Peer to peer communication over internet implementation in java

Question

I am trying to build a system in java, where one computer acts as the server and the other one as the client and connect them together (i.e send data b/w them) on the public network. Here is my pseudo code -

Server.java

ServerSocket server = new ServerSocket(55955);
Socket socket = server.accept();

Client.java

 Socket socket = new Socket("<ip address of server>", 55955);
//code to send and receive response from server

The code works fine if I have my server and client running on my computer.But if i run the server on a different machine (a friend's pc on the internet) and try to use that ip on the client running my machine, its not working. I did a lot of googling and did not find a appropriate solution other than opening up the firewall or ports on the machine.

Is there any other option to connect b/w the computers on the internet without any forwarding or opening the ports ?
There are a few peer to peer systems (eg: skype)etc that does the same thing I am trying to achieve (and in my best of my knowledge its without opening any ports on the host machine). Please correct me if I am wrong.

Thank you.

1. Connection without forwarding/opening could be possible if this specific machine is exposed to the Internet without any protection by firewall or NAT - and this is almost improbable nowadays. 2. Skype uses TCP port 443 which is just open by default. — Nowhere Man, Jun 28 '20 at 09:55
@AlexRudenko Thank you. Sorry I am newbie on networking. One question that arise to my mind is that if it listens on tcp port 443, how can it differentiate b/w other requests. For eg: If I have another application such as Zoom, which also listens on port 443. How can we differentiate ? — Mukesh Keshu, Jun 28 '20 at 10:10
these links https://stackoverflow.com/questions/1694144/can-two-applications-listen-to-the-same-port https://serverfault.com/questions/653513/how-to-handle-multiple-apps-via-port-443 provide some details how this can be done. — Nowhere Man, Jun 28 '20 at 10:26

score 2 · Accepted Answer · answered Jun 28 '20 at 13:12

Thank you. I am adding the solution which I was finally able to work with.

This can be achieved using NAT Hole punching. Found these awesome articles which explains the approach.

Peer to peer communication over internet implementation in java

1 Answers1