An extranet is a private network that uses Internet protocols, network connectivity. An extranet can be viewed as part of a company's intranet that is extended to users outside the company, usually via the Internet. It has also been described as a "state of mind" in which the Internet is perceived as a way to do business with a selected set of other companies (business-to-business, B2B), in isolation from all other Internet users. In contrast, business-to-consumer (B2C) models involve known servers of one or more companies, communicating with previously unknown consumer users.
During the late 1990s and early 2000s, several industries started to use the term extranet to describe central repositories of shared data made accessible via the web only to authorized members of particular work groups. Some applications are offered on a Software as a Service (SaaS) basis by vendors functioning as Application service providers (ASPs).Specially secured extranets are used to provide virtual data room services to companies in several sectors (including law and accountancy). For example, in the construction industry, project teams may access a project extranet to share drawings and documents, make comments, issue requests for information, etc. In 2003 in the United Kingdom, several of the leading vendors formed the Network for Construction Collaboration Technology Providers (NCCTP) to promote the technologies and to establish data exchange standards between the different data systems. The same type of construction-focused technologies have also been developed in the United States and Australia.