from etharp.h:86
struct etharp_hdr
{
PACK_STRUCT_FIELD(u16_t hwtype);
PACK_STRUCT_FIELD(u16_t proto);
PACK_STRUCT_FLD_8(u8_t hwlen);
PACK_STRUCT_FLD_8(u8_t protolen);
PACK_STRUCT_FIELD(u16_t opcode);
PACK_STRUCT_FLD_S(struct eth_addr shwaddr);
PACK_STRUCT_FLD_S(struct ip4_addr_wordaligned sipaddr);
PACK_STRUCT_FLD_S(struct eth_addr dhwaddr);
PACK_STRUCT_FLD_S(struct ip4_addr_wordaligned dipaddr);
};