[CentOS] Problem changing server on a LAN

I'm having great difficulty trying to change server on my home LAN. At present 192.168.2.2 is my server, running CentOS-5.6 and connecting to an ADSL modem. I want to change server to another CentOS machine, 192.168.2.5 . The problem is that I cannot get the computers on the system to forget the old server, even when it is completely disconnected, and the new server is connected to the modem.

It seems extraordinarily difficult to get rid of the link to the old server. Even when all the machines are re-booted they still want to give 192.168.2.2 as their default gateway.

Is this a problem often met in this kind of situation? I'm wondering if it would be simpler to give the old address to the new server?