ARP message format
ARP uses two types of messages: ARP request and ARP reply. Figure 1 shows the format of ARP request/reply messages. Numbers in the figure refer to field lengths.
Figure 1: ARP message format
Hardware type—Hardware address type. The value 1 represents Ethernet.
Protocol type—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 the type of ARP message. The value 1 represents an ARP request, and the 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.