What is my ip adress?

[Arjen]

Fill in your local ip when behind a router (cmd -> ipconfig). Give your friends your WAN-ip ( http://whatismyipaddress.com/ ). Fill in your WAN-ip when you're not behind a router.
Open these ports:

UDP: 1200 (used for friends service)
UDP: 27000 to 27015 inclusive
TCP: 27020 to 27050 inclusive
UDP: 27015 and 27020 (default HLDS, SRCDS and HLTV port)
TCP: 27015 (SRCDS Rcon port)