We have three Xbox 360s in one house. One only connects to the local network and the other two are connect the Xbox live. The local Xbox 360 and the other Xbox 360 (which is connect to Xbox live) are a standard xbox 360 and mine is a Xbox 360 slim.
Now for some reason the Xbox 360 slim has NAT restriction and we cant figure out why. We have opened ports 53, 80, 1863, 3074 and enabled Upnp and still nothing works. The only other thing i can think of is port forwarding to the Xbox 360 slims ip but we want to stay away form that if possible.