If these are IP addresses issued to you by your ISP, then they are the only ones who can fix this problem. Something tells me though that the lease time for the IP Address is set to expire almost immediately, which is very strange indeed. Usually a lease can be either one day or one week, for me my lease on my IP address is set to expire after 4 days. Put simply, if you only want one IP address, they have to go into thier tables and fix this problem. Call them up if you have to, because dealing with issues like this via email is damn near impossible.
If you're using XP, you can use ipconfig/all to view your lease length. be sure to run the command prompt first, then type in the command, otherwise it will close so fast you won't see it.