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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?

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    Not every L2,protocol uses MAC addresses – Ron Trunk Apr 30 at 10:26
  • It is the IEEE protocols that use MAC addresses, and some use 48-bit MAC addresses and some use 64-bit MAC addresses. – Ron Maupin Apr 30 at 13:27
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From Wikipedia:

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.

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    Also, there are both 48-bit and 64-bit MAC addresses. – Ron Maupin Apr 30 at 13:25

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