1-2ARP Message FormatFigure 1-1 ARP message formatThe following explains the fields in Figure 1-1.z Hardware type: This field specifies the hardware address type. The value “1” represents Ethernet.z Protocol type: This field specifies the type of the protocol address to be mapped. The hexadecimalvalue “0x0800” represents IP.z Hardware address length and protocol address length: They respectively specify the length of ahardware address and a protocol address, in bytes. For an Ethernet address, the value of thehardware address length field is "6”. For an IP(v4) address, the value of the protocol address lengthfield is “4”.z OP: Operation code. This field specifies the type of ARP message. The value “1” represents anARP request and “2” represents an ARP reply.z Sender hardware address: This field specifies the hardware address of the device sending themessage.z Sender protocol address: This field specifies the protocol address of the device sending themessage.z Target hardware address: This field specifies the hardware address of the device the message isbeing sent to.z Target protocol address: This field specifies the protocol address of the device the message isbeing sent to.ARP Address Resolution ProcessSuppose that Host A and Host B are on the same subnet and Host A sends a packet to Host B, asshown in Figure 1-2. The resolution process is as follows:1) Host A looks into its ARP table to see whether there is an ARP entry for Host B. If yes, Host A usesthe MAC address in the entry to encapsulate the IP packet into a data link layer frame and sendsthe frame to Host B.2) If Host A finds no entry for Host B, Host A buffers the packet and broadcasts an ARP request, inwhich the sender IP address and the sender MAC address are the IP address and the MACaddress of Host A respectively, and the target IP address and the target MAC address are the IPaddress of Host B and an all-zero MAC address respectively. Because the ARP request is abroadcast, all hosts on this subnet can receive the request, but only the requested host (namely,Host B) will respond to the request.3) Host B compares its own IP address with the destination IP address in the ARP request. If they arethe same, Host B saves the source IP address and source MAC address in its ARP table,encapsulates its MAC address into an ARP reply, and unicasts the reply to Host A.
