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OpenNap is a peer-to-peer service server software. It was created as an open source Napster server, extending the Napster protocol to allow sharing of any media type, and adding the ability to link servers together.
The original Napster peer-to-peer file sharing service was a protocol which allowed users to transfer files directly between their clients. The protocol was reverse engineered by a developer nicknamed Drscholl and several other programmers. The first OpenNap servers appeared in 1999, operating in the same manner as Napster servers did. A client program connects to a centralized OpenNap server, search for, share and download files. The OpenNap server keeps track of all available files and provides clients the ability to search the index of available files and initiate a direct transfer between the clients. The files available via OpenNap servers are stored on the clients, never passing through the server. In addition, instant messages (private chat) and group chat services similar to IRC are also available.
OpenNap servers can be interconnected with each other, to form OpenNap networks. In order to find as many files as possible, many clients connected to multiple OpenNap networks. In 2000-2001 an indexing service for all OpenNap servers was created, called Napigator. Napigator allowed server administrators to add their server to a central list, so they could be easily (and often automatically) found by client software.
As the RIAA began to successfully dismantle Napster in the end of 2000, the population of OpenNap began to surge. Even though it appeared OpenNap would become the next Napster, it suffered from the same vulnerability as Napster: centralized servers. When the RIAA finally dismantled Napster in 2001, it then aimed its focus on OpenNap.
During OpenNap's peak in February 2002, the RIAA on behest of its member companies, began sending “Cease and Desist” notices to the biggest OpenNap networks. One by one, the networks began to collapse and OpenNap was reduced from a population of over 250,000 to little more than 50,000 in less than five months.
There are only a few private and public opennap servers still in existence.
List of historic client software 
- Lopster (or WinLop, the Windows port)
- Napigator (enabled the original Napster client to connect to opennap servers)
- TekNap (an opennap port of BitchX)
- Utatane (P2P)
Currently there are no actively maintained OpenNap clients anymore.
External links