Hope that helps.The following ports must be open in your firewall and router for Steam and Steam Games to work:
[font=Verdana, Helvetica]UDP 1200 (used for friends service)
UDP 27000 to 27015 inclusive
TCP 27020 to 27050 inclusive[/font]
Computers running Dedicated Servers need these ports open:
UDP: 27015 and 27020 (default HLDS, SRCDS and HLTV port)
TCP: 27015 (SRCDS Rcon port)