Administrator Guide

Dynamic Host Conguration Protocol (DHCP)

DHCP is an application layer protocol that dynamically assigns IP addresses and other conguration parameters to network end-stations

(hosts) based on conguration policies determined by network administrators.

DHCP relieves network administrators of manually conguring hosts, which can be a tedious and error-prone process when hosts often

join, leave, and change locations on the network and it reclaims IP addresses that are no longer in use to prevent address exhaustion.

DHCP is based on a client-server model. A host discovers the DHCP server and requests an IP address, and the server either leases or

permanently assigns one. There are three types of devices that are involved in DHCP negotiation:

DHCP Server This is a network device oering conguration parameters to the client.

DHCP Client This is a network device requesting conguration parameters from the server.

Relay Agent This is an intermediary network device that passes DHCP messages between the client and server when the

server is not on the same subnet as the host.

Topics:

• DHCP Packet Format and Options

• Assign an IP Address using DHCP

• Implementation Information

• Congure the System to be a DHCP Server

• Congure the System to be a Relay Agent

• Congure the System to be a DHCP Client

• Congure Secure DHCP

• Source Address Validation

DHCP Packet Format and Options

DHCP uses the user datagram protocol (UDP) as its transport protocol.

The server listens on port 67 and transmits to port 68; the client listens on port 68 and transmits to port 67. The conguration parameters

are carried as options in the DHCP packet in Type, Length, Value (TLV) format; many options are specied in RFC 2132. To limit the number

of parameters that servers must provide, hosts specify the parameters that they require, and the server sends only those parameters.

Some common options are shown in the following illustration.

Figure 33. DHCP packet Format

The following table lists common DHCP options.

342 Dynamic Host Conguration Protocol (DHCP)