Playing Half-life 2 with NAT on the same server

For the ones with the same problem me and my brother are having in Half life 2 when connecting from the same external ip, behind NAT, getting disconnections when the other one joins the same server:

clientport 27006
or any other number then 27005

If you want this to be executed by all mods on startup you have to create a autoexec.cfg containing the previous command in every mod config dir found at:

C:\Program Files\Valve\Steam\SteamApps\<username>\<game>\hl2\cfg

Special attention goes to

C:\Program Files\Valve\Steam\SteamApps\<username>\source sdk base\hl2\cfg

as this one is used by a lot of mods as a basis and thus the autoexec.cfg get executed by a lot of games.

Happy fragging with your internet connection sharer!