Call of Duty 5 Problems

[Blackmoore]

My brother and I both bought Call of Duty 5 World at War and whenever we try to connect to a co-op game with a friend it only allows one of us to connect to his game at a time. When my brother joins and then I try to join it boots him out and lets me in and the other way around, but it won't let us both connect to the same game, even though we both have our own legal copies of the game. This has only occurred when we try to play together on our home router. It worked previously when we played at a friends house connected via his Belkin router. So I would assume there is a block in the router somewhere. Any ideas?

We own a Linksys RangePlus Wireless Router WRT110 and we are both hardwired to the router using Windows Vista.