No, Gnutella does not rely on a central server, and no user registration exists. The network is completely anonymous.

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2) I read the Gnutella protocol but i found this: "A Gnutella servent connects itself to the network by establishing a connection with another servent currently on the network. The acquisition of another servent’s address is not part of the protocol definition and will not be described here (Host cache services are currently the predominant way of automating the acquisition of Gnutella servent addresses)." does anybody know how the first host is found? what technique is used? what is the host caching service? Thanx in advance

In LimeWire (and almost all Gnutella clients) there are several address that are contacted first in order to receive a brief list of currently online hosts. The primary ones are router.limewire.com:6346 and connect1.gnutellanet.com:6346. This allows LimeWire to connect quickly to the network. These are not central servers (and therefore, the network does not pivot on its existence, but they are good for network connectivity. Currently, there is no direct scanning or searching performed on the part of the individual clients for other hosts, but eventually, this ability will need to be added to the Gnutella protocol. :-) Sounds like an open source project to me!