I had a game last night with John on Modern Warfare 2, and noticed that infinity ward have some directions on NAT negotiation errors, which makes sense given it’s a p2p game. They direct you to your routers main support page for more detailed instructions (suggesting a search for “Open NAT”), but one point they mention, which I haven’t seen us trying, is checking UPnP is turned on (Universal Plug and Play). It may be on by default, but worth checking anyways.