An extranet could be described as two or more intranets with network connectivity. Generally, and as with intranets, an extranet will be based on internet protocols. The underlying network technology does not really matter, for instance it may be that organisations use the internet for carrying data but restrict access to resources from the general public via firewalls. A virtual private network could be set up over the Internet to achieve the same result.
An argument could be made that "Extranet" is just a buzz word for describing what institutions have been doing for decades, that is, interconnecting to each other to create private networks for sharing information.
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 authorised members of particular work groups. For example, in the construction industry, project teams could login to and access a 'project extranet' to share drawings and documents, make comments, issue requests for information, etc.