The physical network consists of the cables (coaxial cable, twisted pair, fiber optic, and telephone lines) that connect the different hardware residing on the network, the adapter cards used on the attached hosts, and any concentrators, repeaters, routers or bridges used in the network. (A host is a computer attached to the network.)
Physical networks vary both in size and in the type of hardware used. The two common kinds of networks are local area networks (LANs) and wide area networks (WANs). A LAN is a network where communications are limited to a moderately sized geographic area of 1 to 10 km (1 to 6 miles), such as a single office building, warehouse, or campus. A WAN is a network providing data communications capability throughout geographic areas larger than those serviced by LANs; for example, across a country or across continents. An intermediate class of networks exists also, called metropolitan area networks (MANs). This guide does not generally distinguish MANs; they are grouped with WANs.
LANs commonly use Standard Ethernet, IEEE 802.3 Ethernet, or token-ring hardware for the physical network, while WANs and asynchronous networks use communications networks provided by common carrier companies. Operation of the physical network in both cases is usually controlled by networking standards from organizations such as the Electronics Industry Association (EIA) or the International Telecommunication Union (ITU).