Distributed hash table
A distributed hash table (DHT) is a class of a decentralized
DHTs form an infrastructure that can be used to build more complex services, such as
DHT research was originally motivated, in part, by
These systems differed in how they located the data offered by their peers. Napster, the first large-scale P2P content delivery system, required a central index server: each node, upon joining, would send a list of locally held files to the server, which would perform searches and refer the queries to the nodes that held the results. This central component left the system vulnerable to attacks and lawsuits.
Gnutella and similar networks moved to a flooding query model – in essence, each search would result in a message being broadcast to every other machine in the network. While avoiding a
Distributed hash tables use a more structured key-based routing in order to attain both the decentralization of Freenet and gnutella, and the efficiency and guaranteed results of Napster. One drawback is that, like Freenet, DHTs only directly support exact-match search, rather than keyword search, although Freenet's routing algorithm can be generalized to any key type where a closeness operation can be defined. 
In 2001, four systems—