32.1. Introduction to the Dynamic Host Configuration Protocol

DHCP is a protocol that allows hosts to request and be assigned an IP address on a local area network. It is used to simplify the process of IP assignment, as a single server can manage the addresses of multiple clients. It is also useful for systems like laptops that are moved between multiple networks, as they do not need to be reconfigured for each LAN to which they are connected.

DHCP is usually used on Ethernet networks, although it can be used on any type of LAN that supports broadcast traffic such as 802.11b and Tolken Ring. It is not used for address assignment for dial-up connections—the PPP protocol has its own method of telling clients their IP addresses. Because broadcasts are not normally forwarded by routers, a DHCP server can only assign addresses to hosts on a single LAN, unless you have a router that is configured to forward DHCP packets.

A DHCP server can also supply other information to clients in addition to an IP address. The addresses of DNS servers and the network gateway can be sent, along with the DNS domain, NIS server, NIS domain, static routes, and much more. DNS and routing information allows clients to fully integrate themselves into the network to which they are connected without needing any manual configuration.

When a server assigns an IP to a client, it is given a lease on that address for a certain amount of time, during which no other client will be assigned the same address. When the lease expires, the client must contact the server again. Typically, it will be assigned the same IP address as before and the lease will be extended for the same time period. If a client does not contact the server when its lease is up, the server assumes that the client has been shut down and marks the address as available to be assigned to other hosts.

Most operating systems include support for configuring a network interface to use DHCP to get its IP address. Chapter 16 explains how to set it up for Linux systems, and it is relatively simple to configure Windows and MacOS clients to use it as well. DHCP has become the standard protocol for address assignment on IP networks, replacing the older BOOTP protocol used by some UNIX operating systems.