An extranet is a computer network that allows controlled access from outside of an organization's intranet. Extranets are used for specific use cases including business-to-business. In a business-to-business context, an extranet can be viewed as an extension of an organization's intranet that is extended to users outside the organization, usually partners, vendors and suppliers, in isolation from all other Internet users.
It is in context of that isolation that an extranet is different from an intranet or internet. In contrast, business-to-consumer (B2C) models involve known servers of one or more companies, communicating with previously unknown consumer users. An extranet is similar to a DMZ in that it provides access to needed services for channel partners, without granting access to an organization's entire network.
Relationship to an intranet An extranet could be understood as an intranet mapped onto the public Internet or some other transmission system not accessible to the general public, but managed by more than one company's administrator(s). For example, military networks of different security levels may map onto a common military radio transmission system that never connects to the Internet. Any private network mapped onto a public one is a virtual private network (VPN), often using special security protocols.
For decades, institutions have been interconnecting to each other to create private networks for sharing information. One of the differences that characterizes an extranet, however, is that its interconnections are over a shared network rather than through dedicated physical lines.
A site can be in more than one VPN; for example, in an intranet and several extranets. We regard both intranets and extranets as VPNs. In general, when we use the term VPN we will not be distinguishing between intranets and extranets. Even if this argument is valid, the term "extranet" is still applied and can be used to eliminate the use of the above description.