Internetworking

Internetworking involves connecting two or more distinct computer networks or network segments together to form an internetwork (often shortened to internet), using devices which operate at layer 3 (Network layer) of the OSI Basic Reference Model (such as routers or layer 3 switches) to connect them together to allow traffic to flow back and forth between them ^[1]. The layer 3 routing devices guide traffic on the correct path (among several different ones usually available) across the complete internetwork to their destination.

Note: Routers were originally called gateways, but that term was discarded in this context, due to confusion with functionally different devices using the same name.

It is interesting to note that some people inaccurately refer to the connecting together of networks with bridges as internetworking, but the resulting system mimics a single subnetwork, and no internetworking protocol (such as IP) is required to traverse it. However, a single computer network may be converted into an internetwork by dividing the network into segments and then adding routers or other layer 3 devices between the segments ^[1].

The original term for an internetwork was catenet. Internetworking started as a way to connect disparate types of networking technology, but it became widespread through the developing need to connect two or more local area networkswide area network. The definition now includes the connection of other types of computer networks such as personal area networks. via some sort of

The most notable example of internetworking in practice is the Internet, a network of networks running different low-level protocols, unified by an internetworking protocol, the Internet Protocol (IP).

IP only provides an unreliable packet service across an internet. To transfer data reliably, applications must utilize a Transport layer protocol, such as TCP, which provides a reliable stream (These terms do not mean that IP is actually unreliable but instead that it sends packets without contacting and establishing a connection with the destination router beforehand. The opposite applies for reliable). Since TCP is the most widely used transport protocol, people commonly refer to TCP and IP together, as "TCP/IP". Some applications occasionally use a simpler transport protocol (called UDP) for tasks which do not require absolutely reliable delivery of data, such as video streaming.

source: http://en.wikipedia.org/wiki/Internetworking