Unless the user has modified the default configuration or has special networking circumstances, most home routers should work with the game by default and should not require you to unblock additional ports. Those behind a more advanced firewall or router configuration (such as at a business or college) may need to unblock the below ports to ensure the game can be played.
Hi-Rez games use ports 9000-9999 TCP, and 9002-9999 UDP.
Steam uses the following ports. These are not strictly required to run Hi-Rez games but it is a good practice to have them open for Steam games:
UDP 27000 to 27015 inclusive (Game client traffic)
UDP 27015 to 27030 inclusive (Typically Matchmaking and HLTV)
TCP 27014 to 27050 inclusive (Steam downloads)
Dedicated or Listen Servers
TCP 27015 (SRCDS Rcon port)
Steamworks P2P Networking and Steam Voice Chat
UDP 3478 (Outbound)
UDP 4379 (Outbound)
UDP 4380 (Outbound)
Additional Ports for Call of Duty: Modern Warfare 2 Multiplayer
UDP 1500 (outbound)
UDP 3005 (outbound)
UDP 3101 (outbound)