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.
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
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.
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.