The Physical Network

In essence, a network is a group of computers or devices connected together by communication links. In networking terms, every computer or device (printers, routers, switches, and so on) connected to the network is called a node. Nodes are connected by links, which could be cables or wireless links (such as infrared or radio signals), and they can interact with any other node by transmitting messages over the network.

We can differentiate networks according to their size:

• A LAN, or Local Area Network, connects nodes over a limited area. This area can be as large as the site of a big company, or as small as connected computers in someone's home. The most commonly used LAN technology is the Ethernet network (see next section).
• WAN is the acronym for Wide Area Network. Multiple LAN sites are connected together by a WAN. WAN technologies that you might know of include Frame Relays, T1 lines, ISDN (Integrated Services Digital Network), X.25, and ATM (Asynchronous Transfer Monitor). In the next section, we'll further discuss the means of connecting to a WAN.
• A MAN, or Metropolitan Area Network, is very similar to a WAN in that it connects multiple LANs. However, a MAN restricts the area of the network to a city or suburb. MANs use high-speed networks to connect the LANs of schools, governments, companies, and so on, by using fast connections to each site, such as fiber optics.

When talking about networks, the term backbone is often used. A backbone is a high-speed network that connects slower networks. A company can use a backbone to connect slower LAN segments. The Internet backbone is built up of high-speed networks that carry WAN traffic. Your Internet provider either connects directly to the Internet backbone, or to a larger provider that connects directly to the Internet backbone.