Substract Mac addresses

submac.c

#include <stdint.h>
#include <stdio.h>
#define ETH_ALEN 6

int main(void)
{
    uint8_t  mac1[ETH_ALEN] = {0x00, 0x00, 0x00, 0x00, 0x00, 0xFF};
    uint8_t  mac2[ETH_ALEN] = {0x00, 0x00, 0x00, 0x00, 0x01, 0x00};

    uint64_t a = (uint64_t)mac1[5] +
        ((uint64_t)mac1[4] << 8) +
        ((uint64_t)mac1[3] << 16) +
        ((uint64_t)mac1[2] << 24) +
        ((uint64_t)mac1[1] << 32) +
        ((uint64_t)mac1[0] << 40);

    uint64_t b = (uint64_t)mac2[5] +
        ((uint64_t)mac2[4] << 8) +
        ((uint64_t)mac2[3] << 16) +
        ((uint64_t)mac2[2] << 24) +
        ((uint64_t)mac2[1] << 32) +
        ((uint64_t)mac2[0] << 40);

    printf("%lu\n", b - a);
}