This should solve your problem
For most devices that use NAT, port forwarding is not required to
connect to Xbox Live. This is especially true if you use an Xbox Live
certified device. Sometimes, you may have to configure port settings on
a non Xbox Live certified router or gateway.
If there is a
firewall device between the Xbox console and the network device, you
may have to configure the firewall to enable communication on specific
network ports. If the NAT status in the Network Status area of the
Dashboard is "moderate" or "strict," you may have to configure port
The following ports must be available for Xbox Live to operate correctly:
� UDP 88
� UDP 3074
� TCP 3074
To open the ports on your router see this handy website here
which gives you a step by step guide.
Before opening the ports you must give your Xbox 360 a static IP address (manual) be sure to use an IP address
that no other item on your network is using and is fairly high up in
your range, for example if your router's address is 192.168.0.1 then
you should use something like 192.168.10 for your Xbox 360.
can also enable DMZ (Demilitarized Zone) on your router, which will
open up all the ports to your Xbox 360, however NEVER use this on a
PC's IP, see your router's manual on how to do this.
Once the correct ports are open for your Xbox 360, then your NAT should say open.