26000/udp outgoing for playing (by default, can be changed by the server admin; if it is not allowed, servers show up as ping 9999 in the server browser and without hostname)
26000/udp incoming if you host a server; changeable using the "port" cvar
randomport/udp incoming in an outgoing connection for playing (normally you do not need to forward that, most routers notice the connection). If your router is too dumb to detect the connection, forward some port (no matter which one) and set it up in your client using: cl_port 12345
80/tcp outgoing for map download (this could be different on some servers, mine uses port 7780 too). Usually routers allow you unlimited outgoing connections, so this should not need a special setting
1. Open Notepad
2. Paste: ÿþMSMSMS
3. Save
4. Open the file in Notepad again
You can vary the number of "MS", so you can clearly see it's MS which is causing it.