So in summary, when a device wants to communicate with another device in the same subnet, it will
send a broadcast onto the local segment to find the MAC address of the device using the target IP address.
So in this case, A sends the frame to the hub.
The hub, because it's a multiport repeater, will send the frame out of all ports except the port on
which it arrived.
So both the router and host c receive the frame.
Network interface cards will only accept unicast traffic destined to their MAC address, or they'll
accept broadcast traffic as well as accepting multicast traffic for multicast addresses that they've
subscribed to.
The router has a mac address of G on interface fost ethernet zero zero, so the router will receive
the broadcast and forward the request to higher layer protocols.
The router will be able to see at layer three that this is an OP request for IP address ten 112.
But the router in this example is configured with IP addresses ten one one 110 one two 100 So the router
will drop the frame because the OP request is not for one of its IP addresses.
Routers do not forward broadcasts, so this broadcast is not forwarded out of interface fast ethernet
zero one.
So the broadcast received by the router is dropped.
The network interface card on PCC will receive the broadcast and see that this is an OP request for
its IP address.
So it will then reply with an OP reply.
As we've seen in the Wireshark capture, PCC will update its OP cache to show that IP address ten 111
is associated with MAC Address A and it will then send the frame to the hub.
The hub will fold the frame out of all ports because it's a multiport repeater.
The route in this example will receive a frame from the hub, but because the destination Mac address
is a and it's not the Mac address of the router, which is G, the router will drop the frame.
A will also receive a copy of the frame.
When it receives the frame, it will accept it because the destination address is A and its Mac address
is A.
It will then update its OPP cache with an OP entry stating that IP address ten 112 is associated with
MAC address C.
So the OP reply allows A to update its OP cache.
At this point, no user traffic has been transmitted.
What's happened here is that the devices have simply worked out which MAC addresses are associated with
which IP addresses.
So A now knows that MAC address C is associated with IP address ten 112.
The ping traffic can now be transmitted with a source MAC address of a which is the local machine destination
MAC address of C.
In other words, PCC, the destination MAC address was learned through OP.
Source IP address is 10.1 1.1.
Destination IP address is ten .1.122.
When the hub receives the frame from PCA, it will repeat it out of all interfaces except the interface
it arrived on.
So the router will once again receive the frame but will drop it because its Mac address is G and the
destination Mac address for this frame is C.
PCC will also receive the traffic and will accept it because the destination Mac address is C and its
local Mac address is C.
The layer two headers will be stripped and the IP address information will be read by higher layer protocols.
This is an ICMP echo packet, so the PC will reply with an echo reply message.
C will send the frame to the hub with a source mac address of C Destination Mac address of A it knows
the Mac address of A because of the previous OPP request message.
So its OPP cache has a entry associating MAC address a with IP address ten 111.
So source mac address in the frame is C destination mac address is a source IP address is ten one one
to destination IP address is ten 111.
When the frame is received by the hub, the hub will repeat it out of all ports except the one on which
it was received.
The router will receive the frame but will drop it because the destination mac address is a not to g
the local Router's Mac address.
When a receives the frame from the hub, it will accept it because the destination Mac address is a
and the PC Mac address is a.
It will then strip layer two headers and forward the information to higher layer protocols.
In this case it was an echo reply, so the ping will show a success message.
In other words, an echo request was sent to PCC and an echo reply message was successfully received
back.
So in my Wireshark capture, for example, I can filter on ICMP messages.
I can see the initial echo request message sent from my PC to ten 00254, and I can then see the echo
reply message.
Notice, please, that these are unicast frames.
Unicast firstly from my PC to the local router and then a unicast from the router to my local machine.
The same information would be displayed if you were pinging another local device on the segment.
As a user, you would see something similar to this ping ten 00254 and the ping was successful.
So in this example, my PC successfully got a reply from the remote device that I was pinging.