550 CHAPTER 30: ARP CONFIGURATION■ Hardware address length: Length of a hardware address in bytes. For anEthernet address, the value of the hardware address length field is “6”.■ Protocol address length: Length of a protocol address, in bytes. For an IP(v4)address, the value of the protocol address length field is "4".■ OP: Operation code. This field specifies the type of ARP message. For example,the value "1" represents an ARP request, "2" represents an ARP reply, and "4"represents a reverse ARP request.■ Sender hardware address: This field specifies the hardware address of thedevice sending the message.■ Sender protocol address: This field specifies the protocol address of the devicesending the message.■ Target hardware address: This field specifies the hardware address of the devicethe message is being sent to.■ Target protocol address: This field specifies the protocol address of the devicethe message is being sent to.ARP Process Suppose that Host A and Host B are on the same subnet and that Host A sends amessage to Host B, as shown in Figure 156. The resolution process is as follows:1 Host A looks in its ARP mapping table to see whether there is an ARP entry forHost B. If Host A finds it, Host A uses the MAC address in the entry to encapsulatethe IP packet into a data link layer frame and sends the frame to Host B.2 If Host A finds no entry for Host B, Host A buffers the packet and broadcasts anARP request, in which the source IP address and source MAC address arerespectively the IP address and MAC address of Host A and the destination IPaddress and MAC address are respectively the IP address of Host B and an all-zeroMAC address. Because the ARP request is sent in broadcast mode, all hosts on thissubnet can receive the request, but only the requested host (namely, Host B) willprocess the request.3 Host B compares its own IP address with the destination IP address in the ARPrequest. If they are the same, Host B saves the source IP address and source MACaddress into its ARP mapping table, encapsulates its MAC address into an ARPreply, and unicasts the reply to Host A.4 After receiving the ARP reply, Host A adds the MAC address of Host B into its ARPmapping table for subsequent packet forwarding. Meanwhile, Host Aencapsulates the IP packet and sends it out.
