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Firewalls and Routers
If your computer is behind a router with a firewall, you'll need to open the ports that Steam uses to connect to the Internet. Most embedded firewalls require you to open ports manually (they don't learn automatically). Routers without firewalls rarely need to have ports opened for game client (but usually do for game servers).
If your configuration software requires port numbers, please use the following:
UDP 1200 (used for friends service)
UDP 27000 to 27015 inclusive
TCP 27020 to 27039 inclusive
TCP 27040 and 27041 only for CyberCafe Owners
Computers running Dedicated Servers need these ports open:
UDP: 27015 and 27020 (default HLDS, SRCDS and HLTV port)
TCP: 27015 (SRCDS Rcon port)
For help on port forwarding for your Router, please consult this excellent website:
http://www.portforward.com.
For assistance with your Firewall you will need to go to the support site for your particular Firewall. Windows XP SP1 firewall does not require any specific configuration changes for Steam to work but XP SP2 firewall does.