How skype punches holes in firewalls

http://it.slashdot.org/article.pl?sid=06/12/15/191205

An anonymous reader writes "Ever wondered, how P2P software like Skype directly exchanges data — despite the fact, that both machines are sitting behind a firewall that only permits outgoing traffic? Read about the hole punching techniques, that make a firewall admin's nightmares come true."