ARP Message Format

As the ARP is strongly dependent on the addressing of the physical network,
there is no fixed format for the ARP messages. In general, the ARP message is
encapsulated within the frame used by the network infrastructure, as shown
in Figure 3.5.
To identify a frame as the carrier of an ARP message, the type field of
the frame header takes a specific value. For example, for Ethernet, the type
field takes the hexadecimal value 806.
The ARP message is designed for any type of physical network, so its
length is variable depending on the type of addressing. Figure 3.6 shows an
ARP message including the Ethernet hardware addresses.
The hardware type field indicates the type of physical network (for Ethernet
it takes a value of 1). The protocol type field specifies the protocol of
the network used, which in the IP case takes the hexadecimal value of 800. The operation field indicates whether the packet contains an ARP
request or an ARP reply.
The HLen and IPLen fields specify the length of the hardware (physical)
and IP addresses, respectively. When an ARP request is generated, the
host that sends the request fills in the sender hardware address, sender IP
address, and target IP address fields. The host that receives the request fills in
the remaining field, target hardware address (that is, its physical address),
exchanges the target IP and sender IP address fields, and sends back the message
as an ARP reply (operation field equal to 2).