peer-to-peer and opening ports

You mean, if one or both of the machines are behind a firewall, and/or have a non routable IP? In this case I would guess there is no way, without having a 3rd aprty server to rely the messages between the machines. But if one of the machines has a routable IP and is not firewalled, then that machine can open two connections, one for sending and one for reciving data.

If both machines are behind NAT, but the NAT allows UDP through and do "cone NAT" (i e, are well-conforming NAT implementations), then you can do peer-to-peer communication without bouncing all the data off a third server -- although you still need the third server to initially set up the connection.

Check the Forum FAQ about "introducers" and "NAT punch-through".

Quote:

Original post by hplus0603 If both machines are behind NAT, but the NAT allows UDP through and do "cone NAT" (i e, are well-conforming NAT implementations), then you can do peer-to-peer communication without bouncing all the data off a third server -- although you still need the third server to initially set up the connection. Check the Forum FAQ about "introducers" and "NAT punch-through".

Following up on what hplus mentioned, you can also do "NAT punch-through" with TCP although I have heard that slightly less NATs allow you to punch through. There is paper on TCP and UDP punch through at http://www.brynosaurus.com/pub/os/nat.pdf and another site at http://midcom-p2p.sourceforge.net/ that contains some information on NAT compatibility with P2P networking over TCP and UDP.

Any yes, you do need a server that has a public IP address to intialize the connections.