ARP operation codes (the oper field).
Includes the original ARP request/reply pair (RFC 826) and the RARP variants (RFC 903) for completeness.
REQUEST: number
ARP request — "who has TPA, tell SPA".
REPLY: number
ARP reply — sender's MAC for the requested protocol address.
RARP_REQUEST: number
RARP request — "who am I", given my hardware address (RFC 903).
RARP_REPLY: number
RARP reply — protocol address for the requested hardware address.