January 08, 2012

1.1.2 Routers

A router links computers to the Internet, so users can share the connection. A router acts as a dispatcher, choosing the best path for information to travel so it's received quickly.

Switches create a network. Routers connect networks.

A router is a network device that connects several networks together and relays data between them.
A router is comprised of the following components: network interfaces, routing protocol, routing table, router operating system, routing policy or set of rules.

A router is a device that forwards data packets between computer networks. Routers work by providing a path between the networks. A router is connected to two or more data lines from different networks. When a data packet comes in on one of the lines, the router reads the address information in the packet to determine its ultimate destination. Then, using information in its routing table or routing policy, it directs the packet to the next network on its journey.

Routers perform traffic directing functions on the Internet.
Routers store information about the networks to which they're connected. Most routers can be configured to operate as packet-filtering firewalls. Many of the newer routers also provide advanced firewall functions.

Routers, in conjunction with a Channel Service Unit/Data Service Unit (CSU/DSU), are also used to translate from LAN framing to WAN framing (for example, a router that connects a 100BaseT network to a T1 network). This is needed because the network protocols are different in LANs and WANs
Routers establish communication by maintaining tables about destinations and local connections. A router contains information about the systems connected to it and where to send requests if the destination isn't known.

Routers usually communicate routing and other information using one of three standard protocols: Routing Information Protocol (RIP), Border Gateway Protocol (BGP) and Open Shortest Path First (OSPF).
An administrator should take a layered approach to protecting the network. The router should be only one part of that approach.

The routes themselves can be configured as static or dynamic. If they are static, then they are edited manually and stay that way until changed. If they are dynamic, then they learn of other routers around them and use information about those to build their routing tables.

When two or more computers are connected together they can share resources freely. We refer to this construct as a network. You can set up multiple such networks and each would be able to share resources only between its own set of computers. I.e. network #1 would allow sharing between its own set of computers, network #2 would allow sharing between its own set of computers. Suppose you wanted a computer in network #1 to communicate with a computer in network #2.

You could do it in one of two ways:
  • Put all computers in network #1 and network #2 together
  • Somehow connect network #1 and #2 together that allowed the communication but also maintained the separate identities of the two networks.
There are good reasons to follow the 2nd option and to do that we use a router.

References:
  • http://www.cisco.com/cisco/web/solutions/small_business/resource_center/articles/connect_employees_and_offices/what_is_a_network_switch/index.html
  • http://en.wikipedia.org/wiki/Router_(computing)
  • http://www.ciscorouting.com/routingbasics.html
  • CompTIA Security+ Study Guide: Exam SY0-301, Fifth Edition by Emmett Dulaney

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