Broadcast Domain and VLANs for Cisco CCNA

In this tutorial were gonna look up broadcast domains and introduce the idea of a broadcast domain and vlan.


First we review what Broadcast domain is, and were gonna used that to describe what vlan is. Let’s see how we gonna create two or more broadcast domains in one single physical switch.



If you remember the concept of a broadcast domain, the idea of a broadcast domain is that if one device sends out a frame with the destination mac address of all apps. The switch is going to forward that frame with all the active ports on that switch except for the port where the broadcast message was received.

In this case, the Blue computer is sending out broadcast and all other devices in the network are hearing about it. If we add some ip addresses to this, all of the network portion of the ip address should be the same in the single broadcast domain.(see image above)

The Broadcast Domain is a layer 2 function that relates to a layer 3 property. The layer 2 function here is that all apps in the destination mac address is a broadcast message. The layer 3 property is all the devices must have the same network portion of the address in order to work properly in a broadcast domain


If we change the ip addresses of our 3 work stations and still connected to the same switch. (image above)

If my device wants to ARP to find out the mac address of the device, the 3 other devices of 172.16.x.x network can also hear that arp (see image below)9-29-2016-2-20-31-am

And if wants to ARP the 3 other devices of 10.0.x.x network can also hear that arp (see image below)


The switch at layer 2 sees 1 single broadcast domain and forwards messages accordingly. But in layer 3 we actually have 2 networks here and this design is Not Recommended for implementation.



Having multiple layer 3 networks in one single layer 2 broadcast domain is not recommended.


The main reason why this is not recommended is that we can’t send a message from to without going through a router first. Although the broadcast message at layer 2 will arrive on that network station, we cannot actually communicate on that device unless we put a router on a middle.

Remember: When we have 2 devices that are connected to each other, they must share the same layer 3 network address or else we will not be able to send messages back and fort.

How to Solve this Problem?

To solve this,

  1. We need to create 2 separate broadcast domain. Take 2 switches and connect. or


2.  With VLANs we can actually set this 2 broadcast domain up in a one single switch. We just call it a VLAN


so in the image above we have two vlans, vlan 1 and vlan 2.

What happen with this vlan is the messages sent on vlan 1 would not be ever-ever reach vlan 2. The switch would not allow the message to go from 1 vlan to another vlan unless we go through a router first, it’s called InterVLAN Routing.  

-tutorial up next.

Mike Gil Kim

Howdy, It's mikegilkim a tech blogger and a gamer. I've created this blog to help other people solve it technical problems. I don't usually update or create an article because I'm blogging based on experience.