The process of routing begins when a host computer transmits a packet of information that is destined for a computer that is not physically attached to the network that the sending computer resides on. Put simply, this means any computer not on the same Ethernet network. This packet of information, which could be part of an email message, or part of a file transfer, is sent to what is referred to as the default router, or the gateway router. This is a router that gets all packets whose destination address is different from the sender's network address.
A router is basically a very fast computer. It has a processor, memory, software, and one or more input and output devices. The input and output devices are network interface modules. There must be at least two interfaces: one input, and one output, but there can be several inputs and several outputs. The simplest router setup, one where only two networks are interconnected, would have one Ethernet interface, and one WAN (Wide Area Network) interface. Packets sent from the local area network (LAN) with destination addresses that referred to the network at the other end of the WAN link, would all be sent out on the WAN interface. This would be the case with a PPP (Point-to_Point Protocol) communications link or a Frame Relay connection.
When used in a local area network, a router can be used to send traffic to network segments that have a different topology. For example, a router could be used to connect an Ethenet segment with a Token Ring segment. In this scenario the router would have a Token Ring interface and an Ethernet interface. It may also have a serial interface if it is connected to a WAN.
The simplest routing protocol is static routing. Static routing requires that each individual destination address be entered into the router's memory, along with the address of the next router in the chain, and the physical interface associated with that next router. The next router in the chain is refered to as the "next hop".
When a router receives a packet of information, it looks inside of the packet to determine the destination address of the data. The router then examines its internal routing table (or database) to determine where to send the packet. The router then sends the packet out on the interface that has been specified for the next hop.
Static routing works for internetworks that are relatively small, but can become unmanagable as the internetwork grows in size and requires more and more individual routes.
Larger internetworks require the use of RIP, or Routing Information Protocol, to keep track of new routes. Very large networks require the use of a link-state protocol such as OSPF (Open Shortest Path First). OSPF is a more efficient routing protocol which does not suffer from the limitations of RIP.
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