A darknet (or dark net) is an overlay network that can only be accessed with specific software, configurations, or authorization, often using non-standard communications protocols and ports. Two typical darknet types are friend-to-friend networks (usually used for file sharing with a peer-to-peer connection) and privacy networks such as Tor.
Originally coined in the 1970s to designate networks which were isolated from ARPANET (which evolved into the Internet) for security purposes, darknets were able to receive data from ARPANET but had addresses which did not appear in the network lists and would not answer pings or other inquiries.
The term gained public acceptance following publication of "The Darknet and the Future of Content Distribution", a 2002 paper by Peter Biddle, Paul England, Marcus Peinado, and Bryan Willman, four employees of Microsoft who argued that the presence of the darknet was the primary hindrance to the development of workable DRM technologies and inevitability of copyright infringement.
Journalist J. D. Lasica in his 2005 book Darknet: Hollywood's War Against the Digital Generation describes the darknet's reach encompassing file sharing networks. Consequently, in 2014, journalist Jamie Bartlett in his book The Dark Net would use it as a term to describe a range of underground and emergent sub cultures, including
- Social media racists
- Self Harm communities
- Darknet drug markets
As of 2015[update], "The Darknet" is often used interchangeably with the dark web due to the quantity of hidden services on Tor's darknet. The term is often used inaccurately and interchangeably with the deep web due to Tor's history as a platform that could not be search indexed. Mixing uses of both of these terms has been described as inaccurate, with some commentators recommending the terms be used in distinct fashions.
Darknets in general may be used for various reasons, such as:
- To better protect the privacy rights of citizens from targeted and mass surveillance
- Protecting dissidents from political reprisal
- Whistleblowing and news leaks
- Computer crime (hacking, file corruption etc.)
- Sale of restricted goods on darknet markets
- File sharing (warez, personal files, pornography, confidential files, illegal or counterfeit software etc.)
All darknets require specific software installed or network configurations made to access them, such as Tor which can be accessed via a customised browser from Vidalia, aka the Tor browser bundle or alternatively via a proxy server configured to perform the same function.
- Tor (The onion router) is an anonymity network that also features a darknet - its "hidden services". It is the most popular instance of a darknet.
- I2P (Invisible Internet Project) is another overlay network that features a darknet whose sites are called "Eepsites".
- Freenet is a popular darknet (friend-to-friend) by default; since version 0.7 it can run as a "opennet" (peer nodes are discovered automatically).
- RetroShare can be run as a darknet (friend-to-friend) by default to perform anonymous file transfers if DHT and Discovery features are disabled.
- GNUnet is a darknet if the "F2F (network) topology" option is enabled.
- Zeronet is open source software aimed to build an internet-like computer network of peer-to-peer users of Tor.
- Syndie is software used to publish distributed forums over the anonymous networks of I2P, Tor and Freenet.
- OneSwarm can be run as a darknet for friend-to-friend file-sharing.
- Tribler can be run as a darknet for file-sharing.
- Riffle is a client-server darknet system that simultaneously provides secure anonymity (as long as at least one server remains uncompromised), efficient computation, and minimal bandwidth burden. 
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