Installation guide
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Virtual LANs (VLANs) are logical, independent workgroups within a network. These
workgroups communicate as if they had a physical connection to the network. However,
VLANs are not limited by the hardware constraints that physically connect traditional
LAN segments to a network. As a result, VLANs allow the network manager to segment
the network with a logical, hierarchical structure. VLANs can define a network by
application or department. For instance, in the enterprise, a company might create one
VLAN for multimedia users and another for e-mail users; or a company might have one
VLAN for its Engineering Department, another for its Marketing Department, and
another for its guest who can only use Internet not Intranet. VLANs can also be set up
according to the organization structure within a company. For example, the company
president might have his own VLAN, his executive staff might have a different VLAN,
and the remaining employees might have yet a different VLAN. VLANs can also set up
according to different company in the same building to save the money and reduce the
device establishment.
The Figure 5-1 shows the IEEE 802.1Q tag frame and its insertion point within the
Ethernet and 802.3 frames. The 802.1Q tag contains 3 priority bits and 12 VLAN ID bits.
The 3 priority bits are for 802.1P. Ethernet switches and endpoints must be capable of
interpreting the 802.1Q tag to make use of the tag. If an Ethernet switch or an endpoint
cannot interpret the 802.1Q tag, the presence of the tag may cause problems.
Figure 5-1. Untagged and 802.1Q-Tagged Ethernet frames