Configuring a router for multiple clients

http://support.steampowered.com/cgi-bin/steampowered.cfg/php/enduser/std_adp.php?p_faqid=120

You need to change one or the other of your clients so they have different clientport numbers.

If you want to run multiple HL Clients behind a router, you need a few things:

1. Each machine must have a unique Steam account with different CDKeys.
2. Each machine must have a different clientport setting.
3. Each machine must have a unique MAC address (usually don't need to worry about this).
4. Make sure your router has the latest working firmware installed. NOTE: the latest Linksys router firmware has problems, the last known good version is 1.45.7, please refer to this FAQ for more information.
5. If your router has a firewall you need to allow the following ports:

UDP 1200 UDP 27000 to 27015 inclusive TCP 27030 to 27039 inclusive

Some firewalls only block incoming ports but many newer firewalls also block outgoing ports.

The default clientport is 27005 and you can only have one game with that port active on the same lan.

What you need to do is go to your launch properties (do this for each computer):

Steam > Games > Right-click your MOD > Properties > Launch options

then for each one, add:

 +clientport 270xx

where xx is: 05, or 06, 07, 08, 09, 10 ... etc.

Each computer must have a different clientport.

Then click OK and you should be good to go.

NOTE: This solution only works if you launch the game through the "Play Games" Menu!

If there are already options in there, be sure to leave a space between the last one and the "+".

If you wish to make a more general solution for your game where this will work if you have a desktop shortcut, you will need to add this switch to your userconfig.cfg file in the game directory (create a file with notepad if you don't have one there):

clientport 270xx where xx is: 05, or 06, 07, 08, 09 or 10

If you use this switch in the cfg file then you don't put the "+" in front.