Now, the way i understand it, is that x-boxes by default search for a dhcp server each time they are turned on, and it's most likely that the router also functions as a dhcp server itself. So, unless im missing something the problem doesn't have anything to do with the ip...unless it's a software issue(in this case a game) that requires each connection to have unique ip(remember that there are other things that can be, and are used in conjunction with ip to determine the correct source/destination, like ports). But if it is then i guess your only solution is,as sugested before, to get an extra ip...