Ports to forward:
Modern Warfare 3
TCP: 3074
TCP: From 27014 to 27050
UDP: 3074
UDP: From 27000 to 27030
Steam Client
UDP 27000 to 27015 inclusive (Game client traffic)
UDP 27015 to 27030 inclusive (Typically Matchmaking and HLTV)
TCP 27014 to 27050 inclusive (Steam downloads)
UDP 4380
Additional Ports for Call of Duty: Modern Warfare 2 Multiplayer (might be worth adding these too if your having issues with MW3!)
UDP 1500 (outbound)
UDP 3005 (outbound)
UDP 3101 (outbound)
UDP 28960
Further Tips:
Can’t find server browser or dedicated server option – you have to enable these under dedicated server options first before they become visible.
Set classes up for use in dedicated servers prior to joining. To do this goto find private game and create them in there – they will then be selectable.
How to join a dedicated server
1. Enable the server browser in the options menu. Options > dedicated server
2. When you enable that option, you will have servers available in the multiplayer menu
3. Select servers
4. Hit connect to.
Port forwarding advice for your router:
Search for you model, skip the advert and then select the program you wish to portforward. You will need your router IP address – eg. 192.168.0.1 and your USERNAME and PASSWORD.