I'm hoping someone maybe able to help with this...
I currently have my system set up as so;
A modem, with an ethernet cable running to a router (linksys WRT54GS) in my living room. This then has an ethernet cable connected up to an xbox 360. I also have a couple of laptops that connect up wirelessly.
I can connect to xbox live fine and play games online. The problem arises when I try to connect to parties that my mates invite me too. I have found, if any of my mates are with virgin media I can't connect at all to them. But if they are with another ISP, it works fine (like my mate with tiscali).
Now, if I connect the ethernet cable that runs from my modem direct to the xbox, then it works fine. I have also found this problem only started when I upgraded from 2MB to 10MB.
All been done. I opened up ports 88 (UDP) and 3074 (both). I even set my xbox to get network details manually as was told this may help. Someone mentioned enabling DMZ for the IP the xbox uses so I've done that. I've enabled UPnP as was told that should help. I upgraded the firmware and still the same problem.
I'm certain its not a router problem as it used to work fine. Its very frustrating. When I load up the game (Modern Warfare 2), it actually says my NAT is set to open.
My other friends on virgin have no problems connecting to each other and like I said, when I plug my modem straight to the xbox, it works fine.