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The OSI Reference Model

The routing table used in a router includes the following information:

Network addresses These are protocol-specific network addresses. A
router must maintain a routing table for individual routing protocols
because each routing protocol keeps track of a network with a different
addressing scheme. Think of it as a street sign in each of the different lan-
guages spoken by the residents that live on a particular street. So, if there
were American, Spanish, and French folks on a street named "Cat," the
sign would read: Cat/Gato/Chat.

Interface The exit interface a packet will take when destined for a spe-
cific network.

Metric The distance to the remote network. Different routing protocols
use different ways of computing this distance. I'm going to cover routing
protocols in Chapter 5, but for now, know that some routing protocols
use something called a hop count (the number of routers a packet passes
through en route to a remote network), while others use bandwidth, delay
of the line, or even tick count (1/18 of a second).

And as I mentioned earlier, routers break up broadcast domains, which

means that by default, broadcasts aren't forwarded through a router. Do you
remember why this is a good thing? Routers also break up collision domains,
but you can also do that using layer-2 (Data Link layer) switches. Because
each interface in a router represents a separate network, it must be assigned
unique network identification numbers, and each host on the network con-
nected to that router must use the same network number.

Here are some points about routers you should really commit to memory:

Routers, by default, will not forward any broadcast or multicast
packets.

Routers use the logical address in a Network layer header to determine
the next hop router to forward the packet to.

Routers can use access lists, created by an administrator, to control
security on the types of packets that are allowed to enter or exit an
interface.

Routers can provide layer-2 bridging functions if needed and can
simultaneously route through the same interface.

Layer-3 devices (routers in this case) provide connections between
virtual LANs (VLANs).

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