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peer-to-peer (or P2P) computer network is a network that relies primarily
on the computing power and bandwidth of the participants in the network
rather than concentrating it in a relatively low number of servers. P2P
networks are typically used for connecting nodes via largely ad hoc connections.
Such networks are useful for many purposes. Sharing content files (see file
sharing) containing audio, video, data or anything in digital format is
very common, and realtime data, such as telephony traffic, is also passed
using P2P technology..
A pure peer-to-peer network does not have the notion of clients or servers, but only equal peer nodes that simultaneously function 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 for a non peer-to-peer file transfer is an FTP server where the client and server programs are quite distinct, and the clients initiate the download/uploads and the servers react to and satisfy these requests.
Some networks and channels, such as Napster, OpenNAP, or IRC @find, use a client-server structure for some tasks (e.g., searching) and a peer-to-peer structure for others. Networks such as Gnutella or Freenet use a peer-to-peer structure for all purposes, and are sometimes referred to as true peer-to-peer networks, although Gnutella is greatly facilitated by directory servers that inform peers of the network addresses of other peers.
Peer-to-peer architecture embodies one of the key technical concepts of the internet, described in the first internet Request for Comments, "RFC 1, Host Software"  dated 7 April 1969. More recently, the concept has achieved recognition in the general public in the context of the absence of central indexing servers in architectures used for exchanging multimedia files.
The concept of peer to peer is increasingly evolving to an expanded usage as the relational dynamic active in distributed networks, i.e. not just computer to computer, but human to human. Yochai Benkler has developed the notion of commons-based peer production to denote collaborative projects such as free software. Associated with peer production are the concept of peer governance (referring to the manner in which peer production projects are managed) and peer property (referring to the new type of licenses which recognize individual authorship but not exclusive property rights, such as the GNU General Public License and the Creative Commons License).
" A type of network in which each workstation has equivalent capabilities and responsibilities. This differs from client/server architectures, in which some computers are dedicated to serving the others." (Source: Webopedia)
A peer-to-peer network may be as simple as two computers at home sharing a printer, or as complex as thousands of computers exchanging millions of files through Internet P2P software.
Popular Internet P2P file-sharing has evolved through three generations, to the form it exists as today:
Initially, P2P networks relied on a central directory of files available for download. The United States courts eventually ruled that the company or individual controlling such a list would be liable for any illegal activities that might occur because of this information, including copyright violations. This resulted in the closure of the original Napster.
The P2P networks that followed in Napster’s wake (e.g. Gnutella and Kazaa) made use of decentralized file lists. This arrangement is still open to potential copyright abuse because of technical limitations that prevent P2P networks from filtering out copyrighted content.
Current P2P file-sharing networks are built upon optimized versions of the previous P2P generations, retaining or incorporating the features that made such networks reliable and efficient. There are 2 types of current generation P2P networks:
Friend-to-friend (e.g. Metanet, WASTE, MUTE)
The network is defined by degrees of familiarity or relationships between users. Users depend on the endorsement of other users for access to files.
Anonymous (e.g. GNUnet, Entropy, Freenet)
Much like the first and second generations of P2P networks, file-sharing is primarily based on the availability of files.