6 understanding DHCP

Understanding DHCP
PREPARED BY HAMEDA HURMAT

Introducing the DORA Process
Advantages of DHCP
Disadvantages of DHCP
Knowing Ipconfig Lease Options
Scope
Super scope

There are two ways to have clients and servers get TCP/IP addresses:
You can manually assign the addresses.
The addresses can be assigned automatically.
Manually assigning addresses is a fairly simple process. An administrator goes to each of the
machines on the network and assigns TCP/IP addresses.
The problem with this method arises when the network becomes midsized or larger. Think of an
administrator trying to individually assign 4,000 TCP/IP addresses, subnet masks, default
gateways, and all other configuration options needed to run the network.
DHCP’s job is to centralize the process of IP address and option assignment. You can configure a
DHCP server with a range of addresses (called a pool ) and other configuration information and
let it assign all of the IP parameters—addresses, default gateways, DNS server addresses, and so
on.

Introducing the DORA Process
DORA stands for Discover, Offer, Request, and Acknowledge.
1. Discover: When IP networking starts up on a DHCP-enabled client, a special message called a DHCPDISCOVER is
broadcast within the local physical subnet.
2. Offer: Any DHCP server that hears the request checks its internal database and replies with a message called a
DHCPOFFER, which contains an available IP address. The contents of this message depend on how the DHCP
server is configured—there are numerous options aside from an IP address that you can specify to pass to the
client on a Windows Server DHCP server.
3. Request: The client receives one or more DHCPOFFERs (depending on how many DHCP servers exist on the
local subnet), chooses an address from one of the offers, and sends a DHCPREQUEST message to the server to
signal acceptance of the DHCPOFFER. This message might also request additional configuration parameters.
Other DHCP servers that sent offers take the request message as an acknowledgment that the client didn’t accept
their offer.
4. Acknowledge: When the DHCP server receives the DHCPREQUEST, it marks the IP address as being in use (that
is, usually, though it’s not required). Then it sends a DHCPACK to the client.
the client’s IP network initialization will fail, and the client will use Automatic Private IP Addressing (APIPA).

Advantages of DHCP
Configuration of large and even midsized networks is much simpler. If a DNS server address or some
other change is necessary to the client, the administrator doesn’t have to touch each device in the
network physically to reconfigure it with the new settings.
Once you enter the IP configuration information in one place—the server—it’s automatically
propagated to clients, eliminating the risk that a user will misconfigure some parameters and require
you to fix them.
IP addresses are conserved because DHCP assigns them only when requested.
IP configuration becomes almost completely automatic. In most cases, you can plug in a new system
(or move one) and then watch as it receives a configuration from the server. For example, when you
install new network changes, such as a gateway or DNS server, the client configuration is done at only
one location—the DHCP server.
It allows a preboot execution environment (PXE) client to get a TCP/IP address from DHCP. PXE
clients (also called Microsoft Windows Deployment Services [WDS] clients) can get an IP address
without needing to have an operating system installed. This allows WDS clients to connect to a WDS
server through the TCP/IP protocol and download an operating system remotely.

Disadvantages of DHCP
Unfortunately, there are a few drawbacks with DHCP:
DHCP can become a single point of failure for your network. If you have only one DHCP server
and it’s not available, clients can’t request or renew leases.
If the DHCP server contains incorrect information, the misinformation will automatically be
delivered to all of your DHCP clients.
If you want to use DHCP on a multisegment network, you must put either a DHCP server or a
relay agent on each segment, or you must ensure that your router can forward Bootstrap
Protocol (BOOTP) broadcasts.

Ipconfig Lease Options
ipconfig /renew Instructs the DHCP client to request a lease renewal. If the client already has a
lease, it requests a renewal from the server that issued the current lease.
ipconfig /release Forces the client to give up its lease immediately by sending the server a DHCP
release notification. The server updates its status information and marks the client’s old IP
address as “available,” leaving the client with no address bound to its network interface.

Scope
DHCP uses scopes as the basis for managing and assigning IP addressing information.
Each scope has a set of parameters, or scope options, that you can configure.
Scope options control what data is delivered to DHCP clients when they’re completing the DHCP
negotiation process with a particular server.

Super scope
A super scope enables the DHCP server to provide addresses from more than one scope to
clients on the same physical subnet.
clients within the same subnet have more than one IP network and thus need IPs from more
than one address pool.

Exclusions These are IP addresses within the range that you never want automatically
assigned. These excluded addresses are off-limits to DHCP.
Reservations These are IP addresses within the range for which you want a permanent DHCP
lease. They essentially reserve a particular IP address for a particular device.
Address Pool The range of IP addresses that the DHCP server can assign is called its address
pool.
DHCP Relay Agent If no DHCP server is available on the client’s network, you can use a DHCP
relay agent to forward DHCP broadcasts from the client’s network to the DHCP server. The relay
agent acts like a radio repeater, listening for DHCP client requests and retransmitting them
through the router to the
server.

6 understanding DHCP

More Related Content

What's hot (20)

Similar to 6 understanding DHCP (20)

More from Hameda Hurmat (20)

Recently uploaded (20)

6 understanding DHCP