Notable distributed networks that use DHTs include Bit Torrent's distributed tracker, the Kad network, the Storm botnet, Ya Cy, and the Coral Content Distribution Network.
Some prominent research projects include the Chord project, Kademlia, PAST storage utility, P-Grid, a self-organized and emerging overlay network, and Coop Net content distribution system.
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).
However, in order to route traffic efficiently through the network, nodes in a structured overlay must maintain lists of neighbors that satisfy specific criteria.
This makes them less robust in networks with a high rate of churn (i.e.
However, news servers communicate with one another as peers to propagate Usenet news articles over the entire group of network servers.
The peer-to-peer movement allowed millions of Internet users to connect "directly, forming groups and collaborating to become user-created search engines, virtual supercomputers, and filesystems." Tim Berners-Lee's vision for the World Wide Web was close to a P2P network in that it assumed each user of the web would be an active editor and contributor, creating and linking content to form an interlinked "web" of links.
The early Internet was more open than present day, where two machines connected to the Internet could send packets to each other without firewalls and other security measures.
In particular, when a peer wants to find a desired piece of data in the network, the search query must be flooded through the network to find as many peers as possible that share the data.
Flooding causes a very high amount of signaling traffic in the network, uses more CPU/memory (by requiring every peer to process all search queries), and does not ensure that search queries will always be resolved.