A computer network may be defined as the coordination or interconnection of a number of individual computers. It is established by the network layer in the Open Systems Infrastructure model, popularly known as the OSI model. In a broad sense, computer networks can be classified on the basis of size, structure, mode of connectivity, topology, etc. Network Size Taking into consideration the size of the implemented network, networks are classified into three main divisions; namely, the Local Area Network (LAN), Wide Area Network (WAN), and the Metropolitan Area Network (MAN). In certain fields, they are further classified as the Personal Area Network (PAN), Campus Area Network (CAN), and the Global Area Network (GAN). LAN: A network of computers which covers only a small area, like an office or a building, can be referred to as a Local Area Network (LAN). In this case, all the individual computers have a common protocol layer for their connectivity with the basic functionality taking place at the network layer. In general, LANs are based on Ethernet technology. WAN: The Wide Area Network (WAN) is basically an extension of the Local Area Network, except for the fact that the size of the network extends to a very large area. The major functionality of the WAN takes place at the three lower layers of the OSI model; namely, the network layer, data link layer, and the physical layer. MAN: The Metropolitan Area Network (MAN) is a combination of two or more individual Local Area Networks, with the criterion that the boundary of the network should not exceed the city limits. It integrates the network as a single unit within a city. PAN: The Personal Area Network (PAN) is the smallest of all networks, with most of these networks not exceeding a size of 25 feet. CAN: The Campus Area Network (CAN) is similar to a Wide Area Network, but its size is limited to a certain social limit. For example, the network in a university which is formed by the collaboration of more than one Local Area Network. GAN: The Global Area Network (GAN) is a large-scale Wide Area Network, which is implemented globally. It consists of a large number of sub-networks which are connected using smaller wireless networks. Structure Of The Network As far as the structure and relationship between the elements of a network are concerned, the network can be classified into three divisions; namely the Client-Server model, Peer to Peer model, and the Active Networking model. Each of the classifications has a separate and unique structure, which differentiates it in connectivity and working mode. Mode of Connectivity In terms of connection mechanisms, computer networks can be divided into wired networks and wireless networks. The wired networks are further divided into sub-divisions like Optical Fiber and Ethernet networks. Topology In general, a computer network is technically classified based on the topology of the total network, which includes the structure of the networks as well. These divisions include the Bus network, Ring network, Star network, Mesh network, Star-bus network, and the Tree network. Routers: A router simply functions as an intermediary device which forwards the data packets that it receives, to the next available or neighboring router, or a terminal. Switches: Switches are the virtual controllers of routers and bridges. These are operated from each of the individual layers of the OSI layer. Bridges: Technically, a bridge acts a connector between the data link layer and network layer. The three types of bridges are: Local bridges, Remote bridges, and Wireless bridges. Network Cards: One of the most important hardware components of any given computer network, the network card connects the computer to it, thus enabling communication between the two.