Intranets are fundamentally "small" Internets. They employ the same network facilities which the Internet does, however access is limited to a restricted sphere. For example, a company can set up an intranet in the confines of the company itself. Access can be firmly controlled and restricted to authorized employees & staff. There is no connection to the Internet or any other outside network. Functions such as web sites, file uploads & downloads, and e-mail is accessible on intranets in the confines of the network.
As frivolous sites are no longer obtainable, there is no employee time lost because of accessing them. There is, certainly, the restriction of the networking area. The very benefit of limiting access to all of the facilities obtainable on the Internet also limits communication to other wanted locations. This is where the extranet steps in.
An extranet is network which connects a number of intranets in a truly mini-Internet Access is extended to all of the intranets linked through the extranet, however, again, not to the Internet. Extranets needs a constant Internet connection & a hypertext transfer protocol (http) server.