How do VLANs affect the broadcast domain?

Question

I'm repeating a "LAN Switching and Wireless" module in college and I'm looking over past exam papers in preparation for the repeat exam. One of the questions is

What is a broadcast domain? Using an example, explain how VLANs affect the broadcast domain.

I know that a broadcast domain is a logical division of a network whereby all nodes can reach each-other on the data-link layer by broadcast. And I realise that each VLAN is essentially its own broadcast domain. But I don't know what example I could give that could answer this question properly.

Answer 1

Each VLAN creates its own broadcast domain in 1 or more physical switches. If you have a switch taken out of the box it tends to put all ports in vlan 1 so for say a 24 ports the broadcast domain includes all 24 ports. If you were to create a vlan 2 and configure half the ports to be members of vlan 2 you then have 2 broadcast domains each of 12 ports. if you create a vlan 3 and put half the ports that are still in vlan 1 in it you would end up with 3 broadcast domains 2 with 6 ports and another 1 with 12 ports.