A router is a computer networking device that buffers and forwards data packets across an internetwork toward their destinations, through a process known as routing. It is a device that maintains different routes of the network and finds the best route between any two networks, even if there are several networks to traverse.

A router consists of a computer with at least two network interface cards supporting the IP protocol. The router receives packets from each interface via a network interface and forwards the received packets to an appropriate output network interface. Received packets have all link layer protocol headers removed, and transmitted packets have a new link protocol header added prior to transmission.

The first router was created at Stanford University by a staff researcher named William Yeager in January of 1980. His boss at the time told him that he was the "network guy" and to find a way to connect the computers in the computer science department, medical center and department of electrical engineering. He first wrote a network operating system and routing code to run on a DEC PDP11/05. He used Alan Snyder's Portable C compiler but it generated too much code so he modified the compiler to improve the code generators. That still wasn't good enough so he wrote an optimizer for PDP11/05 assembler that reduced the code size further.

The routing occurs at layer 3 (the Network layer) e.g. IP) of the OSI seven-layer protocol stack, where a router acts as a junction between two or more networks to buffer and transfer data packets among them. A router is different from a switch. A switch connects devices to form a Local area network One easy illustration for the different functions of routers and switches is to think of switches as neighborhood streets, and the router as the intersections with the street signs. Each house on the street has an address within a range on the block. In the same way, a switch connects various devices each with their own IP address on a LAN. However, the switch knows nothing about IP addresses except its own management address. Routers connect networks together the way that on-ramps or major intersections connect streets to both highways and freeways, etc. The street signs at the intersection show which way the packets need to flow.

So for example, a router at home connects the Internet Service Provider's network (usually on an Internet address) together with the LAN in the home (typically using a range of private IP addresses, see network address translation) and a single broadcast domain. The switch connects devices together to form the LAN. Sometimes the switch and the router are combined together in one single package sold as a multiple port router.

In order to route packets, a router communicates with other routers using routing protocols and using this information creates and maintains a routing table. The routing table stores the best routes to certain network destinations, the "routing metrics" associated with those routes, and the path to the next hop router. See the routing article for a more detailed discussion of how this works.

Routing is most commonly associated with the Internet Protocol. They are specialized computers that send your messages and those of every other Internet user speeding to their destinations along thousands of pathways.

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