ipf_nat6_ip6subtract
extern u_32_t ipf_nat6_ip6subtract(i6addr_t *, i6addr_t *);
sumd = ipf_nat6_ip6subtract(&a2, &a3);
sumd = ipf_nat6_ip6subtract(&a2, &a3);
sumd += ipf_nat6_ip6subtract(&a4, &a1);
sumd += ipf_nat6_ip6subtract(&a4, &a1);