Introduction.  A router is a device that forwards/filters traffic based on the IP address of the message.  Routers have interfaces to which LAN segments are attached to move packets of data between the attached LAN segments.  Routers are probably the single most important devices in internetworking.  They provide the flexibility and programmed decisions that make it possible to get even incompatible LAN segments to interact.  Routers operate predominantly at layer 3 of the OSI Reference Model.

When they are used to connect different segments in a LAN these segments are usually then called subnets.  They are probably most often used to provide connectivity across WANs.

The following diagram represents these basic capabilities.



The above diagram could represent two different LAN segments in the same engineering firm.  The segment on the left is used by engineers engaged in computer aided design, which is very highly resource intensive.  The segment on the right is used by marketing.  To keep the CAD engineers from bogging down everyone's use of the network, they have been put on their own segment.  The router essentially protects everyone else's use of the network from their CAD engineers demands, yet allows interaction when it is desired.  For example, maybe marketing needs to start developing the ads for a new design.

Routers have routing tables that contain the network addresses of other routers.  They must be connected to at least two different network interfaces serving as a gateway between these networks.  The network address of the particular router that serves this function is called the default gateway.

One of the other major aspects of routers is illustrated in the following diagram.



Here we have LAN segments that all using different operating systems and even some different network types.  A routers ability to be configured to support different protocols simultaneously is probably its most important feature.  This is largely done through what are called interface modules.  These modules are actually physical printed circuit boards that handle particular networking protocols.

The following list displays the most important router capabilities.

  • Routers can simultaneously support different protocols effectively making almost all computers compatible for many aspects of computing via internetworks.
  • They can seamlessly connect LANs to WANs.
  • They filter out unwanted traffic.
  • They act as security by checking traffic against permission lists.
  • They assure reliability by providing multiple paths through the networks.
  • They can learn about new paths and select the best ones.

Notice that most of these capabilities reflect adaptability and adaptable interactions with other routers.  These capabilities are important when considering where routers should be placed in networks.

When multiple paths are available to bridges they always choose one to prevent looping.  When multiple paths are available to routers for each packet they make the decision based on some sort of metric like hop distance or link availability.

Routers can also be configured to deter broadcast messages rather than forward them like bridges.

Routing Protocols.  While we will go into this in much more depth later I want to quickly survey some ideas about routing protocols.  Routers work only with routable protocols such as

  • IP
  • IPX
  • OSI
  • XNS
  • DECnet
  • DDP

A protocol such as NetBeui is nonroutable because it lacks the appropriate addressing scheme, among other things.

It can be important to classify routing protocols based on the following characteristics.

  • Static versus dynamic routing
    • static protocols require the administrator to enter addresses into the routing table and keep it current
    • dynamic routing is developed to allow routers to interact to automatically and dynamically exchange routing information
      • RIP - Routing Information Protocol
      • OSPF - Open Shortest Path First
      • NLSP - NetWare Link Services Protocol
  • Interior versus exterior
    • IGP - Interior Gateway Protocols - operate within administratively autonomous portions of networks
      • usually within an organization or something like a university campus
    • EGP - Exterior Gateway Protocols - operate across administratively autonomous portions of networks

Brouters can work as either routers or bridges.  Most modern routers satisfy this criterion.  This way they can deal with routable protocols such as TCP/IP and nonroutable protocols such as NetBEUI.

The next webpage will present some routers from a variety of companies.