In order to understand a VLAN, you need to know what a LAN is. A LAN is a network where all devices are in the same broadcast domain (broadcast address to all devices in a network, everyone hears everyone). In a LAN, each network element can communicate with the entire network without going through a router. Without VLAN, a switch considers all its interfaces as being in the same LAN and therefore in the same broadcast domain. While with VLANs, a switch can put some of its interfaces in one broadcast domain and others in another broadcast domain. The same switch then has several broadcast domains. So several logical separations on the same physical support. It's actually like a school. You have a building (switch) with its students (network elements). If you don't use classroom (vlan) like an auditorium, a teacher will talk with his students and everyone will hear him, even if they are not concerned. Whereas if we use classroom, a specific teacher will be able to talk to particular students.
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