What Is A Router?
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 layer 3). The primary function of a router is to connect networks together and keep certain kinds of 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.
WHY IS A ROUTER NECESSARY?
Routers perform the following functions:
- Restrict network broadcasts to the local LAN
- Act as the default gateway.
- Move data between networks
- Learn and advertise loop free paths between sub-networks.
Restrict Broadcasts to the Local 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 PC wants to talk to a PC on another network, it does so by sending your data to the default gateway (your router). The router receives your data, looks for the remote address of that far-off PC makes a routing decision and forwards your data out a different interface that is closer to that remote PC. There could be several routers between you and the remote PC, so several routers will take part in handing off the packet, much like a fireman's bucket brigade.
A router can take in an Ethernet frame, strip the ethernet data off, and then drop the IP data into a frame of another type. In this way a router can also perform 'protocol conversion', provided it has the appropriate hardware and software to support such a function. The whole point, however, is to forward the data from the interface it receives data on, to another interface that retransmits the received data onto another interface serving another network.
Move Data between Networks
Routers have the capability to move data from one network to another. This allows two networks managed by different organizations to exchange data. They create a network between them and exchange data between the routers on that network. Because a router can accept traffic from any kind 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. In technical terms, a token ring network and an ethernet network can communicate over a serial network. Routers make all this possible.
Learn and Advertise Routes
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.
DO I REALLY NEED A ROUTER?
If you have a small network, with few hosts, you probably don't need a router unless you are connecting your network to another network. For example, the Internet is a very large network, so you would need a router (and a lot of other things) to connect your network to the Internet.
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