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2-16
Chapter 2
Planning a TCP/IP Network Infrastructure
If you were to have thousands of computers all connected to the same LAN,
each com-
puter would have to devote an inordinate amount of time to processing
broadcast mes-
sages. In addition, there would be a high collision rate because so many
computers
would be contending for the network medium at the same time. More collisions
mean
more packet retransmissions. The result would be a slow, inefficient
network. By split-
ting that network into multiple LANs, you create individual broadcast and
collision
domains, reducing the number of broadcasts each system has to process and
the num-
ber of collisions that occur.
Routing and Network Topology Design
In Lesson 3 of Chapter 1, “Planning a Network Topology,” you learned that
net-
work designers often split the network into a series of horizontal networks,
each
of which is connected to a backbone network using a router. This design pro-
vides an efficient routing solution. No matter how many horizontal networks
you
have in your installation, a transmitted packet never has to travel through
more than
two routers to get to any destination on the network (as shown in Figure
1-7). Each
packet passes through one router to get from its origin network to the
backbone
and through a second router to get from the backbone to the destination
network.
Connecting the horizontal networks in series would require packets to pass
through a separate router for each network they traverse.
The number of LANs you create and the number of computers in each LAN
depend on the data-link layer protocol you select for your network. Some
proto-
cols have specific limitations on the number of computers they support on a
sin-
gle LAN while others have implied limits based on other factors, such as the
maximum number of hubs you can use. In many cases, however, a network’s
LAN configuration is based on geographical or political factors. For
example, if
you are designing a network for a multi-story office building, creating a
separate
LAN for each floor might be the most convenient solution. In other cases,
design-
ers create a separate LAN for each department or division in the
organization.
Another advantage of routers is that they can connect networks running
completely
different protocols at the data-link layer. Whenever a packet arrives at a
router, it trav-
els up through the protocol stack only as high as the network layer (see
Figure 2-3).
The router strips off the data-link layer frame from the packet and
processes the IP
datagram contained inside. When the router has determined how to forward the
data-
gram to its next destination, it repackages the datagram in a new data-link
layer frame
prior to transmission. This new frame can be the same as, or different from,
the original
frame on the packet when it arrived on the router. So if your network
infrastructure
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