ip changed when the system is running

[Xiaoyang Gu <xiaoyang.backup@gmail.com>]

hi
   I have a little pain problem. It is about the ip address. My debian box got an ip from the DHCP server when booting the system. But some time later, maybe ten hours, the ip address changed automaticlly. I didn't shutdown or reboot my computer, or plug out the cable ... I don't know why. But other computers using M$'s Windows don't have this problem, their ip address never changed when the power is on. 
   When ip changed, I need to run a auth client again, so that my box can get outgoing connections. So when the problem accurs, all the program related with network may lost their connections. It sometimes makes me crazy!
   Any suggestions? Thanks for help!
   						xiaoyang