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 Address field, the questionable IP
address in the Destination Protocol Address field, and the Destination Hardware
Address field containing zeroes.