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 a 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 and ICMP traffic but on PC 1 what I'm gonna do now is send a broadcast
to 10 11 255 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 PC1 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 send 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 on the
OSI model.
We talk about bits at layer 2 in the OSI model we talk about frames at layer 3 we talk
about packets and at layer 4 we talk about segments and then we typically talk about data at
higher layers.
I'm often using terms interchangeably here but if you wanna be very precise about terminology at
layer 1 it's bits, layer 2 its 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 a collision taking place here so there's a problem with the frames because of the collisions,
only one device can access the network at any time. 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 gonna make sure that PC 2 can ping PC 4 make sure that it's ARP cache is populated so
both
PC 2 and PC1 have PC 4s 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 can 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 gonna have problems with lots of collisions taking place as you add more and more devices to
a hub.
So be careful with hubs.
They are single collision domains and single broadcast domains so we can save for 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.