Napster was the best known example of this type of P2P application

In this design, the P2P file-sharing service uses a large server (or server farm) to provide the directory service

The P2P application contacts the directory service, informing the directory service of its IP address and the names of objects in its local disk that it is making available for sharing

When an active peer obtains a new object or removes one, it informs the directory server, which then updates its database

Another way the directory server updates is by periodically sending messages to see which peers are still active, and when it determines that a peer is no longer connected, it removes the peer's IP address from the database