If there is a firewall or a router between the Xbox console and the networking hardware, and the Xbox LIVE test reports a NAT problem, you may have to make configuration changes to enable communication on the specific network ports that are required by Xbox LIVE. This will not guarantee that this will give you an "open" NAT status, but it may help.
If you are using an Xbox LIVE-compatible or Windows Vista-compatible device with the correct firmware installed, you do not have to manually open any ports. For a device to gain "compatible" status, the device and the firmware must perform specific functions with Xbox LIVE. These functions include (but are not limited to) setting NAT to "Open" and having the ability to connect multiple consoles to a single router. For more information about how to find out if your networking hardware is compatible with Xbox LIVE, click the following article number to view the article in the Microsoft Knowledge Base:
944293 How to find out if your networking hardware is compatible with the Xbox LIVE Service
If your device is not on the compatible list, you can try opening the following ports:
For information about how to configure port settings on a firewall, a router, or a gateway, please see the documentation that was included with that device or contact your network administrator or local technical support for help. You can also check the xbox.com official forums to see if other users have the same problem.
If you use a Windows Vista-certified or Xbox LIVE-certified device and the NAT status in the Network Status area of the Dashboard is "moderate" or "strict," verify that the device has the current firmware. For information about Xbox LIVE-certified devices, such as the correct firmware version for the device, please visit the following Xbox Web site: http://winqual.microsoft.com/HCL/ProductList.aspx?m=v&cid=712&g=d