LAN Games Restricted to Class C Clients

http://support.steampowered.com/cgi-bin/steampowered.cfg/php/enduser/std_adp.php?p_faqid=54

Game Servers can be LAN Servers or they can be Internet Servers.

In order for Internet players to connect to your Server, you must enable your Server to be an Internet Server. You do this by entering "sv_lan 0" in the Server console, in the server.cfg or in the listenserver.cfg file.

Typically, only Listenservers (where the Server is also a Client) are LAN Servers by default.

If you are behind the same router as the Server, then you must connect to the your Server using the internal IP address of the Server and not the external IP address.