Call of duty, unable to play 2 comps on same network

Me and my brother both play call of duty, but we are not able to play at the same time. We are playing on 2 computers on the same network which is hard wired through a router. It is not a server issue. We are not able to play at the same time even in different servers. If one person is playing and then the other connects to any server, the first person will get disconnected and unable to reconnect to the server for several minutes. Any help with this issue will be greatly appreciated. Also I checked our local ip addresses and they are different.

That is not the case b/c we used to be able to play at same time in our old house, plus i know ppl who play using the same, also the problem exists for cod1 and 2 (the only games we both play).