So in summary when the 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 multi port repeater will send the frame
out of all ports except the port on which it arrive.
So both the router and host C received the frame.
Network Interface Card will only accept unicast traffic destined
to the 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 F0/0
so the router will receive the broadcast
and forward the ARP request to high layer protocols.
The router will be able to see at layer 3 that this is an ARP request
for IP address 10.1.1.2 but the router in this example is configured
with IP addresses 10.1.1.100 and 10.1.2.100 so the router will drop
the frame because the ARP request is not for one of one its IP addresses.
Routers do not forward broadcast, so this broadcast is not forwarded
out of interface F0/1.
So the broadcast received by the router is dropped.
The network interface card on PC C will received the broadcast
and see that this is an ARP request forwards IP address.
So it will then reply with an ARP reply
as we’ve seen in the Wireshark capture.
PC C will update its ARP cash to show that IP address 10.1.1.1 is
associated with MAC address A and it will then send the frame to the hub
The hub will forward the frame out of all ports
because it’s a multiport repeater.
The router 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 A receives the frame it will accept it
because the destination address is A and its MAC address is A.
it will then update its ARP cache with and ARP entries
stating that IP address 10.1.1.2 is associated with MAC address C.
so the ARP reply allows A to update its ARP cache.
At this point no user traffic has been transmitted
what’s happen 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 10.1.1.2