Your computer uses this number to do stuff on the Internet, such as retrieve a web page. Any system that translates names into Internet numbers is a name space. When another Napster user wants to communicate with you, the Napster server translates this name into the Internet address of your computer. The Napster server acts as a name server and a search engine, all using proprietary protocols. Users sign up on a Napster server with whatever name they want to use.
Registration is instant, free, and requires no contact or other personal information.
Napster heralds a new, balkanized Internet that relies less on web browsers. Even if you don't have a permanent IP address, you can still register on Napster and play, after a fashion. Napster's search engine searches this name space and links to files, all using proprietary protocols. Napster's chat service also uses this space. Web browsers have limitations as application interfaces, but they offer compensating advantages of stability and trust.