DYNAMIC HOST CONFIGURATION PROTOCOL

Available online at www.sciencedirect.com
University Of Limerick
Procedia Economics and Finance 00 (0661) 200–700
DYNAMIC HOST CONFIGURATION PROTOCOL
Venkateshan Alamelu Srinivasan
University Of Limerick,Co.Limerick,Ireland
Abstract
This report discusses about the structure, configuration of the addresses and message exchange sequence in DHCP. A
Dynamic Host Configuration Protocol (DHCP) server is a well-known server deployed at a network to manage Internet
Protocol (IP) addresses temporarily rentable to hosts in the network [3]. Without DHCP, network administrators should
manually enter the IP address for each machine and network devices and then the address is manually changed each time
the device is moved to a different part of the network [2]. Hence in order to make this task simpler the DHCP has been
devised which allocates the address both manually and automatically.
© 2012 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of Global
Science and Technology Forum Pte Ltd
Keywords: DHCP; IP Address; BOOTP; Address Allocation; client; server; relay; host
1.Introduction
The Dynamic Host Configuration Protocol (DHCP) provides a mechanism for transmitting configuration
parameters to hosts using the TCP/IP protocol suite. IP addresses can be configured statically or dynamically.
Typically we configure static IP addresses on system gadgets like switches, routers, firewalls and servers
while we dynamically assign IP address to machines, laptops, tablets, cell phones. Once an association has
acquired a block of address, it can allot individual IP address to the host and router interfaces in its
organization. The role of the DHCP is to get an IP address for the host. In addition to the host IP address task,
DHCP also allows a host to learn additional information, such as its subnet mask, the address of its first-hop
router (often called the default gateway), and the address of its nearby DNS server.
A system administrator will physically configure the IP address into the router (often remotely, with a
network management tool) regularly. Host address can also be configured manually, but more regularly this

2 Venkateshan Alamelu Srinivasan/ Procedia Economics and Finance 00 (089) 972–9597
undertaking is currently done using the DHCP (RFC 2131) [8]. DHCP permits the host to acquire an IP
address automatically. A network administrator can configure DHCP so that a given host receives the same IP
address each time it interfaces to the network, or a host may be assigned a temporary IP address that will be
different each time the host connects to the network.
1.1. Background
DHCP is derived from an earlier protocol called BOOTP, and a portion of the packet fields are thus not
strictly relevant to host configuration [10]. BOOTP is a static configuration protocol. The DHCP has been
devised to provide dynamic configuration. DHCP is an augmentation BOOTP. It upgrades BOOTP and is
backward compatible with BOOTP [10]. This implies a host running the BOOTP client can ask for a static
setup from a DHCP server.
1.2. Structure
As IP address management has got to be exceptionally troublesome with the ever growing number of attached
stations, subnets and networks and also strict tenets have been imposed on IP address assignment due to lack
of address space and internet router's capacity, a move from inherent statically assigned addressing has led to
the development of a dynamic protocol. DHCP was developed in 1997(RFC 2131)[8][9]
It is often referred to as plug-and-play protocol due to DHCP's ability to automate the network-related aspects
of connecting a host into a network. This capability makes it very attractive to the network administrator who
would generally need to perform these errands manually. DHCP is also enjoying widespread application in
residential Internet Access Networks and in wireless LANs, where host join and leave the network frequently.
Let’s consider an example of a student who carries a portable computer (laptop) from a dormitory room to a
library to a classroom. It is likely that at each location, the student will be connecting into a new subnet and
hence will need a new IP address at each location. DHCP is preferably suited to this situation as there are
numerous users coming and going and addresses are needed for only a limited amount of time. DHCP is
similarly useful in residential ISP access networks. Let’s Consider an example of a residential ISP that has
2000 customers, but not more than 400 customers are ever online at the same time[7]. In this case, rather than
needing a block of 2048 addresses, a DHCP server that assigns addresses dynamically needs only a block of
512 addresses (for example, a block of the form a. b. c. d/23). As the hosts join and leave, the DHCP server
needs to update its list of available IP addresses. Each time a host joins, the DHCP server allocates an
arbitrary address from its current pool of available addresses and each time the host leaves, its address is
returned to the pool [7]. The figure below shows DHCP server [7] attached to subnet 223.1.2/24, with the
router serving as the relay agent for the arriving clients attached to subnets 223.1.1/24 and 223.1.3/24.

Fig.1: DHCP client-server scenario.
1.3 DHCP Client Server Interaction:
DHCP is a client-server protocol and is a four step process. A client is typically a recently arriving host
wanting to obtain network configuration information, including an IP address for itself. In simplest case, each
subnet will have a DHCP server. If no server is available on the subnet, a DHCP relay agent (typically a
router) that knows the address of a DHCP server required for that network. The four steps are
 DHCP Server Discovery.
 DHCP Server Offers.
 DHCP Request.
 DHCP Ack.
Fig.3.DHCP Protocol with Network Topology[1]
1.3.1DHCP Server Discovery
Since the objective of DHCP is to minimize the amount of manual configuration needed for a host to function,
it would rather crush the purpose if each host had to be configured with the address of a DHCP server.Thus
the first problem faced by DHCP is that of server discovery. The principal errand of a recently arriving host

is to discover a DHCP server with which to interact. This is carried out using DHCP discover message, which
a client sends within UDP packet to port 67[5]. The UDP packet is encapsulated in an IP datagram. But to
whom should this datagram be sent?. The host doesn't even know the IP address of the network to which it is
attaching, much less the address of a DHCP server for this network. The DHCP client creates an IP datagram
containing its DHCP discover message along with the broadcast destination IP address of 255.255.255.255
and a 'this host' source IP address of 0.0.0.0.The DHCP client passes the IP datagram to the link layer,which
then broadcasts this frame to all nodes attached to the subnet.
OPERATION HType HLen Hops
XID
Secs Flags
Ciadrrr
Yiaddr
Siaddr
Giaddr
Chaddr (16 bytes)
Sname (64 bytes)
File (128 bytes)
Options
Fig.4. DHCP Packet Format.
1.3.2 DHCP Server offers
The DHCP server maintains a pool of accessible addresses that it distributes to the hosts on
demand.This significantly reduces the amount of configuration an administrator must do, since now it is only
necessary to allocate a range of IP address (all with the same network number) to each network.A DHCP
server receiving a DHCP discover message reacts to the client with a DHCP offer message that is broadcast to
all nodes on the subnet,again utilizing the IP broadcasts address of 255.255.255.255[5]. Since several DHCP
servers can be available on the subnet, the customer may end up in the lucky position of having the capacity
to look over among several offers. Since several DHCP servers can be present on the subnet, the client may
find itself in the enviable position of being able to choose from among several offers.Each server offer
message contain the transaction ID of the received discover message, the proposed IP address for the client,
the network mask,and an IP address lease time-the amount of time for which the IP address will be valid. It is
common for the server to set the lease time to several hours or days.

Fig.5.DHCP relay agent receives a broadcast DHCPDISCOVER message from a host and sends a
0unicast DHCPDISCOVER message to the DHCP server.[5]
1.3.3.DHCP Request
The recently arriving client will choose from one or more server offers and respond to its chosen offer with a
DHCP request message, echoing back the configuration parameters. [7]
1.3.4DHC Pack
The server responds to the DHCP request message with a DHCP ACK message,confirming the requested
parameters. [7]
Once the client receives the DHCP ACK, the interaction is complete and the client can use the
DHCP designated IP address for the lease span. Since a client may use its address beyond the lease's
expiration, DHCP gives a component that permits a customer to replenish its lease on an IP address.
Fig.6.DHCP Message Exchange Sequence.
The DHCP is intended to provide a centralized approach to the configuration and maintenance of an IP
address space. It permits the network administrator to configure various clients on the network from a single
location. DHCP also allows IP address leases to be dynamically assigned to workstations, eliminating the
need for static IP address allocation by network and systems management staff. Pools of accessible IP address
are maintained by servers that implement DHCP server functionality (DHCP servers have permanent IP
address).
1.4 DHCP Mechanisms: DHCP supports three mechanisms for IP address allocation.
 Dynamic Allocation.
 Automatic Allocation.

 Manual Allocation.
1.4.1Dyamic Allocation
DHCP assigns an IP address to a host for a limited period of time.
1.4.2. Automatic Allocation
DHCP assigns a permanent IP address to a host.
1.4.3. Manual Allocation
The network administrator assigns the host's IP address, and DHCP is used to convey the assigned
address to the host.
At the point when a DHCP client workstation boots, it broadcasts a DHCP request asking for any
DHCP server on the network. Toward the end, the client may decide to accept it with an IP address and
configuration parameters. A DHCP server on the network that is authorized to configuration this client will
offer an IP address by sending a reply to the client. After receiving this offer, the client may choose to
acknowledge it or it may hold up for some extra offers from different servers on the system. At the end, the
client decides on a specific offer and sends a request to accept the offer made by the server. Realizing that it
has been picked, the server will then send back an acknowledgement with the offered IP address and any
other configuration parameters that the client might have requested. The server does not force any parameters
on the client and it is dependent upon the client to request the parameters that it is willing to accept. In this
case DHCP dynamically assigns IP address to hosts, it is clear that hosts cannot keep addresses indefinitely,
as this would eventually cause the server to exhaust its address pool. In the meantime, a host cannot be
depended upon to give back its address, since it might have crashed, been unplugged from the network, or
been turned off. Thus, DHCP allows addresses to be leased for some period of time. Once the lease expires,
the server is free to return that address to its pool. A host with a leased address clearly needs to replenish the
lease periodically if actually it is still associated with the network and functioning correctly.
1.5 IP Address Lease Time
Under the dynamic allocation scheme the IP address offered to the client has an associated lease
time. The lease time dictates how long the IP address is legitimate. During the lifetime of the lease, the client
will repeatedly ask the server to renew. In the event if the client chooses not to replenish or if the client
machine is shut down, the lease will eventually expire. Once the lease expires, the IP address comes back to
the pool of available IP addresses and may be given to alternate machine. DHCP dynamic allocation works
well for stations that stay temporarily connected to the network.
A more sophisticated use of DHCP saves the network administrator from even having to assign
addresses to individual hosts. Besides dynamic assignment of address, DHCP servers can assign static IP
addresses to clients. In this case, addresses are assigned with an infinite lease.Net managers need a way to
keep the same IP address from being used to more than one user on the network. DHCP servers commonly
verify addresses by using a PING to figure out whether an IP address is already in use.
1.6 Security Considerations
The majority of the working framework give a path to a framework administrator, or even a client,
manually configure the IP information needed by a host. However there are a few evident drawbacks to such
manual configurations. One is that simply a considerable measure of work to configure all the hosts in an
extensive system specifically, particularly when you consider that such hosts are not reachable over a network
until they are configured. The configuration process is very error-prone, since it is necessary to ensure that
every host get the right network number and that no two hosts receive the same IP address. Due to these

reasons, computerized design systems are needed and the primary method uses a protocol known as the
dynamic host configuration protocol.
DHCP depends on the existence of a DHCP server that is responsible for providing configuration
information to hosts. There is no less than one DHCP server for an administrator domain. At the least difficult
level, the DHCP server can function just as a centralized repository for host configured data.For instance
,consider the problem of administering address in the internetwork of a large company. DHCP spares the
system executives from needing to stroll around to each host in the organization with a list of addresses and
network map in hand and configuring each host manually. Instead, the configuration information for each host
can be put away in the DHCP server and automatically retrieved by each host when it is booted or connected
to the network. However, the administrator would still pick the address that each host is to receive, he would
just store that in the server. In this model,the configuration information for each host is stored in a table that is
indexed by some form of unique client identifier, typically the 'hardware address'. DHCP may also introduce
some more complexity into network management, since it makes the binding between physical hosts and IP
address substantially more dynamic. This may make the network manager's job more troublesome if, for
example, it becomes necessary to locate a malfunctioning host.
1.7 References
1. Fred Halsal, 5th edition, "Computer Networking and the Internet",P.367.
2. Ralph.E.Droms, Ted Lemon, 2002,"DHCP Handbook 2", P.588 [Online] available at:
http://dl.acm.org/citation.cfm?id=862868.
3. Tzu-Chi Huang, Kuo-Chih Chu, 2011, "Networking Without Dynamic Host Configuration Protocol
Server in Ethernet and Wireless Local Area Network", Journal of Network and Computer Applications
34, 2011, 2027-2041.
4. Charles E.Perkins, Kevin Luo, I.J.Watson, "Using DHCP with Computers", P.341-353, [Online]
available at: http://dl.acm.org/citation.cfm?id=276427.
5. Larry L.Peterson, Bruce S.Davie, 3rd edition, "Computer Networks", P.264-266.
6. R.Droms, 2004, Network Working Group, "Stateless Dynamic Host Configuration Protocol (DHCP)
Service for IPv6”, [Online] Available at: http://tools.ietf.org/html/rfc3736.
7. James F.Kurose, Keith W.Ross, 6th Edition ,"Computer Networking",P.371-374
8. R.Droms, Network Working Group, 1997,"Dynamic Host Configuration Protocol", [Online] available at:
http://www.freesoft.org/CIE/RFC/2131/.
9. Rita Puzmanova, "Routing and Switching”, 2001.
10. R.Droms,"Interoperation between DHCP and BOOTP", 1993, [Online] available at:
http://tools.ietf.org/html/rfc1534.
11. H. Schulzrinne, "Dynamic Host Configuration Protocol (DHCP for IPV6)",2002,[Online] available
at: http://tools.ietf.org/html/rfc336.

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