C++ P2P Listen on a port without port forwarding

Question

I'm trying to write a C++ chat program that is based on Peer To Peer technique, with no need to a server. Say peers connect to each other using their IP addresses as identifier.

Can I listen to incoming connections without configuring port forwarding on the router?

Answer 1

It is possible if peers are not behind a NAT. If they are you have to make port mapping(forward ports). You can easily write a function to check if a peers machine is behind NAT or not. And if it is, you can reconsider using server as a transfer place of the messages between peers.

Edit: You can also think about using public VPN as a proxy(with port forward included). However, it is hard to find free one. Even if you are willing to pay for it, you have no assurance that no one will listen to it and you will be dependent of the uptime of the VPN servers.