In reply to a post by RobertoS:

Would using IPv6 solve the problem?

Not specifically IPv6, but more individually addressable IPs would solve it. One for each XBox and one for the router. Or you get really clever and allocate them all to the router, and have separate NAT for each XBox to a different public IP. Not something I'd want to configure on any home class router, something my Cisco certified colleagues do in their sleep I think