" The difference between a physical network and a virtual, or logical, network can be shown in the following example:
The students in a school are divided into two groups. In the first group, each student is given a red card, for identification. In the second group, each student is given a blue card. The principal announces that students with red cards can only speak to other students with red cards and that students with blue cards can only speak to other students with blue cards. The students are now logically separated into two virtual groups, or VLANs.
Using this logical grouping, a broadcast goes out only to the red card group, even though both the red card group and the blue card group are physically located within the same school."