How Skype Works
By MortazaviBlog on Sep 19, 2005
Here's a good document from Skype to consult regarding the working of the Skype P2P system. In this document, there is a discussion of Skype's propriety global index distributed directory, "through which users can find out about each other, place calls, send messages and communicate, all without using any central servers." The system is based on the use of client nodes as "super" P2P nodes. Supernode activity is entirely transparent to the users. However, clients behind firewalls and NATs are ineligble for adopting this role. In a sense, they get a free ride and don't have the honor to serve the network. Unfirewalled "supernodes" perform a key role in maintaining the index and routing calls among firewalled clients (or among clients behind NAT devices).
By the way, you might be interested in know that during the recent eBay acquision of Skype, Skype founders did not travel to the U.S. following legal advice given to them due to proceedings in courts, naming them in the Kazaa file-sharing cases. What does that tell you regarding climate of innovation here?