between private computers (peers/nodes). A limitation was that only music files could be shared. Because this process occurred on a central server, however, Napster was held liable for copyright infringement and shut down in July 2001. It later reopened as a pay service. After Napster was shut down, peer-to-peer services were invented such as Gnutella and Kazaa. These services also allowed users to download files other than music, such as movies and games. Technology evolution Napster and eDonkey2000 both used a central server-based model. These systems relied on the operation of the respective central servers, and thus were susceptible to centralized shutdown. Their demise led to the rise of networks like Limewire, Kazaa, Morpheus, Gnutella, and Gnutella2, which are able to operate without any central servers, eliminated the central vulnerability by connecting users remotely to each other. However, these networks still relied on specific, centrally distributed client programs, so they could be crippled by taking legal action against a sufficiently large number of publishers of the client programs. Sharman Networks, the publisher of Kazaa, has been inactive since 2006. StreamCast Networks, the publisher of Morpheus, shut down on April 22, 2008. Limewire LLC was shut down in late 2010 or early 2011. This cleared the way for the dominance of the Bittorrent protocol, which differs from its predecessors in two major ways. The first is that no individual, group, or company owns the protocol or the terms "Torrent" or "Bittorrent", meaning that anyone can write and distribute client software that works with the network. The second is that Bittorrent clients have no search functionality of their own. Instead, users must rely on third-party websites like Isohunt or The Pirate Bay to find "torrent" files, which function like maps that tell the client how to find and download the files that the user actually wants. These two characteristics combined offer a level of decentralization that makes Bittorrent practically impossible to shut down. File-sharing networks are sometimes organized into three "generations" based on these different levels of decentralization. So-called darknets, including networks like Freenet, are sometimes considered to be third-generation file-sharing networks. Soulseek is a first-generation file-sharing network that has escaped legal trouble and continues to operate in the third-generation era. Peer-to-peer file sharing is also efficient in terms of cost. The system administration overhead is smaller because the user is the provider and usually the provider is the administrator as well.