Tight network security and network-aware applications like Xbox Live are constantly at odds in our computing universe. While there may be firmware updates for wireless routers to automatically solve Xbox Live connection issues in the long run, using your NAT in one of the more secure modes might lead to audio problems or just plain failure to connect your Xbox 360 to Xbox Live. You can do some troubleshooting with a solution posted over at Xbox360Fanboy and details provided from the Test Xbox Live Connection in the Network Settings portion of the System blade. Depending on your network settings, your connection may fail in a Moderate or Strict protection mode on the router.
The solution is to open the ports listed over at Xbox360Fanboy.
UDP 88, UDP 3074, and TCP 3074 for those too lazy to click-thru.
In most cases you won’t need to change anything. In the event you need to forward a port used by Xbox Live, you generally need to enter the following data for each port opened:
Application or Description field: Xbox Live
Port Range Start: 88 (or 3074)
Port Range End: 88 (or 3074)
Protocol Type: UDP or TCP
IP Address of your Xbox 360: from Network Settingss
Enable: check a box to enable the port forward