Networking : Internetwork

Two or more networks or network segments connected using devices that
operate at layer 3 (the 'network' layer) of the OSI Basic Reference
Model, such as a router. Any interconnection among or between public,
private, commercial, industrial, or governmental networks may also be
defined as an internetwork.

In modern practice, the interconnected networks use the Internet
Protocol. There are at least three variants of internetwork, depending
on who administers and who participates in them:

* Intranet
* Extranet
* Internet

Intranets and extranets may or may not have connections to the
Internet. If connected to the Internet, the intranet or extranet is
normally protected from being accessed from the Internet without
proper authorization. The Internet is not considered to be a part of
the intranet or extranet, although it may serve as a portal for access
to portions of an extranet