Do all of the peer-to-peer and file-sharing programs work by logging onto a central server(s) in order to get a list of users logged on and then the server contacts all those logged on in order to search for the file or to send an instant message?

Per definition, a peer-to-peer system hasn''t got a central server. There are two different methods to get a (partial) list of participating peers:

1) you contact a (known) participant and tell him to give you a list of peers
2) you send host query packets to a multicast group

Most systems use method 1), because it just works. The obvious disadvantage is that you need to know at least one participant of the network.

Method 2) is the more elegant one, since you obviously don''t need a known host in the network to participate. You could say that the network is always online. However, multicasting isn''t widely supported, especially across different ISPs.

As usual, mixing is the key to success. Nothing''s going to stop you from using both approaches.

cu,
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