> You still haven't answered my question - how can I, sitting behind one NAT, receive a connection from my friend behind another NAT, if neither allows direct i

> You still haven't answered my question - how can I, sitting behind one NAT, receive a connection from my friend behind another NAT, if neither allows direct incoming connections? This is exactly the problem that STUN solves. But you need a third party server, most of the time application-specific, and yet another layer of complexity.