If you use Gnucleus, you can look at the handshake if you select Extened information in the contextual menu of a connection in the connection window (Advanced). There you'll see that in almost every handshake there is an option called Remote-IP like that is followed by your IP. That's the way the other programs detect it.

And I can asure you that it works. Me and a lot of other users are connected to Gnutella like this.