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A router is specialized computer connected to more than one network. It runs software that allows it to move data from one network to another. Routers operate at the network layer (OSI Model - Layer 3). The primary function of a router is to connect networks together and keep Layer 2 broadcast traffic under control. There are several companies that make routers: Cisco, Juniper, Nortel (Bay Networks), Redback, Lucent, 3Com, and HP just to name a few.


Routers perform the following functions:

  1. Restrict network broadcasts to the local LAN
  2. Act as the default gateway.
  3. Move data between networks
    1. Routing
    2. Protocol Translation
  4. Best Path Calculations
  5. Route Advertisement

Restrict Broadcasts to the LAN

Networks use broadcast traffic (transmissions sent to all hosts on the network) to communicate certain kinds of information that makes the network function (ARP, RARP, DHCP, IPX-SAP broadcasts etc.). As the number of hosts on the network increases, the amount of what is called "broadcast" traffic increases. If enough broadcast traffic is present on the network, then ordinary communication across the network becomes difficult.

To reduce broadcasts, a network administrator can break up a network with a large number of hosts into two smaller networks. Broadcasts are then restricted to each network, and the router performs as the 'default gateway' to reach the hosts on the other networks.

Act as the Default Gateway

Especially in today's networks, people are connecting to the Internet. When your computer wants to talk to a computer on another network, it does so by sending your data to the default gateway (your router). The router receives your data, looks for the destination address of the remote computer you're communicating with (a web server for example). The router makes a forwarding decision and forwards your data out a different network interface that is closer to that remote computer. When communicating across the Internet, there are always several routers between you and the remote computer, so several routers will take part in handing off the packet, much like a fireman's bucket brigade.

Move Data between Networks

The primary function of a router is to move packetized data from one network to another. This allows two networks managed by different organizations to exchange data. Because a router can accept data from any type of network it is attached to, and forward it to any other network, it can also allow networks that could not normally communicate with each other to exchange data.


When a packet reaches the router, the router can look at the network layer information in the packet and decide which network to forward the packet to. This process of reading the network layer information, performing a lookup of the network address and then making a forwarding decision based on the destination network address is called routing.

Protocol Translation

A router can take in an Ethernet frame, strip the Ethernet data away and then drop the IP data into a frame of another type. In this way a router can also perform 'protocol conversion' (such as converting Ethernet frames to Serial or Token Ring frames), provided the router has the appropriate hardware and software to support protocol conversion. The whole point of a router however, is to forward packetized data from the interface it received that data on, to another interface on the router that retransmits the received data onto the next network. Switches forward data based on the physical address (usually the MAC address) in the frame. Routers differ from switches because they make a routing decision based on the logical address in a packet, reformat the packet and then retransmit the packet on a different network.

Route Advertisement

Over time, routing protocols (RIP, OSPF, IS-IS, IGRP, EIGRP, BGP) have been invented so that very large network systems with lots of sub-networks can automatically learn where the various networks are tell other routers and move data between them automatically. This is how data makes it across the Internet.

Calculate the Best Path

All routers make decisions about the best path to reach a destination network. This process is determined by the administrator, or by the routing protocol being used. Routers that are connected to several other routers often participate in exchanging routes using a dynamic routing protocols such as RIP, OSPF, IS-IS, EIGRP or BGP. Routers learn about which networks exist from each other and make internal calculations, based in the routing information they receive, as to which path is the shortest to a remote network.


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