Switches remove the physical boundary

Switches remove the physical boundary.
FIGURE 1 4 . 3
Physical LANs connected to a router
E0
E3
E1
E2
VLAN A
VLAN B
VLAN C
VLAN D
VLAN A
VLAN B
VLAN C
VLAN D
VLAN A
VLAN B
VLAN C
VLAN D
Net = A
Net = C
Net = D Net = B
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Chapter 14 
VLANs, Trunks, and VTP
Switches create greater flexibility and scalability than routers can by themselves because
switches define the network VLANs and VLAN port assignments. You can group users into
communities of interest, which are known as VLAN organizations.
Because of switches, we don’t need routers anymore, right? Wrong. In Figure 14.3, notice
that there are four VLANs, or broadcast domains. The nodes within each VLAN can communicate
with each other but not with any other VLAN or node in another VLAN. When configured
in a VLAN, the nodes think they are actually in a collapsed backbone, as in Figure 14.2.
What do these hosts in Figure 14.2 need to do in order to communicate to a node or host on a
different network? They need to go through the router, or other layer 3 device, just as they do
when they are configured for VLAN communication, as shown in Figure 14.3. Communication
between VLANs, just as in physical networks, must go through a layer 3 device.
If the creation of VLANs using the existing addressing scheme does not produce
the segmentation that you need, you may have to bite the bullet and
renumber your network. But it’s not all bad news. Creating a new IP addressing
scheme from the ground up may seem like a huge task, but it is greatly simplified
by using an automatic addressing process such as Dynamic Host Configuration
Protocol (DHCP).