At the moment my friend and me have to use a LTE (4g) stick (usb) to connect to the internet. The stick is connected to my pc and I enabled internet connection sharing. The other pc is connected directly to my pc (wired). It uses my connection.
First everything worked fine. But then we tried to play arma. If only one of us is playing everything works perfectly. But as soon as the second computer connects to the same server, it seems to receive all the data (even the data that should be sent to the first computer).

Other games (Men of War Assault Squad, Battlefield 3, Minecraft) and applications (browser, teamspeak) dont have this issue.

I think the problem is that arma always binds the same local port (the windows ressource manager said arma listens on 2302). Other games/applications let windows decide which local port they should use.