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Internetworking Basics

In Figure 1.1, did you notice that the router is found at center stage, and

that it connects each physical network together? We have to use this layout
because of the older technologies involved--bridges and hubs. Once we have
only switches in our network, things change a lot! The LAN switches would
then be placed at the center of the network world and the routers would be
found connecting only logical networks together. If I've implemented this
kind of setup, I've created virtual LANs (VLANs), which I'll go over thor-
oughly with you in Chapter 6.

Back to Figure 1.1: On the top network, a bridge was used to connect the

hubs to a router. The bridge breaks up collision domains, but all the hosts
connected to both hubs are still crammed into the same broadcast domain.
Also, the bridge only created two collision domains, so each device connected
to a hub is in the same collision domain as every other device connected to that
same hub. This is lame, but it's still better than having one collision domain
for all hosts!

Although bridges are used to segment networks, they will not isolate broad-

cast or multicast packets.

Notice something else: the three hubs at the bottom that are connected

also connect to the router, creating one humongous collision domain and
one humongous broadcast domain. This makes the bridged network look
much better indeed!

The best network connected to the router is the LAN switch network on

the left because, as you'll remember, each port on that switch breaks up col-
lision domains. But it could definitely be improved because all devices are
still in the same broadcast domain. Remember, this can be a bad thing,
because all devices must listen to all broadcasts transmitted, and if your
broadcast domains are too large, the users have less bandwidth and network
response time will suffer.

The best network is a network that is correctly configured to meet the

business requirements of each individual business. LAN switches with rout-
ers, correctly placed in the network, are the best network design. This book
will help you understand the basics of routers and switches so you can make
informed decisions on a case-by-case basis.

So now that you've gotten an introduction to internetworking, and the

various devices that live in an internetwork, it's time to head into internet-
working models.

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