Packet Tracer file (PT Version 7.1):
Get the Packet Tracer course for only $10 by clicking here:
Get my ICND1 and ICND2 courses for $10 here: (you will get ICND2 as a free bonus when you buy the ICND1 course).

For lots more content, visit – learn about GNS3, CCNA, Packet Tracer, Python, Ansible and much, much more.

Can you answer the questions?

What are broadcast and collision domains. Can you prove the answers?

A broadcast domain is a logical division of a computer network, in which all nodes can reach each other by broadcast at the data link layer. A broadcast domain can be within the same LAN segment or it can be bridged to other LAN segments.

In terms of current popular technologies, any computer connected to the same Ethernet repeater or switch is a member of the same broadcast domain. Further, any computer connected to the same set of inter-connected switches/repeaters is a member of the same broadcast domain. Routers and other higher-layer devices form boundaries between broadcast domains.

This is as compared to a collision domain, which would be all nodes on the same set of inter-connected repeaters, divided by switches and learning bridges. Collision domains are generally smaller than, and contained within, broadcast domains.

While some layer two network devices are able to divide the collision domains, broadcast domains are only divided by layer 3 network devices such as routers or layer 3 switches. Separating VLANs divides broadcast domains as well.


We can also answer this question:
How many broadcast domains are there in Network 1?
So if I go back all the way to the original ARP message and then click capture forward, the ARP message is sent to the hub.
Notice it’s broadcast at Layer 2.
So what happens to broadcast traffic? It gets flooded.
So we have one broadcast domain because a broadcast sent to a hub is flooded.

So single broadcast domain in Network 1. We can also prove that by rerunning a simulation.
So let’s rerun the simulation.

I’m only going to look at ARP ICMP traffic. But on PC 1, what I’m going to do now is send a broadcast to

So this is a broadcast, I’ll only send two packets.
Notice the broadcast traffic is sent to the hub. When we look at the packet source address is PC 1. We can see that again by looking at the MAC address.
So notice the MAC address is PC 1.
Destination is a broadcast.
So the destination MAC address is set to that.
Destination IP address is set to a broadcast 255.255.255 in packet tracer, source IP address is PC 1.
Notice the broadcast goes to everyone. So it’s a single broadcast domain. These devices will reply back but the traffic is flooded out of all ports. Notice we’re getting a collision here.

So I’ll reset the simulation and let’s look at another problem if PC 1 sends a ping to PC 4 and PC 2 sends a ping to PC 4, what’s going to happen.
So they’re both sending packets into the network.
In this example, PC 2 sent an ARP because it doesn’t know the MAC address of PC 4.
So here’s the actual frame.

A quick recap of terminology to be precise and to be correct for the CCNA exam at Layer 1 in the OSI model, we talked about bits, at Layer 2 in the OSI model we talked about frames, at Layer 3 we talked about packets and at layer 4 we talked about segments and then we typically talk about data at higher layers.
I’m often using terms interchangeably here, but if you want to very precise about terminology, at Layer 1 it’s bits, Layer 2 it’s frames, Layer 3 it’s packets, at layer 4 it’s segments.

So notice at Layer 2 the frame has a destination address of a broadcast. That’s causing problems with the frame that was sent by PC 1.
We’ve got collision taking place here. So there’s a problem with the frames because of the collisions. Only one device can access the network at anytime.

So here, PC 1 is sending the ICMP message and a reply is sent back to PC 1.
So run the simulation again.
Before I do that, I’m going to make sure that PC 2 can ping PC 4. So make sure that it’s ARP cache is populated. So both PC 2 and PC 1 have PC 4’s MAC address in the ARP cache. And then what I’ll do in simulation mode is get PC 1 to ping PC 4 and get PC 2 to ping PC 4. So they both going to send an ICMP packet.

When that hits the hub, we have a collision. You have a single collision domain when you have a hub. So a hub is a single broadcast domain as well as a single collision domain. We’re going to have problems with lots of collisions taking place as you add more and more devices to a hub. So be careful with hubs. There are single collision domains and single broadcast domains.

So we can say question 11, Network 1 equals a single collision domain. Be careful using hubs. Today we don’t use hubs in wide infrastructures. We use switches which we’ll see in a moment have multiple collision domains.

  • David Bombal