How to set up communication between computers via the Internet?

Question

Computer networks

We need several C# classes that would allow messages to be exchanged between computers over the Internet. All types are grey.
Classes should contain methods like void SendMessage(String message); and String GetMessage(); well, or something like void Listen...(). There are a lot of programs for the local network, but I didn’t find one to work via the Internet. Digging towards openVPN, ssh. But as far as I understand, they implement computer management or file transfer, but I don’t need it.
Task: to make a server that will wait for connections from clients and communicate with them, communication between clients is not necessary.

Answer 1

The thing is that by connecting computers / network via vpn, you end up with the same local network, or several local networks connected to each other. Why everyone loves vpn. And on what you will do it, on openvpn or something else, it doesn’t matter.
Well, one of the easiest ways is not to use vpn at all. I'll tell you now, connect both computers to ipv6, there are plenty of providers for this, and work on real ipv6 addresses!

Answer 2

Badly watched
https://ru.wikipedia.org/wiki/VPN

Answer 3

You can try DDNS