Cisco Cookbook

Chapter 20. DHCP

20.0. Introduction

Dynamic Host Configuration Protocol (DHCP) is often used on networks to allow end devices to automatically retrieve their network configuration when they first connect to the network. It basically expands on the earlier Bootstrap Protocol (BOOTP) and uses the same UDP ports, numbers 67 and 68. The protocol itself is defined in RFC 2131, and the configuration options are defined in RFC 2132.

The most common application for DHCP is to automatically set up IP addresses, net-masks, and default gateways for end devices. However, the protocol can also configure many other options, such as DNS servers, domain names, time zones, NTP servers, and many others. Some software vendors have even added their own configuration options to automatically set up key applications on end devices.

DHCP makes it possible to give a minimal common configuration to all user workstations. You can simply plug the device into the network at any point, and DHCP will take care of getting an IP address that will work at that location. This minimizes errors due to manual configuration, centralizes control over configuration information, and greatly reduces technician costs because anybody can connect a device to the network.

There are three distinct element types in a DHCP network. There must be a client and a server. If these two elements are not on the same Layer 2 network, there also must be a proxy, which usually runs on the router. The proxy is needed because the client device ...