Packet format

To communicate mappings from pairs to 48.bit Ethernet addresses, a packet format that embodies the Address Resolution protocol is needed. The format of the packet follows.

    Ethernet transmission layer (not necessarily accessible to
	 the user):
	48.bit: Ethernet address of destination
	48.bit: Ethernet address of sender
	16.bit: Protocol type = ether_type$ADDRESS_RESOLUTION
    Ethernet packet data:
	16.bit: (ar$hrd) Hardware address space (e.g., Ethernet,
			 Packet Radio Net.)
	16.bit: (ar$pro) Protocol address space.  For Ethernet
			 hardware, this is from the set of type
			 fields ether_typ$.
	 8.bit: (ar$hln) byte length of each hardware address
	 8.bit: (ar$pln) byte length of each protocol address
	16.bit: (ar$op)  opcode (ares_op$REQUEST | ares_op$REPLY)
	nbytes: (ar$sha) Hardware address of sender of this
			 packet, n from the ar$hln field.
	mbytes: (ar$spa) Protocol address of sender of this
			 packet, m from the ar$pln field.
	nbytes: (ar$tha) Hardware address of target of this
			 packet (if known).
	mbytes: (ar$tpa) Protocol address of target.