A friend asked me about this the other day and I told him to google it. Unfortunately I was unaware that there is a lot of bad information out there about how to achieve an “open NAT” status on your xbox 360. The only ports you need to forward to your xbox are:

TCP port 88
TCP port 3074
UDP port 3074

You do not need to enable DMZ, port forwarding and UPnP at the same time to get it to work. You can use any ONE of those options. Here’s what they mean. Port forwarding means that you set your gateway device (router) to forward connections to the above mentioned ports to your Xbox’s IP address. Enabling DMZ on your gateway generally means that your gateway will forward all connections made to it to the IP address you specify. Lastly, UPnP allows the xbox to automagically configure your gateway device for the appropriate access. If UPnp is enabled on your gateway you should not have to do anything to get an open NAT status on your xbox. Note that some devices have UPnp settings related to media sharing. This is not the same as UPnP firewall settings.

A great guide to this is available at http://www.noddegamra.co.uk/problems-with-xbox-360-live-nat-settings/3/