NAT Types and their meanings are listed below.
Your NAT type is CLOSED. You may not be able to join certain games or hear other players while playing Halo: Reach online. Average matchmaking wait times will be adversely affected.
Your NAT type is MODERATE. You will not be able to play or chat with other players who have CLOSED NAT.
Your NAT type is OPEN. OPEN NAT is the optimal setting. You’re awesome.
We are unable to determine your NAT Type. If you are having issues with the items in Closed or Moderate, trying running the Xbox LIVE connection test in the Xbox Dashboard (My Xbox -> System Settings -> Network -> Test Xbox LIVE Connection)