ARP Message Format

The Hardware Type (HRD) field specifies the Layer 1 technology powering the network and thus also identifies the type of addressing employed.

HRD Value	Hardware Type
1	Ethernet (10 mb)
6	IEEE 802 Network
7	ARCNET
15	Frame Relay
16	Asynchronous Transfer Mode (ATM)
17	HDLC
18	Fibre Channel
19	Asynchronous Trasfer Mode (ATM)
20	Serial Line

The Protocol Type (PRO) field specifies the type of Layer 3 addresses used in the ARP message. The value for this field match the EtherType codes in an Ethernet frame.

The Hardware Address Length (HLN) and Protocol Address Length (PLN) specify the lengths, respectively, of the Layer 2 and Layer 3 address used in the ARP message. ARP supports addresses of different sizes in order to be able to operate with technologies which differ from IP and IEEE 802 MAC addresses.

The Opcode (OP) indicates the type of message being transmitted.

Opcode	Message Type
1	ARP Request
2	ARP Reply
3	RARP Request
4	RARP Reply
5	DRARP Request
6	DRARP Reply
7	DRARP Error
8	InARP Request
9	InArp Reply

The Sender Hardware Address (SHA) is the hardware address of the host issuing the ARP request.

The Sender Protocol Address (SPA) is the Layer 3 address of the device issuing the ARP request.

The Target Hardware Address (THA) is where the hardware address of the sought device goes.

The Target Protocol Address (TPA) is the Layer 3 address of the sought device.