I think that the use is so that we could know the offset where the actual addresses begin in the ARP format, but if so, then the protocol type for example can tell us this information (length of the address according to the type).

So why both are needed? The type and length fields?

  • Did any answer help you? if so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you could provide and accept your own answer.
    – Ron Maupin
    Aug 11 '17 at 4:23

Did you read RFC826? It says:

In theory, the length fields (ar$hln and ar$pln) are redundant, since the length of a protocol address should be determined by the hardware type (found in ar$hrd) and the protocol type (found in ar$pro). It is included for optional consistency checking, and for network monitoring and debugging (see below).

  • I've actually tried searching the rfc for it. Thanks!
    – Alex_t
    Feb 16 '15 at 6:58

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