VLANs make it easy for network administrators to partition a single switched network, enabling systems to be divided into logical groups, and establish rules about how devices in the separate groups are allowed to communicate with each other, for example, imagine that you live in a small town that has around 300 houses with the same characteristics, and some day you need to get an envelope that comes from another state, the delivery man must know how to submit that envelope to its destiny, first of all the envelope need to have a tag, this tag can contain the state, street, house number and receiver's name, with this information does not matter if your house has the same features, the envelope will be arriving at its destination, the same scenario happen with VLANs, the envelopes are equal to frames and are attached to hosts, but the differentiation consists on each frame that has a tag number, and can be delivered to some devices or not, at this point the VLAN's traffic is separated from other VLANs, and forwarding it only where the VLAN is configured. The envelope´s owner will be able to read the information, nobody else can do it, because these differenciation tag provides exact information shipping.
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