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copied!<p>Let's take a step back and just look at the concept here.</p> <p>If you just have a "dumb" switch, it only has one VLAN, and that VLAN is "untagged". That means each packet has a "normal" Ethernet header which does not include the <a href="http://en.wikipedia.org/wiki/IEEE_802.1Q" rel="nofollow">802.1q</a> tag.</p> <p>If you want to multiplex more than one network over a single physical medium, the switch has to have some way to tell the networks apart. The way it does that is by using a VLAN tag. Let's imagine you have two switches connected to each other, and you have three VLANs, 100, 200, and 300.</p> <p>Imagine the two switches are connected to each other with only 1 cable to port <em>A</em> on each switch. You would configure port <em>A</em> on each switch to accept and forward <em>tagged</em> traffic on those three VLANs. Then you would configure the ports you wanted to be in VLAN 100, 200, and 300 as that port's default VLAN, on each port exposed to end users.</p> <p>In summary, a few important points:</p> <ul> <li>VLAN tags are used to multiplex multiple logical layer 2 networks over the same physical link</li> <li>VLAN tags are normally used between infrastructure devices (and possibly servers) and are not seen on end users' networks</li> <li>A switch is typically configured such that when a packet is switched, the appropriate tag is added as the packet traverses the network infrastructure, and then removed before an end user device sees the packet again.</li> </ul> <p>Kind of hard to explain. Hope this helps.</p>

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