Q:

Does it make sense to map a VLAN to each LAN?

I saw a picture of a network-structure with VLANs in it:

Why would somebody structure their network like this? Does this even make sense? I mean, they are in a different LAN, why then mapping a VLAN to each one?

A:

Using VLANs is like breaking a switch into multiple unconnected switches. On a single switch, no hosts on a VLAN will ever see any traffic for a different VLAN. Traffic must pass through a router, where it can be controlled, to get from one VLAN to a different VLAN.
There are many reasons to use VLANs. One is that switches will flood unknown unicast traffic, and broadcast and multicast traffic is sent to all the other interfaces. Someone with bad intentions could be snooping on that traffic for other networks if the switch wasn't broken into VLANs. With VLANs, that type of traffic is restricted to interfaces configured for the VLAN.

