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