From what i know,MAC addresses are made of 48 bits,while IPv4 addresses are 32 bits
Why are there these header fields? Are there MAC addresses with more or less 48 bits?
ARP has been implemented with many combinations of network and data link layer technologies, such as IPv4, Chaosnet, DECnet and Xerox PARC Universal Packet (PUP) using IEEE 802 standards, FDDI, X.25, Frame Relay and Asynchronous Transfer Mode (ATM). IPv4 over IEEE 802.3 and IEEE 802.11 is the most common usage.
ARP is used for different data link protocols, which may not use MAC addresses.