ARP message format
ARP messages include ARP requests and ARP replies. Figure 1 shows the format of the ARP request/reply. Numbers in the figure refer to field lengths.
Figure 1: ARP message format
ARP message fields:
Hardware type—The hardware address type. Value 1 represents Ethernet.
Protocol type—The type of the protocol address to be mapped. The hexadecimal value 0x0800 represents IP.
Hardware address length and protocol address length—Length, in bytes, of a hardware address and a protocol address. For an Ethernet address, the value of the hardware address length field is 6. For an IPv4 address, the value of the protocol address length field is 4.
OP—Operation code, which describes type of the ARP message. Value 1 represents an ARP request, and value 2 represents an ARP reply.
Sender hardware address—Hardware address of the device sending the message.
Sender protocol address—Protocol address of the device sending the message.
Target hardware address—Hardware address of the device to which the message is being sent.
Target protocol address—Protocol address of the device to which the message is being sent.