This format does not allow for more than one resolution to be done in the same packet. This is for simplicity. If things were multiplexed the packet format would be considerably harder to digest, and much of the information could be gratuitous. Think of a bridge that talks four protocols telling a workstation all four protocol addresses, three of which the workstation will probably never use.
This format allows the packet buffer to be reused if a reply is generated; a reply has the same length as a request, and several of the fields are the same.
The value of the hardware field (ar$hrd) is taken from a list for this purpose. Currently the only defined value is for the 10Mbit Ethernet (ares_hrd$Ethernet = 1). There has been talk of using this protocol for Packet Radio Networks as well, and this will require another value as will other future hardware mediums that wish to use this protocol.
For the 10Mbit Ethernet, the value in the protocol field (ar$pro) is taken from the set ether_type$. This is a natural reuse of the assigned protocol types. Combining this with the opcode (ar$op) would effectively halve the number of protocols that can be resolved under this protocol and would make a monitor/debugger more complex (see Network Monitoring and Debugging below). It is hoped that we will never see 32768 protocols, but Murphy made some laws which don't allow us to make this assumption.
In theory, the length fields (ar$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$pro). It is included for optional consistency checking, and for network monitoring and debugging (see below).
The opcode is to determine if this is a request (which may cause a reply) or a reply to a previous request. 16 bits for this is overkill, but a flag (field) is needed.
The sender hardware address and sender protocol address are absolutely necessary. It is these fields that get put in a translation table.
The target protocol address is necessary in the request form of the packet so that a machine can determine whether or not to enter the sender information in a table or to send a reply. It is not necessarily needed in the reply form if one assumes a reply is only provoked by a request. It is included for completeness, network monitoring, and to simplify the suggested processing algorithm described above (which does not look at the opcode until AFTER putting the sender information in a table).
The target hardware address is included for completeness and network monitoring. It has no meaning in the request form, since it is this number that the machine is requesting. Its meaning in the reply form is the address of the machine making the request. In some implementations (which do not get to look at the 14.byte ethernet header, for example) this may save some register shuffling or stack space by sending this field to the hardware driver as the hardware destination address of the packet.
There are no padding bytes between addresses. The packet data should be viewed as a byte stream in which only 3 byte pairs are defined to be words (ar$hrd, ar$pro and ar$op) which are sent most significant byte first (Ethernet/PDP-10 byte style).