Bru - read here...
Posted: Sun Jul 08, 2012 10:41 pm
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Open up Windows Firewall with Advanced Security and create new Inbound and Outbound rules for “iw5mp.exe” (Located in Steam\steamapps\common\call of duty modern warfare 3\) and be sure to explicitly allow all connection both inbound and outbound.
The default location for iw5mp.exe is as follows…
C:\Program Files\Steam\steamapps\common\call of duty modern warfare 3\iw5mp.exe
If you’re on a x64 system, it will be similar, just inside the Program Files (x86) folder instead.
Restart your PC for good measure and fire up the game one more time and verify your NAT type is now Open.