Internetworking

Internetworking is connecting two or more computer networks with some sort
of routing device to exchange traffic back and forth, and guide traffic on
the correct path (among several different ones usually available) across the
complete network to their destination. The devices used in internetworking
are routers (originally called gateways, although that term was dropped due
to confusion with functionally different devices using the same name), and
some types of high end switches.

(Connecting together networks with bridges is sometimes mistakenly called
internetworking, but this is not accurate, as the resulting system mimics a
single subnetwork, and no internetworking protocol, such as IP, is required
to traverse it.)

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 networks via some sort of wide area network. The
definition now includes the connection of other types of computer networks
such as personal area networks.

The most notable example of internetworking in practice is the Internet,
which is 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 the internet; to reliably
transfer data streams, a Transport layer protocol (such as TCP) must be
used. This is part of why we commonly refer to TCP and IP together, as
"TCP/IP". Some applications occasionally use a simpler Transport layer
protocol (called UDP) for tasks which do not require absolutely reliable
delivery of data, such as video streaming.