<P> Therefore, a distributed messaging system that is often likened as an early peer - to - peer architecture was established: USENET . USENET was developed in 1979 and is a system that enforces a decentralized model of control . The basic model is a client - server model from the user or client perspective that offers a self - organizing approach to newsgroup servers . However, news servers communicate with one another as peers to propagate Usenet news articles over the entire group of network servers . The same consideration applies to SMTP email in the sense that the core email - relaying network of mail transfer agents has a peer - to - peer character, while the periphery of e-mail clients and their direct connections is strictly a client - server relationship . </P> <P> In May 1999, with millions more people on the Internet, Shawn Fanning introduced the music and file - sharing application called Napster . Napster was the beginning of peer - to - peer networks, as we know them today, where "participating users establish a virtual network, entirely independent from the physical network, without having to obey any administrative authorities or restrictions ." </P> <P> A peer - to - peer network is designed around the notion of equal peer nodes simultaneously functioning as both "clients" and "servers" to the other nodes on the network . This model of network arrangement differs from the client--server model where communication is usually to and from a central server . A typical example of a file transfer that uses the client - server model is the File Transfer Protocol (FTP) service in which the client and server programs are distinct: the clients initiate the transfer, and the servers satisfy these requests . </P> <P> Peer - to - peer networks generally implement some form of virtual overlay network on top of the physical network topology, where the nodes in the overlay form a subset of the nodes in the physical network . Data is still exchanged directly over the underlying TCP / IP network, but at the application layer peers are able to communicate with each other directly, via the logical overlay links (each of which corresponds to a path through the underlying physical network). Overlays are used for indexing and peer discovery, and make the P2P system independent from the physical network topology . Based on how the nodes are linked to each other within the overlay network, and how resources are indexed and located, we can classify networks as unstructured or structured (or as a hybrid between the two). </P>

In peer to peer network each computer runs both client and server software