I need to renew DHCP Licence every 20 minutes or so. Any ideas?

Question

About once every few months, there is a span of a few days that I need to renew the DHCP Licence every 20 minutes or so to get onto the internet.  Then, everything works, but the DHCP error moves to another computer.  There are 5 computers at my work.  From the router, the signal goes into a gigabit switch, where it is piped into three different rooms.  In the room where we have the most problems, or we seem to as it's the main work room, the signal goes into another gigabit switch, which connects two computers and the Airport Extreme.  The DHCP error happens on the wired computers, a PowerMac G5 and a G4, and the Mac Book, connected to the Airport.  All the computers are running 10.4.11.
Any ideas on how we can avoid this procedure every few months?  Thanks.

Answer 1

Someone correct me if I'm wrong, it's been a while since I read about it, when a device connects using DHCP to a DHCP server it first asks if it can use the IP address it used previously and the DHCP server will reply yes or no (and supply a new Ip address).
There are a number of areas where confusion can occur, if the device ignores the new IP address from the DHCP server, or if the server (incorrectly/ says yes when it should have said no, perhaps for the wrong reasons, ie. the IP address requested is outside the range set aside for DHCP allocation. A flaky DHCP server may do this.
Has the configuration recently been altered ? New firmware for any of the routing devices, not just the AirPort. Or the configuration altered ?
Or are there 2 DHCP servers, or 2 servers offering DHCP services ? This doesn't have to be an issue if the ranges do not overlap, but if they do you may see what you are experiencing.
I changed my DHCP range allocation a few months ago to accomodate some new servers in the reserved range at the bottom end, so the servers could have static IP addresses (I'd only reserved 10 initially). What a nightmare with some of the DHCP Macs, most were fine that were up and running at the switch, but others that booted afterwards and those that booted into multiple OS's sometimes refused to initially pick up a new IP address from the correct range. A quick pull of the cable, to get a self-assigned IP and the put it back in seemed to work. AirPort attached devices were the hardest, my router STILL thinks my PBG4 is attached on two IP addresses (it's connected ONLY by AirPort, no Ethernet, etc), 192.168.0.12 and 192.168.0.24. 24 is the correct one but apparently there's a device called PowerBookG4.local on the .12 address. Everything works so I'll sort it at the next power-down of the networking kit in March.