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3-5. Virtual LAN
What is a VLAN?
It is a subset of a LAN. Before we discuss VLAN, we must understand what LAN is. In general, a
LAN is composed of different physical network segments bridged by switches or bridges which attach to
end stations in the same broadcast domain. The traffic can reach any station on the same LAN. Beyond
this domain, the traffic cannot go without a router’s help. This also implies that a LAN is limited. If you
need to communicate with the station outside the LAN, a router is needed which always lies on the edge
of the LAN.
For a layer 2 VLAN, it assumes it is a logical subset of a physical LAN separated by specific rules
such as tag, port, MAC address and so on. In other words, they can communicate with each other
between separated small physical LANs within a LAN but not between any two separated logical LANs.
In the figure above, all stations are within the same broadcast domain. For these stations, it is
obviously that the traffic is getting congested while adding more stations on it. With the more and more
users joining the LAN, broadcast traffic will rapidly decrease the performance of the network.
Fig. 3-7
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