How to Create a Virtual LAN
To create multiple physical LANs, or broadcast domains, you
need multiple LAN switches. However, adding more LAN switches can be expensive.
Luckily, LAN switch vendors include a feature in their products that allows you
to create multiple broadcast domains in a single switch, essentially allowing
you to create multiple LANs, but without the additional hardware. These
broadcast domains are called virtual LANs (VLANs). VLANs are defined more
formally as follows:
A broadcast domain, created by a switch, using a subset of the
physical ports on the switch.
Earlier in the chapter, you read that a broadcast domain is the
group of devices for which a broadcast frame sent by one device is received by
all devices in the group. And as you well know now, a physical LAN is the same
thing as a broadcast domain.
So, what's the difference between a physical LAN and a VLAN?
Very little. The key lies in the fact that the network engineer can configure
the switch and tell it to treat some physical ports as if they are in one
broadcast domainone VLANand then configure other ports to be in a different
broadcast domainin other words, a different VLAN. When you want multiple LANs,
instead of buying a new switch to create a new physical LAN, you could just
configure VLANs. Figure 7-5 shows an
example, with the same network as in Figure 7-3.
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