The example in Figure ,
host Cerf's IP address does not appear in host Stevens' ARP table.
Host Stevens must send out an ARP request for the IP address
172.16.10.25, host Cerf's IP address. Once again, host Stevens knows
it can do an ARP request directly for host Cerf, because it had
determined that they are both on the same subnetwork.
As also shown in Figure ,
the ARP request is encapsulated within an Ethernet frame.
ARP request from host Stevens at
172.16.10.10
The ARP Request is a Layer 2 broadcast, which means there are all
binary 1's in the destination MAC address. This is normally written in
hexadecimal as all F's (FF-FF-FF-FF-FF-FF). 
The IP address Host Stevens is looking for is in the field Target IP
Address. All hosts on the LAN receive and process this ARP Request
because it is a Layer 2 broadcast. The hosts examine the Target IP
Address to see if their IP address matches.
ARP reply from host Cerf at
172.16.10.25
The host with the IP address that matched 172.16.10.25 will reply.
This reply is unicast, meaning only the host that matches the MAC
address will process it. Notice that in the ARP reply, the information
in the "Sender" and "Target" fields is reversed
from the ARP request. This is because the source is now host Cerf
instead of host Stevens. 
Data Transmission from host
Stevens
Host Stevens receives the ARP reply and
enters host Cerf's IP address and MAC address into its ARP Table. Host
Stevens now encapsulates the IP packet into the Ethernet frame and
sends the packet directly to host Cerf. 
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