How do I make a TCP server work behind a router (NAT) without any redirection configuration needed

Question

The scenario is the following. I have two machines A and B: A: Client (behind NAT) B: Server (behind NAT)

I want B to be able to listen on any given port, so that A

Answer 1

To answer the example of Peer to Peer programs, and in general: There is a technology called Universal Plug and Play which NAT routers can use to allow clients behind them to expose ports to the outside. That's what bittorrent clients can use so the other clients can directly connect to them.