Stinky on 01 October 2010
They should autoconfigure with upnp. The first system online will redirect the default port 3074 UDP, subsequent systems will redirect on a random port. This can be confirmed on the router's UPNP mapping page if it has one, and also on the 360's information page in settings.
Even though the random port reports as not fully open, other 360s can connect to it without problems. So don't worry about it.
