P2P application using PeerChannel

Hi,

I wrote a simple chat using wcf PeerChannel and CustomPeerResolver.
Everything works fine between one computer with vista and a public ip,
and a second one with vista but private ip. But I cant establish a
connection between two computers behind NAT (first xp sp2 with pnrp2,
second vista, both private ip), peer resolver is of course
accessilbe.
I have read already a lot of articles about pnrp, ipv6 and teredo but
there were so much informations I got no idea where to start. If
somebody could help me I would really appreciate it.