As Toto has said, IP addresses are issued dynamically and you cannot prevent them from changing if the change is caused by something like resegmentation. In general IP addresses tend to stay the same particularly if you hardly ever turn your PC off or router if you are connecting through one. It could be a question of you having a firewall that isn't allowing DHCP requests to be processed resulting in you losing the lease of the IP. Have a look at Robin Walkers site regarding DHCP issues and also the page on that site regarding firewall issues.
http://homepage.ntlworld.com/robin.d...tips/dhcp.html