Found this shit on another forum
"your NAT settings on your router/switch might be set too high. Go onto the dashboard into settings and run a network test, it should let you know if your NAT settings are Restricted or not.
go into your router by typing the IP address of the router into the address bar of your internet browser (if you dont know it, go Start->Run, then type CMD and click ok. When you get the black DOS window, type IPCONFIG and hit return. When the results come up, the Default Gateway is the address of the router), then go into your advanced settings in the port forward/application sharing section. If there is a NAT setting there, drop it down to normal or light and you'll be flying. If there isnt a direct NAT setting, you'll need to open a few ports:
www.portforward.com will give you the precise settings for your router for Xbox Live. it will also give you a step by step guide on how to do it. "
Hope it helps but try googling shit like that next time.