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A router is a device that transmits packets of data through a network. A router is connected to a minimum of two networks (could be LAN or WAN or even a LAN and its Internet Service Provider’s network). So the basic function of a router is to connect networks. Hence they are placed on gateways where two (or more) networks connect. A router links a computer to the Internet, thereby enabling the users to share the connection. It functions like a dispatcher, transmitting data via the best path so that it is received quickly.
Minimal filtering of data is done via routers. Forwarding tables and headers are used by routers to determine the best path for transmitting the packets. A router has different interfaces corresponding to different physical types of network connections, like fibre optic, copper cables, wireless transmission etc. Routers use protocols like Internet Control Message Protocol (ICMP) for communication and configuration of the best route between two hosts. Routers used in home and offices (small) are the most familiar ones that simply forwards data, such as email, web pages, videos and instant messaging (IM) between the personal computers and the Internet. More powerful and advanced routers are those which connect large businesses or ISP networks up to the sophisticated core routers that transmit data at rapid speed along the optical fiber lines of the Internet backbone. These are called enterprise routers.
A router has two stages of operation called planes:
• Control plane: The selection of route for forwarding data packet and the interface connection is done by a routing table. This is done by static routes (private pre-configured channels), or through learning routes using protocols of dynamic routing. These routes are stored in the Routing Information Base (RIB). This RIB is stripped from the unimportant directives by the control-plane logic and a Forwarding Information Base (FIB) built to be used by the forwarding-plane.
• Forwarding plane: The router passes data packets between incoming and outgoing connecting interfaces. It routes them to the appropriate network type using information contained in the packet header using the data recorded in the control plane.
Although routers are typically dedicated hardware devices, software-based routers have also become prevalent these days. The owner's cable or a digital subscriber line (DSL) router connecting to the Internet through an ISP is an example of a router