Set your router to DMZ mode, or manually configure your ports in your router for Gamespy Arcade and SFC.
To manually configure ports
If you are behind a router you will need to forward ports 2300-2400 TCP&UDP and 47624 TCP to login. (allow pings from the WAN) Port forwarding applies to a static LAN config. The following site may be of assistance in this:
http://www.portforward.com/Some ADSL and cable modems have router capabilites in them that the ISP does not tell you about and often poorly understand themselves.
Understand what a private IP address is:
http://en.wikipedia.org/wiki/Private_IP_addressDetermine if you have one: "ipconfig /all" at the command prompt on WinXP/NT or Start..Run.."winipcfg" on Win9x.
You will have to check the Gamespy Arcade web site in their help section on Firewall/Routers to find which ports to open for their service.