Port Forwarding Information
If you have trouble connecting to game servers or friends, it may be necessary for you to open ports on your router. This requires you to have admin access to your router. Consult your router's manual or manufacturer's website for information on how to access your router's settings (every brand is a little different).
Connection problems are typically tied to the user's NAT type. This setting can be seen in the 'Play' menu of both games. MW2 defaults to 'Strict' most often and MW3 defaults to 'Moderate'. Opening ports will ideally set the NAT to 'Open', though MW2 will sometimes still display 'Strict', even when connectivity improves.
The first step with either game is to ensure the router's UPnP setting is 'On'. For most routers this is the default state. The following link gives a little more information: http://www.infinityward.com/nat
Call of Duty: Modern Warfare 2:
TCP Port 80
Port 28960, 3300, 3101, 3104, 3005, 27019
Call of Duty: Modern Warfare 3: Game Port UDP: 52765, 56553
Port 3074, 3076, 27019
UDP 27000 – 27015 UDP 27015 – 27030 TCP 27014 – 27050 UDP 4380
UDP 3478 UDP 4379 UDP 4380