Sometimes your Xbox will say your NAT type is open when simply that is not the case, and your NAT type is actually moderate/strict.
This can be caused by a number of things: Bad internet connection, a temporary fault with your home network, an incompatible router or even a corrupted file on your Xbox. Most fixes are relatively simple, such as power-cycling the router (unplug for 30 seconds, plug back in again), forwarding your ports to your Xbox (guides all over the internet) or deleting the impartial corrupted file on the HDD of the Xbox (they have a yellow triangle next to them).
For starters you should take a look at the support articles [http://support.xbox.com/en-GB/xbox-live/connecting/nat-type-strict] for this issue, then if you still require help: Go ask the Xbox Ambassadors [http://support.xbox.com/en-GB/ContactUs/clickToChatWithAmbassador?service=xbox-live&category=connecting&ccfissue=&ewt=0&navlistpath=/en-GB/NavigationList] (volunteer support community) to see what they know. If they can't offer a working solution, give official support a shout; it's all done through in-browser instant messaging and they've all helped me on numerous times. Queues's aren't that long for support, especially in the mornings; couple of minutes, tops, sometimes even instant reply.
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I've had this problem many-a-times over the past year. It sounds very much like a firewall or NAT issue, though more likely a NAT issue. Follow those links and see if they help
