Hardware Type - The physical medium of transmission. 1 for Ethernet, 6 for 802.11 (wifi).

Protocol Type - Uses same constants as EtherType. Identifies the Layer 3 address that needs to be mapped to an Ethernet address.

Hardware Address Length - Size of Layer 2 address defined in Hardware type.

Protocol Address Length - Size of Layer 3 address defined in Protocol Type.

Op Code - 1 for Request, 2 for Reply. Other codes exist for protocols that make use of an Arp Header

Sender Hardware Address, Sender Protocol Address - The sender's addresses.

Target Hardware Address, Target Protocol Address - The target's addresses, The hardware address is filled in upon receipt of the ARP Request.