability for all other clustering-enabled Windows
services.

Automatic Client Configuration
The DHCP client in Windows 2000 and Windows XP has the ability, in the absence of a DHCP server,
to configure an IP address and subnet mask automatically when the client starts on a small private
network. This feature is called Automatic Private IP Addressing (APIPA). APIPA assigns a unique
address from the range 169.254.0.1 through 169.254.255.254 with the subnet mask of 255.255.0.0.

The DHCP client service uses a two-step process to configure the client with an IP address and other
configuration information. Upon installation, the client attempts to locate a DHCP server and obtain a
configuration from it. Most TCP/IP networks use DHCP servers that are configured to dispense
information to clients on the network. For Windows–based operating systems, if the first attempt to
locate a DHCP server fails, the DHCP client configures itself with a selected IP address and APIPA
address.

If the DHCP client has previously obtained a lease from the DHCP server and the lease has not
expired, the client tries to renew any unexpired lease with the DHCP server. If the client fails to locate
any DHCP server, it attempts to ping the default gateway listed in the lease. If this action is successful,



the client assumes it has not moved to a different network and uses that lease. The client then seeks to
automatically renew the lease when half of the lease time has expired.

If the attempt to ping the default gateway fails, the client assumes that it moved to a network that has no
DHCP services available, and it configures itself with an APIPA address. It then automatically keeps
trying to locate a DHCP server every five minutes.

Alternate Client Configuration
With DHCP client alternate configuration, administrators can easily move a computer among two or
more networks, one configured with static IP addresses and one or more configured with DHCP.
Alternate configuration provides simplified computer migration (for example, a laptop) among networks
without requiring reconfiguration of network adapter parameters, such as IP address, subnet mask,
default gateway, preferred and alternate DNS servers, and WINS servers.

Configuration Options

Table 2 shows options that are available for configuring TCP/IP properties for a local area network
(LAN) connection.

Table 2. Configuring TCP/IP Properties for LAN Connections

Option Description
Static IP address When you click Use the following IP address, you can provide values for static IP
configuration address settings, such as IP address, subnet mask, default gateway, preferred and
alternate DNS servers, and WINS servers. However, if you click Obtain an IP address
automatically to change the configuration of your network adapter to a DHCP client
configuration, all static IP address settings are lost. Additionally, if you move the
computer and configure it for another network, when you return to the original network
you must reconfigure the computer with the original static IP address settings.
Dynamic IP address When you click Obtain an IP address automatically, your computer acts as a DHCP
configuration without client and obtains IP address, subnet mask, and other configuration parameters from
alternate configuration your network DHCP server. If the DHCP server is unavailable, the network adapter is
configured using IP autoconfiguration.
Dynamic IP address When you click Obtain an IP address automatically, click the Alternate
configuration with Configuration tab, and type an alternate configuration, you can move your computer
alternate configuration from one statically configured network (such as a home network) to one or more
dynamically configured networks (such as a corporate network) without changing any
settings. If a DHCP server is unavailable (for example, when your computer is
connected to your home network), the network adapter is automatically configured with
your alternate configuration, and the computer functions correctly on the network.
When you move the computer back to the dynamically configured network and the
DHCP server is available, the network adapter is automatically configured with the
dynamic configuration assigned by the DHCP server. The alternate configuration is
used only when the DHCP client cannot locate a DHCP server.

DHCP Server Discover Attempts

If you use DHCP without an alternate configuration and the DHCP client cannot locate a DHCP server,
IP autoconfiguration is used to configure the network adapter. The DHCP client continues to attempt to



discover a DHCP server on the network every five minutes. If a DHCP server is found, the network
adapter is assigned a valid DHCP IP address lease.

If you use DHCP with an alternate configuration, and the DHCP client cannot locate a DHCP server, the
alternate configuration is used to configure the network adapter. Normally, there are no additional
discovery attempts. However, a DHCP server discovery attempt will occur in the following cases:
• The network adapter is disabled and then enabled again.

• Media (such as network cabling) is disconnected and then reconnected.

• The TCP/IP settings for the adapter are changed, and DHCP remains enabled after these changes.

If a DHCP server is found, the network adapter is assigned a valid DHCP IP address lease.

Command-Line Management
The Netsh DHCP component offers a command-line tool that helps with the administration of DHCP
servers and provides an equivalent alternative to console-based management. Command-line
management is useful in the following situations:
• When managing DHCP servers in wide area networks (WANs), you can more effectively manage
across slow-speed network links by using commands in interactive mode at the Netsh command
prompt.

• When managing a large number of DHCP servers, you can script and automate recurring administrative
tasks by using batch mode.

For a comprehensive Netsh DHCP command reference, including syntax, parameters, and examples,
see the topic titled “Netsh commands for DHCP” in Windows Server 2003 Help and Support Center.



DHCP Overview
DHCP is derived from the Internet standard BOOTP (RFCs 951 and 1084), which allows dynamic
assignment of IP addresses as well as remote booting of diskless workstations. In addition to
supporting the dynamic assignment of IP addresses, DHCP supplies all configuration data required by
TCP/IP, plus additional data required for specific services.

As noted, this functionality simplifies tasks for the network administrator, who can now manually
configure just one computer—the DHCP server. Whenever a new computer starts on a network
segment that is served by the DHCP server (or an existing computer is restarted), the computer asks
for a unique IP address and the DHCP server assigns one from the pool of available addresses.

As Figure 2 shows, this process requires only four steps:
1. The DHCP client asks for an IP address (a DHCP Discover message).
2. The DHCP Server offers an address (a DHCP Offer message).

3. The DHCP client accepts the offer and requests the address (a DHCP Request message).

4. The DHCP Server officially assigns the address to the client (a DHCP Acknowledge message).

Figure 2. DHCP Automates the Assignment of IP Addresses

The DHCP server places an administrator-defined time limit, called a lease, on the address assignment.
Halfway through the lease period, the DHCP client requests a lease renewal, and the DHCP server
extends the lease. As a result, when a computer stops using its assigned IP address (for example, upon
relocation to another network segment), the lease expires and the address returns to the pool for
reassignment.

Server, Clients, and Relay Agents
Three basic components make up DHCP in Windows Server 2003:
• DHCP Servers

• DHCP Clients

• DHCP/BOOTP Relay Agents



DHCP Servers

The Microsoft DHCP Server service includes the DHCP snap-in, which is an easy-to-use graphical user
interface management tool that allows network administrators to define DHCP client configurations. The
DHCP server also includes a database for managing assignment of IP addresses and other
configuration parameters.

As listed in Appendix A, the DHCP Server service supports more than 30 DHCP options, which are
defined by the RFC 2132. The DHCP server can assign the following TCP/IP configuration parameters:
• IP addresses for each network adapter in a client computer.

• Subnet masks to identify the IP network portion from the host portion of the IP address.

• Default gateways (routers), which connect a single network segment to others segments.

• Additional configuration parameters, optionally assigned to DHCP clients (such as IP addresses for
DNS or WINS servers that a client might use).

One or more computers on a network must run Windows Server 2003 with TCP/IP and DHCP installed
to provide clients with dynamic IP addresses. After installing the DHCP Server service on a computer
running Windows Server 2003, and creating and activating scopes, the server automatically creates a
DHCP server database.

DHCP Clients

Many low-cost industry standard platforms can act as DHCP clients, as defined in RFC 2132.

The four steps required for a DHCP client to acquire a lease from a DHCP server initiate automatically
when the computer starts. Computers running the following Microsoft operating systems can act as
DHCP clients:
• Windows Server 2003

• Windows XP

• Windows 2000 Server

• Windows 2000 Professional

• Windows NT Workstation 4.0 and earlier

• Windows NT Server 4.0 and earlier

• Windows Millennium Edition

• Windows 98

• Windows 95

• Windows for Workgroups version 3.11 (with the Microsoft 32-bit TCP/IP VxD installed)

In addition to supplying configuration information through DHCP, network administrators can also
override dynamic settings with manual ones. Any information that is manually entered into a client's
TCP/IP configuration overrides dynamic settings.



DHCP BOOTP Relay Agent

BOOTP and DHCP rely on network broadcasts to perform their work. Routers in normal routed
environments do not automatically forward broadcasts from one interface to another. A relay agent
must forward these messages. A router or a host computer configured to listen for DHCP BOOTP
broadcast messages and direct them to a specific server running DHCP can act as a DHCP relay
agent. Using relay agents eliminates the necessity of having a server running DHCP on each physical
network segment. Relay agents direct local DHCP client requests to remote DHCP servers and also
return remote DHCP server responses to the DHCP clients.

Windows Server 2003 includes a DHCP relay agent routing protocol component with the Routing and
Remote Access service. A DHCP relay agent service also comes with Windows Server 2003. Figure 3
shows three common designs.

Figure 3. Three DHCP Configurations Showing the Use of the DHCP BOOTP Relay Agent

Managing DHCP
The DHCP snap-in helps network administrators configure and monitor DHCP servers. Network
administrators can define global and scope-specific configuration settings to identify routers and set
DHCP client configurations.



A computer running Windows Server 2003 and TCP/IP automatically creates a DHCP server database
upon installation of the DHCP Server service. After installation, changing the parameters associated
with the terms in Table 3 can further customize the service.

Table 3. Terminology Associated with Customizing DHCP Parameters

Term Description
Scope A scope is the full consecutive range of possible IP addresses for a network. Scopes typically
define a single physical subnet on your network to which DHCP services are offered. Scopes
also provide the primary way for the server to manage distribution and assignment of IP
addresses and any related configuration parameters to clients on the network.
Superscope A superscope is an administrative grouping of scopes that can be used to support multiple
logical IP subnets on the same physical subnet. Superscopes contain only a list of member
scopes or child scopes that can be activated together. Superscopes are not used to configure
other details about scope usage. For configuring most properties used within a superscope, you
need to configure member scope properties individually.
Exclusion An exclusion range is a limited sequence of IP addresses within a scope, excluded from DHCP
range service offerings. Exclusion ranges assure that any addresses in these ranges are not offered by
the server to DHCP clients on your network.
Address After you define a DHCP scope and apply exclusion ranges, the remaining addresses form the
pool available address pool within the scope. Pooled addresses are eligible for dynamic assignment
by the server to DHCP clients on your network.
Lease A lease is a length of time that a DHCP server specifies, during which a client computer can use
an assigned IP address. When a lease is made to a client, the lease is active. Before the lease
expires, the client typically needs to renew its address lease assignment with the server. A lease
becomes inactive when it expires or is deleted at the server. The duration for a lease determines
when it will expire and how often the client needs to renew it with the server.
Reservation You use a reservation to create a permanent address lease assignment by the DHCP server.
Reservations assure that a specified hardware device on the subnet can always use the same IP
address.
Option types Option types are other client configuration parameters a DHCP server can assign when
providing leases to DHCP clients. For example, some commonly used options include IP
addresses for default gateways (routers), WINS servers, and DNS servers. Typically, these
option types are enabled and configured for each scope. The DHCP console also permits you to
configure default option types that are used by all scopes added and configured at the server.
Most options are predefined through RFC 2132, but you can use the DHCP console to define
and add custom option types if needed.
Options An options class is a way for a server to further manage option types provided to clients. When
class an options class is added to a server, clients of that class can be provided class-specific option
types for their configuration. For Windows 2000 and Windows XP, client computers can also
specify a class ID when communicating with the server. For earlier DHCP clients that do not
support class IDs, the server can be configured with default classes to use instead when placing
clients in a class. Options classes can be one of two types: Vendor classes and user classes.



DHCP Deployment
DHCP has become such an important element of efficient network design that network administrators
want to ensure proper DHCP deployment. Basic considerations of DHCP deployment include:
• Determining the number of DHCP servers to use.

• Determining and configuring scopes.

• Using superscopes.

• Reserving IP addresses.

• Using BOOTP tables.

Determining the Number of DHCP Servers to Use
One active DHCP server and one backup DHCP server can support a large number of clients,
depending on hardware configurations and other issues. However, when deciding on the number of
DHCP servers required, consider the location of routers on the network and whether each subnet
requires a DHCP server. In addition, be sure to account for slower WAN links or dial-up links between
segments utilizing the DHCP service. Network administrators typically deploy a DHCP server on both
sides of these links to serve clients locally.

A network can have practical size constraints based on the size of a broadcast IP address class, such
as the 254-node limit of class C networks. In addition, server configuration issues, such as disk capacity
and CPU speed, can affect performance.

Defining and Configuring Scopes
Administrators create an administrative grouping of computers called a scope for each physical subnet.
Administrators plan scopes based on the needs of specific groups of users, with appropriate lease
durations defined for the related scopes. A scope has the following properties:
• A range of possible IP addresses from which to include or exclude addresses used in DHCP service
lease offerings.

• A unique subnet mask to determine the subnet related to a given IP address.

• A scope name assigned when the scope is created.

• Lease duration values to be assigned to DHCP clients that receive dynamically allocated IP addresses.

• Reservations.

• Options.

A DHCP scope consists of a pool of IP addresses on a subnet, such as 10.223.223.1 through
10.223.223.200, which the DHCP server can lease to DHCP clients. Each physical network can have
only one DHCP scope or a superscope with one or more ranges of IP addresses.

Larger networks might require the following configuration steps to assign several address ranges within
a single scope or subnet for DHCP service:



• Define the scope by using the entire range of consecutive IP addresses that make up the local IP
subnet.

• Set exclusion ranges, which define the IP addresses within the scope that the DHCP server does not
lease. For example, to exclude the first 10 addresses in the previous example scope, create an
exclusion for 10.223.223.1 through 10.223.223.10. Doing so specifies that no DHCP clients ever
receive these addresses for leased configuration.

Depending upon the complexity of your network, a defined scope might require the following additional
configuration steps:
• Select additional exclusion ranges to further exclude any IP addresses that are not to be leased to
DHCP clients. Exclusions should include all devices that cannot use DHCP, such as printers.

• Create reservations by reserving some IP addresses for permanent lease assignment to specified
computers or devices on a network. Reservations should be made only for devices that use DHCP and
have a specialized function on the network, such as special server computers (servers used for DHCP,
WINS, or DNS) and routers.

• Adjust the duration of leases. Note that the default lease duration of eight days might not require
adjustment.

After defining and configuring a scope as outlined above, you must activate the scope before dynamic
service begins for DHCP-enabled clients. After you do this, the server can begin processing IP address
lease requests and offering leases to DHCP-enabled clients on the network.

Using Superscopes
Superscopes add extra flexibility to complex DHCP service configurations by:
• Supporting DHCP clients on a single physical network segment that has multiple logical IP subnets.

• Supporting remote DHCP clients located on the far side of BOOTP/DHCP relay agents.

Superscopes offer greater flexibility when:
• Adding more computers than originally planned.

• Renumbering the network.

• Two DHCP servers manage separate logical subnets on the same physical subnet.

Table 4 shows two DHCP servers that can be reached on the same physical subnet and configured
with a single scope.

Table 4. Single-Scope DHCP Servers A and B on the Same Physical Subnet

DHCP Server Name Starting IP Address Ending IP Address
DHCP-Server A 211.111.111.1 211.111.111.255
DHCP-Server B 222.222.222.1 222.222.222.255



If DHCP-Server A manages a different scope of addresses than DHCP-Server B, and neither has any
information about addresses managed by the other, a problem arises if a client previously registered
with Server A, for example, releases its name during a shutdown and later reconnects to the network
after restarting. The client tries to renew its previously leased IP address.

If DHCP-Server B receives a DHCPRequest message from the client to renew use of an address before
Server A does, Server B, being unaware of that IP address, causes it to reject the request and send a
DHCPNACK message to the client. The client must then renegotiate a DHCP lease by broadcasting a
DHCPDiscover message on the local subnet. Server B can send a DHCPOffer message, offering the
client an address. The client can accept the address by returning a DHCPRequest message for that
address to Server B for approval. When Server B approves the address assignment, it returns a
DHCPACK message to the client.

Several DHCP service problems can occur within this example:
• Nothing prevents a client from having its attempt to renew a previous address rejected each time it
connects to the network.

• The server might, when the client rejects and re-requests an address lease, offer an address that
places the client on a subnet requiring an alternate configuration.

These problems can be averted by using the following steps to implement superscopes on both DHCP
servers, a solution which enables more predictable and effective address management:
1. Create a new scope on a server that contains the respective scope information for the other. For
example, on DHCP-Server A, create a new scope with the range of 222.222.222.1 through
222.222.222.255. Also create an exclusion range for the new scope for all scope addresses
(222.222.222.1 through 222.222.222.255).

2. Repeat the previous step for the other DHCP server. For example, on DHCP-Server B, create a new
scope with the range of 211.111.111.1 through 211.111.111.255, as well as an exclusion range for
this new scope for all scope addresses (211.111.111.1 through 211.111.111.255).

3. Create a superscope on each DHCP server by using the Add Superscope wizard. Add both the old
and the new scopes to the superscope.

4. Activate the new scopes on each server.

By configuring superscopes as described, both DHCP-Server A and DHCP-Server B recognize IP
addresses assigned by the other. This procedure prevents either server from negatively acknowledging
attempts by DHCP clients to renew their same IP address or to obtain an address from the same logical
range of addresses—in other words, a different address within the same logical subnet.

Reserving IP Addresses
Through DHCP, a specific IP address can be reserved for a computer or other IP addressable device
on a network. Reserving selected IP addresses for special-function devices on a network ensures that
DHCP does not duplicate or reassign the address. The following types of devices and computers can
use reservations:
• Other servers running Windows on a network that require static IP addresses, such as WINS servers.

• Any print servers that use TCP/IP print services.



• UNIX or other clients that use IP addresses assigned by another TCP/IP configuration method.

• Any DNS servers.

Each reservation requires a unique identifier for the address of the reserved device, which corresponds
to the media access control (MAC) or physical address for the DHCP client. Ethernet addresses, made
up of a unique sequence of hexadecimal numbers, identify the network adapter hardware for each
network-connected device.

Note: To obtain MAC addresses on Windows NT–based clients, type ipconfig /all at the command prompt
and view the Physical Address field. For Windows 9x–based clients, run Winipcfg.exe, and view the
Adapter Address field.

Using BOOTP Tables
The DHCP Server service offers BOOTP support in the form of pointer records contained in the BOOTP
table. BOOTP, which preceded DHCP, enables diskless clients to obtain their own IP addresses and
other boot information needed for network startup. Many Windows NT–based installations do not
require BOOTP, so the BOOTP table does not need to be configured.

BOOTP allows diskless clients to use User Datagram Protocol (UDP) packets to request and retrieve
an IP address and a small boot image file from a Trivial File Transfer Protocol (TFTP) server.

Data stored in the BOOTP table is returned to any BOOTP network clients that broadcast a BOOTP
request message. If a BOOTP record exists in the BOOTP table, the DHCP server returns a BOOTP
message to the requesting BOOTP client. If no BOOTP records are configured, the DHCP Server
service silently drops BOOTP request messages.

The reply message returned by the DHCP Server service indicates the name and location of a TFTP
server on the network, which the client can contact to retrieve its boot image file. Each record in the
BOOTP table contains the following three fields, which in turn contain the information returned to the
BOOTP client:
• The Boot Image field identifies the generic file name of the boot file requested based on the BOOTP
client’s computer type.

• The File Name field identifies the full path of the boot file returned by TFTP by the BOOTP server to the
client.

• The File Server field identifies the TFTP server used to source the boot file.

You can use the DHCP snap-in to add, remove, and edit records in the BOOTP table.

Although both BOOTP and DHCP allocate IP addresses to clients during startup, they use different
methods of allocation. BOOTP typically provides fixed allocation of a single IP address for each client,
permanently reserving this address in the BOOTP server database. DHCP typically provides dynamic,
leased allocation of available IP addresses, reserving each DHCP client address temporarily in the
DHCP server database.



Best Practices
Best practices optimize the functionality and performance of the DHCP service in Windows Server
2003. These are described below.

Set Appropriate Lease Durations
Because lease renewal processes can affect the performance of DHCP clients and the network,
selecting a different lease duration can improve overall network performance. The following guidelines
will help administrators determine the best configuration for their network.

Lengthening Lease Duration for Large, Fixed Networks

It’s a good practice to increase scope lease length for large, stable, fixed networks that have plentiful
scope address space. Increasing the lease duration lowers the frequency of lease renewal queries
between clients and the DHCP server, thus reducing associated network traffic. Most useful for larger
routed networks, lengthening the default lease period from seven to perhaps 21 days reduces DHCP-
related network broadcast traffic, particularly if client computers generally remain in fixed locations and
scope addresses remain plentiful, such as with less than 80 percent in use.

Shortening Lease Duration for Variable Networks with Fewer IP Addresses

By contrast, for networks that have fewer IP addresses available and either client configurations or
network locations that change, it’s best to reduce the lease duration, which increases the rate at which
addresses return to the available address pool for reassignment to new clients by the DHCP server. A
sales organization, for example, which might issue laptop computers to traveling employees, might find
this practice especially beneficial. When the Routing and Remote Access service supports dial-up
clients on the network, adjust lease time on scopes that serve these clients to less than the default of
eight days.

Most network configurations require a mixture of lease durations. With a single segment where laptops
come and go, shortening the lease on that scope might improve performance, while other parts of a
network with a stable body of clients could set the lease duration somewhat higher.

Reserve Addresses with Reservations
It’s a best practice to use a client reservation to ensure that a DHCP client computer always receives
the same IP address lease at startup. If you have more than one DHCP server that is can be reached
by a reserved client, add the reservation at each of your other DHCP servers. This practice allows the
other DHCP servers to honor the client IP address reservation made for the reserved client. Although
the DHCP server will act upon the client reservation when the reserved address is part of the available
address pool, having the same reservation for the same client on multiple servers will not usually cause
any problems.

Integrate DHCP with Other Services
Both WINS and DNS can register dynamic name-to-address mappings on a network. Operating DHCP
with other name resolution services requires careful planning, and network administrators implementing
DHCP should also develop a strategy for implementing DNS and WINS.



Use Default Client Preference Settings for DNS Dynamic Updates
DHCP in Windows Server 2003 can perform DNS dynamic updates for DHCP clients based on how
clients request these updates. This setting provides the best use of the DHCP service to perform
dynamic updates on behalf of its clients as follows:
• DHCP client computers running Windows 2000, Windows XP, or a Windows server operating system
explicitly request that the DHCP server update only pointer (PTR) resource records used in DNS for the
reverse lookup and resolution of a client's IP address to its name. These clients update their address
(A) resource records for themselves.

• Clients running earlier versions of Windows cannot make an explicit request for DNS dynamic update
protocol preference. When configured to do so, the DHCP Server service updates both the PTR and A
resource records on behalf of the clients.

Keep Audit Logging Enabled for Use in Troubleshooting
By default, the DHCP Server service enables audit logging of service-related events. Audit logging
provides a long-term service monitoring tool that makes limited use of server disk resources.

Use Manual Backup and Restore
Use the Backup command in the Action menu of the DHCP console to perform full backup of the
DHCP service at an interval that helps protects you from significant data loss. The manual backup
method includes all DHCP server data in the backup, including all scope information, log files, registry
keys, and DHCP server configuration information (except DNS dynamic update credentials). Do not
store these backups on the same hard drive on which the DHCP Server service resides, and make sure
that the access control list (ACL) for the backup folder contains only the Administrators and DHCP
Administrator groups as members.

In addition to performing manual backups, back up to other locations, such as a tape drive, and make
sure unauthorized persons do not have access to your backup copies. You can use the Windows
Backup feature for this purpose. For more information, see “Best Practices for Backup” in the Windows
2003 Server Help and Support Center1.

When restoring the DHCP service, you can use a backup created with either the Windows Backup
feature or a copy of the database created with synchronous backup, which is available with the DHCP
service. In addition, you can use the Restore command on the Action menu in the DHCP snap-in to
restore a server running DHCP.

Prevent Undesired Forwarding and Relay of BOOTP and DHCP Message Traffic.
For routed networks, you can either use relay agents or set appropriate timers to prevent undesired
forwarding and relay of BOOTP and DHCP message traffic. If you have multiple physical networks
connected through routers, and you do not have a DHCP server on each network segment, the routers
must be capable of relaying BOOTP and DHCP traffic. If you do not have such routers, you can set up
the DHCP Relay Agent component on at least one server running Windows in each routed subnet. The
relay agent sends DHCP and BOOTP message traffic among the DHCP-enabled clients on a local

1
The Windows Server 2003 product documentation, including online help topics, is available on the TechNet Web site at
http://www.microsoft.com/technet/treeview/default.asp?
url=/technet/prodtechnol/windowsserver2003/proddocs/standard/sag_backconcepts_03.asp



physical network and a remote DHCP server located on another physical network. When using relay
agents, make sure to set the initial time delay in seconds so that relay agents wait before sending
messages to remote servers.

Use the Appropriate Ratio of Servers to Clients
It is important to carefully determine how many DHCP servers are needed to serve all DHCP-enabled
clients on a network. In a small LAN, such as one physical subnet without routers, a single DHCP
server might serve all DHCP-enabled clients. However, routed networks might require several DHCP
servers.

Theoretically, there is no limit to the maximum number of clients that can be served by a single DHCP
server; however, there are practical constraints based on the IP address class of a network and server
configuration issues, such as disk capacity and CPU speed.

Transmission speed between each segment for which DHCP service is provided is an important factor.
With slower WAN links or dial-up links, a DHCP server is typically needed on both sides of these links
to serve clients locally. Another factor is whether DHCP service is used in all or only selected physical
networks. When deploying multiple DHCP servers for an environment, it is advisable to place them on
different network segments for the case where a network segment becomes unreachable. DHCP Relay
agents turn the broadcast into a unicast packet.

Before installing the DHCP server, it is necessary to determine the following:
• The hardware and storage requirements for the DHCP server.

• The computers that can be configured immediately as DHCP clients for dynamic TCP/IP configuration
and which must be manually configured with static TCP/IP configuration parameters, such as static IP
addresses.

• The DHCP option types and their values to be predefined for DHCP clients.

• The DHCP Relay Agent configuration for your network.

Use 80/20 Scope Distribution Balance
You can split a scope between two or three servers so that a network can more easily handle DHCP
traffic floods. In addition, stopped servers will not noticeably affect the network. An 80/20 split offers the
optimum benefit.

For example, consider a Class B scope 132.255.0.0 with an address range from 132.255.0.1 through
132.255.255.255 and a subnet mask of 255.255.0.0. You could set up two servers (SRV1 and SRV2) to
distribute the load as follows:
• SRV1 has a scope of 132.255.0.1 through 132.255.255.255 with a subnet mask of 255.255.0.0. The
exclusion range for this scope is 132.255.128.0 through 132.255.255.255.

• SRV2 has a scope of 132.255.0.1 through 132.255.255.255 with a subnet mask of 255.255.0.0. The
exclusion range for this scope is 132.255.0.1 through 132.127.255.255.

Similarly, you can divide a scope between three servers.



Avoid Deactivating Scopes
Do not deactivate a scope until you are ready to remove it (and its included range of addresses)
permanently from use on the network. After a scope is deactivated, the DHCP server no longer accepts
the scope addresses as valid, which can cause unwanted DHCP negative acknowledgement messages
(DHCPNAKs). You can temporarily deactivate scope addresses by modifying exclusion ranges in an
active scope to achieve the intended result without unwanted messages.

Use Proper Superscope Implementation
Although superscopes can ease DHCP management, they are not required just because a DHCP
server is handling more than one scope (subnet ID). A single DHCP server can serve two or more
physically different subnets separated by a router, where BOOTP/DHCP relay agents are configured to
provide relay of client requests for scopes that serve subnets located away from the DHCP server.
Relay agents are typically included with your routers and, where used, must be configured with IP
addresses for your DHCP servers.

Use Multiple DHCP Servers for the Same Superscope
When using more than one DHCP server to serve a superscope segment, the superscope for each
DHCP server should include all subnets, using placeholder scopes for the subnets to which it does not
supply addresses but must recognize as valid.

For example, consider a segment running four logical IP subnets: 192.168.1.0, 192.168.2.0,
192.168.3.0, and 192.168.4.0, all with a subnet mask of 255.255.255.0. Two servers running DHCP
support this segment, each configured with a superscope covering half of the subnets (the SRV1
superscope contains only subnets 192.168.1.0 and 192.168.2.0; and the SRV2 superscope contains
only subnets 192.168.3.0 and 192.168.4.0). As DHCP requests arrive from clients, addresses can come
from either of the superscopes. However, a problem might arise if a client gets an IP address from
SRV1, and then SRV2 receives its renewal request. SRV2 does not recognize the client’s address as
belonging to that subnet and responds to the client by sending a DHCPNACK.

You can avoid this problem by configuring both SRV1 and SRV2 with all logical IP subnets and using
exclusions to prevent the servers from overlapping address ranges. SRV1 should have a superscope
containing all four subnets and excluding all the addresses of the last two subnets. SRV2 should also
have a superscope containing all four subnets but excluding all the addresses of the first two subnets.

BOOTP Relay Configuration
The correct deployment of DHCP servers prevents BOOTP relay agents from generating duplicate
packets, which can cause the DHCP server to receive several copies of the same Discover or Request
message. For example, Figures 4 and 5 show two BOOTP relay designs that have the same number of
networks, servers, and routers. The Figure 4 design causes eight packets to reach the DHCP servers
for every DHCP message sent by a client. The network design in Figure 5 eliminates duplicate packets
while providing enough fault-tolerant redundancy such that any single part of the network can fail, but
clients continue to received leases.



Figure 4. Inefficient Network Relay Design



Figure 5. A Network Design that Eliminates Duplicate Packets and Provides Fault-Tolerance



Summary
DHCP provides an efficient and reliable TCP/IP network configuration. The DHCP service also helps
prevent IP address conflicts and conserves the use of IP addresses through centralized management of
address allocation. In contrast to manual configuration, where each client computer must have its IP
address information individually set before it can join the network, DHCP offers a form of instant access
for supported clients that use DHCP.

The DHCP service provided in Windows Server 2003 builds on a long history of support for DHCP and
adherence to open industry standards, while introducing features that make DHCP easy to deploy and
manage. Network administrators benefit from the integration of DHCP with DNS as well as enhanced
monitoring and statistical reporting for DHCP servers, new vendor-specific options and user-class
support, multicast address allocation, unauthorized DHCP server detection, and Windows Clustering
Services.



Related Links
See the following resources for further information:
• Windows Server 2003 DHCP Server Role at
http://www.microsoft.com/technet/prodtechnol/windowsserver2003/serverroles/dhcpserver/default.asp

• Windows Server 2003 Networking and Communications Services at
http://www.microsoft.com/windowsserver2003/technologies/networking/default.mspx

• Windows Server 2003 DNS Server Role at http://www.microsoft.com/technet/treeview/default.asp?
url=/technet/prodtechnol/windowsserver2003/serverroles/dnsserver/default.asp

• Windows Server 2003 WINS Server Role at http://www.microsoft.com/technet/treeview/default.asp?
url=/technet/prodtechnol/windowsserver2003/serverroles/winsserver/default.asp

For the latest information about Windows Server 2003, see the Windows Server 2003 Web site at http://
www.microsoft.com/windowsserver2003.



Appendix A:
Predefined Options for DHCP Clients
The following tables describe the predefined options available for configuring servers running DHCP to
automatically configure DHCP clients. Options listed in bold are options that Windows–based DHCP
clients request from the server by default.

Table A-1. Basic Options

Code Option Name Meaning
0 Pad Causes subsequent fields to align on word boundaries.
255 End Indicates end of options in the DHCP packet.
1 Subnet Mask
2 Time offset Specifies the Universal Coordinated Time (UCT) offset in seconds.
3 Router Specifies a list of IP addresses for routers on the client's subnet.¹
4 Time server Specifies a list of IP addresses for time servers available to the client.¹
5 Name servers Specifies a list of IP addresses for name servers available to the client.¹
6 DNS servers Specifies a list of IP addresses for DNS name servers available to the client.¹
7 Log servers Specifies a list of IP addresses for MIT_LCS User Datagram Protocol (UDP) log
servers available to the client.2
8 Cookie servers Specifies a list of IP addresses for RFC 865 cookie servers available to the client.¹
9 LPR servers Specifies a list of IP addresses for RFC 1179 line-printer servers available to the
client.¹
10 Impress Specifies a list of IP addresses for Imagen Impress servers available to the client.¹
servers
11 Resource Specifies a list of RFC 887 Resource Location servers available to the client.¹
location
servers
12 Host name Specifies the host name of up to 63 characters for the client. The name must start
with a letter, end with a letter or digit, and have as interior characters only letters,
numbers, and hyphens. The name can be qualified with the local DNS domain
name.
13 Boot file size Specifies the size of the default boot image file for the client, in 512-octet blocks.
14 Merit dump file Specifies the ASCII path name of a file where the client's core image is dumped if a
crash occurs.
15 Domain name Specifies the DNS domain name that the client should use for DNS host name
resolution.
16 Swap server Specifies the IP address of the client's swap server.

2
List is specified in order of preference.



17 Root path Specifies the ASCII path for the client's root disk.
18 Extensions Specifies a file retrievable through TFTP, containing information interpreted like the
path vendor-extension field in the BOOTP response, except that the file length is
unconstrained and references to Tag 18 in the file are ignored.

The following table lists IP layer parameters on a per-host basis.

Table A-2. IP Layer Parameters per Host

19 IP layer Enables or disables forwarding of IP packet for this client: 1 enables forwarding; 0
forwarding disables it.
20 Nonlocal Enables or disables forwarding of datagrams with nonlocal source routes. 1 enables
source routing forwarding; 0 disables it.
21 Policy filter Specifies policy filters that consist of a list of pairs of IP addresses and masks
masks specifying destination/mask pairs for filtering nonlocal source routes. Any source
routed datagram with a next-hop address that does not match a filter is discarded by
the client.
22 Max DG Specifies the maximum size datagram that the client can reassemble. The minimum
reassembly value is 576.
size
23 Default time- Specifies the default time-to-live (TTL) that the client uses on outgoing datagrams.
to-live The value for the octet is a number ranging from 1 through 255.
24 Path MTU Specifies the timeout in seconds for aging Path Maximum Transmission Unit (MTU)
aging time-out values (discovered by the mechanism defined in RFC 1191).
25 Path MTU Specifies a table of MTU sizes to use when performing Path MTU Discovered as
plateau table defined in RFC 1191. The table is sorted by size from smallest to largest. The
minimum MTU value is 68.

The following table lists IP parameters on a per-interface basis. These options affect the operation of
the IP layer on a per-interface basis. A client can issue multiple requests, one per interface, to configure
interfaces with their specific parameters.

Table A-3. IP Parameters per Interface

26 MTU option Specifies the MTU discovery size for this interface. The minimum MTU value is 68.
27 All subnets Specifies whether the client assumes that all subnets of the client's internetwork use
are local the same MTU as the local subnet where the client is connected. 1 indicates that all
subnets share the same MTU; 0 indicates that the client should assume some
subnets may have smaller MTUs.
28 Broadcast Specifies the broadcast address used on the client's subnet.
address
29 Perform mask Specifies whether the client should use Internet Control Message Protocol (ICMP) for
discovery subnet mask discovery. 1 indicates that the client should perform mask discovery; 0
indicates that the client should not.



30 Mask supplier Specifies whether the client should respond to subnet mask requests using ICMP. 1
indicates that the client should respond; 0 indicates that the client should not.
31 Perform Specifies whether the client should solicit routers using the router discovery method in
router RFC 1256. 1 indicates that the client should perform router discovery; 0 indicates that
discovery the client should not.
32 Router Specifies the IP address to which the client submits router solicitation requests.
solicitation
address
33 Static route Specifies a list of IP address pairs that indicate the static routes the client should
install in its routing cache. Any multiple routes to the same destination are listed in
descending order or priority. The routes are destination/router address pairs. (The
default route of 0.0.0.0 cannot be used as a destination for a static route.)

The following table lists link layer parameters on a per-interface basis. These options affect the
operation of the data link layer on this basis.

Table A-4. Link Layer Parameters per Interface

34 Trailer Specifies whether the client should negotiate use of trailers (RFC 983) when using
encapsulation the ARP protocol. 1 indicates that the client should attempt to use a trailer; 0
indicates that the client should not.
35 ARP cache Specifies the timeout, in seconds, for ARP cache entries.
time-out
36 Ethernet Specifies whether the client should use Ethernet version 2 (RFC 894) or IEEE
encapsulation 802.3 (RFC 1042) encapsulation if the interface is Ethernet: 1 indicates that the
client should use RFC 1042 encapsulation; 0 indicates that the client should use
RFC 894 encapsulation.

The following table shows TCP parameters. These options affect the operation of the TCP layer on a
per-interface basis.

Table A-5. TCP Parameters

37 Default time-to- Specifies the default TTL the client should use when sending TCP segments. The
live minimum value of the octet is 1.
38 Keep-alive Specifies the interval in seconds the client TCP should wait before sending a keep-
interval alive message on a TCP connection: 0 indicates that the client should not send
keep-alive messages on connections unless specifically requested by an
application.
39 Keep-alive Specifies whether the client should send TCP keep-alive messages with an octet of
garbage garbage data for compatibility with older implementations: 1 indicates that a
garbage octet should be sent; 0 indicates that it should not.



The following table shows application layer parameters. These miscellaneous options are used to
configure applications and services.

Table A-6. Application Layer Parameters

40 NIS domain Specifies the name of the Network Information Service (NIS) domain as an ASCII
name string.
41 NIS servers Specifies a list of IP addresses for NIS servers available to the client.3
42 NTP servers Specifies a list of IP addresses for Network Time Protocol (NTP) servers available
to the client.¹

Table A-7. Options for Vendor-specific Information.

43 Vendor-specific Specifies binary information used by clients and servers to exchange vendor-
info specific information. Servers not equipped to interpret the information ignore it.
Clients that do not receive the information attempt to operate without it.

Table A-8. NetBIOS over TCP/IP

44 WINS/NBNS Specifies a list of IP addresses for NetBIOS name servers (NBNS).¹
servers
45 NetBIOS over TCP/ Specifies a list of IP addresses for NetBIOS datagram distribution servers
IP NBDD (NBDD).¹
46 WINS/NBT node Allows configurable NetBIOS over TCP/IP clients to be configured as
type described in RFC 1001/1002, where 1=b-node, 2=p-node, 4=m-node, and
8=h-node.
47 NetBIOS scope ID Specifies a string that is the NetBIOS over TCP/IP Scope ID for the client,
as specified in RFC 1001/1002.
48 X Window system Specifies a list of IP addresses for X Window font servers available to the
font client.4
49 X Window system Specifies a list of IP addresses for X Window System Display Manager servers
display available to the client.¹

3

4


Dhcp2003

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