So this is how to play Portal in Multiplayer!

1. Open Portal

2.Make sure you have the cheats console enabled (LABELED DEVELOPER CONSOLE)

3.Start new game, open console by pressing T

4. Type "killserver"

4a. Type "hostname (your host-name)

5. Type "maxplayers (number)"

6.Type "map (i.e testchmb_a_08)"

7. Type "sv_cheats 1"

8. Type "bind "n" noclip". You can turn noclip on/off by pressing N.

9. Type "status". If you see the public IP, you have sucsessfully made a Portal 1 multiplayer server!

Tell your friend the public IP.

10. Type "bind "y" messagemode". This will let you chat with others on the server.

For guest-

1. Make sure you have developer console on.

2. Start new game, go to console.

3. Type "connect (server public IP)"

4. Press N once in the server.

Here is a map I recommend:

https://www.moddb.com/members/biduba/addons/portal-multiplayer-01