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A router is a network device that connects networks, forwarding packets to and from them as needed. This prevents sniffing by limiting the network segments through which a packet passes. It determines how, and where to forward packets based on internal routing tables that may be hard-coded on the router and not subject to change, or may be dynamically updated by other routers on the network as the best routes to different destination networks change.
Generally, a router contains at least two network interfaces, and in larger networks, often more. Connected to each interface is a specific network or network subnet. Packets come into the router from each interface; the router compares each packets destination address with its internal routing tables, and sends the packet out the appropriate interface, on its way to its destination. Often the only router on a small business network is one with only two interfaces, one for the internal network containing the organizations workstations and other devices, and one for the companys Internet connection.
Originally deciding which network to send a packet to, to move it toward its ultimate destination, was all the functionality that routers provided. In the case of the small business router described above, a router would often do little more than serve as an expensive connector between the internal network and the Internet, with very little in the way of routing decisions to make.
In larger networks, routers can be employed to segment traffic and regulate traffic among segments, helping to ensure that network performance is as optimal as possible and limiting the amount of traffic that can be spied on by those who install a network sniffer to inspect packets traveling along the wire. By limiting the number of nodes through which a packet passes, routers also reduce the potential for man-in-the-middle attacks.
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