DHCP Server problems

This deals with the DHCP Server in the Internet Gate, distributing IP addresses to clients on the LAN.

One of the nightmares on a local network (and indeed on any network) is when two PC:s or other devices (we call them hosts) start using the same IP address. The consequenses are characteristic but not so obvious for other than experienced network users: Suddenly an application, like “surfing”, stops working for some time for one host. Then it starts working again, after a minute or so, but now the problem has moved to another host on the LAN. After some time again the problem jumps back to the first host, and so on.

The reason for this jumping error is that if two hosts act as if they have the same IP address it will confuse the router, in this case the Internet Gate who is set to work as their default gateway. The Internet Gate, as any router and host on a network, has a memory, the “ARP cache”, that pairs an IP address with the host's MAC address. This memory is used for forwarding packets, destined to the particular IP address, to the correct unit. But for various reasons this memory has to be refreshed now and then, and this refresh is done by asking around on the network: “who has this particular IP address”? If there is more than one host out there answering this question, the Internet Gate may well get different answers from time to another, and as a result the traffic will jump around accordingly.

This IP address collision can happen if hosts on a LAN are assigned their IP address statically by a less careful network administrator. But if all hosts are getting their IP address dynamically by a DHCP server this should not happen, since the DHCP server is responsible in the first place for handing out unique IP addresses. Still, that can happen in two cases:

1. The DHCP server is not alone on the network.
2. The DHCP server has forgotten all about the IP addresses that have been distributed.