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It's NOT bannable! It's safe!
It's using Windows Firewall!
It works like this:
If your friend's IP is for example 188.24.135.56 then it will create Firewall Rule which is applied to all IPs from 1.0.0.0 to 187.0.0.0 and from 189.0.0.0 to 255.255.255.255. In short, your friend's IP is bypassed/ignored (that's how only your friend's IP is not blocked and he is allowed to join session).
I played today with a friend, IP whitelist worked for me.
First I added his IP, then did all the steps, after that I did exactly what is said in "NOTE" which is bold font by the way. His IP was something like 5.24.145.138 and program stays opened during gameplay (because by default settings it removes firewall rules on exit).
PS. Program is coded on 1080x720 resolution, I'm sorry if the fonts or form size is somehow different on your PC, don't know how to make it universal!
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