This link may be of some use to you
http://www.unofficialguidetolive.co....net-connection
Below copied from the same site
The only way to get Open NAT on multiple Xboxes connected to the same router is via UPnP or Port Triggering and it has to be the right router and right firmware.
Not many routers will give both of your Xboxes a NAT status of Open. This is crucial for the best gaming experience. If your router cannot do this, both of your Xboxes may not even be able to play in the same game. Your router must fully support UPnP for this type of setup. Lots of routers have this functionality, but not many actually implement it correctly to allow an Open NAT on multiple Xboxes
The size of your internet connection is crucial. If you want to use two Xboxes on the same Internet connection I would recommend 2Mb or above. Obviously the more Xboxes you use, the more bandwidth you require.
Cable Customers
All of the networking equipment listed on the Microsoft sites above here and here will allow you to use two or more Xboxes "out of the box" with no extra configuration, if you use the firmware version listed for the particular router on that page.
(UK users look here although if you can get a router from the US list, do, as the UK version is very limited, they US one's will still work here)
Also the following routers that may not be on the official list also work with two or more Xboxes:-
Belkin F5D7231uk4 (version 2) (firmware F5D7231-4_UK_5.01.03) - Ensure you enable UPnP, details for Belkin here
Dlink DIR-655 Extreme N Gigabit router (tested with firmware 1.03, higher versions of firmware may also work)
Dlink DIR-300/301 Wireless G Router(Also known as DKT-110 when bundled with wireless PC adapter
Dlink DIR-825 (firmware 2.01EU, higher versions may also work)
Linksys WRT160N v3 DD-WRT (Third Party Firmware) NEWD K2.6 Mini Generic
http://www.dd-wrt.com/routerdb/de/do..._mini.bin/3407
Linksys WRT310N v1 DD-WRT (Third Party Firmware) generic mini version build 14929
Not sure if this is all up to date but hopefully it might help