A local area network or LAN is a collection of two or more computers that share a common protocol and broadcast media such as a cable or in the case of WiFi a radio signal. The protocol is usually Ethernet and the connection by cat5 or 6 twisted pair cable with RJ45 connectors although today WiFi is becoming increasingly popular.
Computers talk to each other over the common link by packaging the data in Ethernet frames and addressing them with the recipients network address. The network address for each computer, which are referred to as hosts, is a burnt in MAC (media access control) address on the network adapter card and is globally unique. Hosts can be any network capable device such as a printer, scanner or server. Because each host has a unique address all the hosts on the common segment can identify each other and exchange messages. There are three ways that computers can talk uncast, multicast and broadcast. With unicast the sender directly addresses the recipient using its MAC address, this is a one to one relationship. Multicast uses a group address to reach a subset of the computers on the LAN. Computers register an interest in being a group member and receive all communications sent to that group address. A Broadcast is as its name suggests is a message sent by one host but delivered to all hosts on the LAN. Because broadcasts are so important for the operation of the LAN and also the main limiting factor in the size of a LAN, a local area network is often defined as being a broadcast domain. Boadcasts are used for host discovery and network control but they can be very wasteful of bandwidth therefore a LAN is typically restricted to less than 100 hosts. LAN’s can be joined together to form larger LAN’s by routers, which restrict the broadcast domain by blocking LAN broadcasts from entering or leaving their own LAN. To be able to communicate through a router to a host on another LAN a host must use another communication protocol called IP and have an IP address.