If you have a linksys router, heres what you can do..
Login to your router and look for something that says DMZ Host. It might be under a tab called advanced, so look there too. Once found, you should see a box that has "192.168.1.__". Now turn on your 360 and go to the dashboard. Go to the very right tab on the dashboard and hit network settings, then hit test network settings. Now once its through back out, and go to edit settings. Under where it says "automatic" you should see an ip address. Enter the last three digits into the missing spot on your computer on the DMZ Host page, then hit apply. Let your router restart, and then test your connection. Your NAT should now be open... and also for anyone with a netgear router just enable UPNP, which should fix it. If anyone needs help with this pm me