The networks are formed by connections between groups of computers and devices associate that allow to the users the electronic transference of information. The local area network, represented in the left part, is an example of the configuration used in many offices and companies. The different computers denominate workstations and a cable or telephone line connected the servants communicates between himself through. These are computers like the workstations, but they have administrative functions and they are dedicated in exclusive right to supervise and to control the access of the workstations to the network and the shared resources (like the printers). The red line represents a main connection between network servers; the blue line shows the local connections. A modem (modulator/demodulators) allows the computers to transfer information to traverse of the normal telephone lines. The modem digital convert a signal to analogical vice versa and, and allows the communication between very distant computers to each other.