Hole Punching

Hole punching is a technique by which a network connection between two computers can be made, even if both are behind restrictive firewalls. It is a special case of tunneling.

This technique is used for applications such as online games, peer- to-peer networks and IP telephony. Both Clients connect to with an open third computer, which gets so both external and internal address information. Because the clients have built up the connections, the server knows the IP addresses and port numbers of both clients. This information gives the server on which the other client. Through this procedure, the firewall accepts packets from both peers, so that a direct connection can be established.