If you are having some problems connecting to certain people over Xbox Live on your 360 the boys at XBox 360 Fanboy have some tips.
“Go to the System blade, select Network Settings and then Test Xbox Live Connection. You'll be able to see what sort of NAT your system is behind, and if it's set to Moderate or Strict then you will only be able to connect to certain other users.
To fix this, set your NAT to Open by opening the following ports on your router: 88 (UDP), 3074 (UDP), and 3074 (TCP).”
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