Internet Core Protocols: The Definitive Guide

DHCP ARP

One of the key differences between DHCP and RARP is that DHCP uses a shared pool of addresses when assignments are made, rather than using fixed entries like RARP does. This allows a network manager to share a limited number of IP addresses among a large number of hosts, instead of having to assign fixed addresses to each host. Although this makes life a little easier for the administrator in one sense, it can also make life more difficult, particularly when it comes to making sure that more than one device is not trying to use the same address at the same time. This is particularly problematic when users do not release the address they have been assigned and another device starts trying to use it, or when a user has manually configured his system to use a specific address that is also being assigned out of the DHCP pool.

In order to minimize the opportunity for this problem to occur, RFC 2131 (the DHCP RFC) stated that devices should verify the integrity of the address they are getting from the DHCP server before they accept the assignment. RFC 2131 also defined a specific ARP request format to be used for this purpose, in order to keep other systems from getting confused by the ARP request.

With DHCP ARP, the requesting device issues a normal ARP request, except that instead of putting its own IP address in the Source Protocol Address field, it puts in “0.0.0.0”. The rest of the packet looks like a normal ARP request, with the local hardware address in the Source Hardware ...