The unfortunate truth about NAT type is that sometimes it is out of your hands. If you’re on a network that you didn’t set up, like some apartment buildings or dorms, it may be hard to resolve. But if this is your home internet connection, you can most likely resolve your issues by logging in to your router and enabling UPnP. The steps for doing this on every router are a little different, but it’s usually fairly simple to find.
Along the same lines, many newer routers also feature an option called teredo tunneling. Enabling that may also resolve your issue.
But Xbox 360 users, fear not! If you’re still having problems after you enable UPnP and teredo tunneling, Microsoft hasn’t given up on you. They suggest some ports you can open as well as other tips to get “NAT type Open” on your connection to Xbox Live.
This became such a huge problem, that Microsoft presented a PowerPoint discussing the issue at Gamesfest 2010, which you can download from Microsoft.com.