/* first calculate the prefix length */
    if(slash) {
        pfx_len = (int)strtoul(slash + 1, NULL, 10);
        if(!pfx_len && memcmp(in_zero, in_net, sizeof(struct in6_addr)))
            goto end;

        /* if it's an ipv4 address (ie, the first 80 bits are zero), then
         * we need to add 96 to the prefix length.
         */
        if(!memcmp(in_zero, in_net, 10))
            pfx_len += 96;

        if(pfx_len > 128 || pfx_len < 0)
            goto end;
    }
    else {
        pfx_len = 128;
    }
    /* now we need to fill the mask and then bitwise-and it properly */
    for(i = 0; i < 3; i++) {
        if(pfx_len >= 32) {
            mask->s6_addr32[i] = 0xffffffff;
            pfx_len -= 32;
        }
        else if(!pfx_len) {
            mask->s6_addr32[i] = 0;
        }
        else {
            m = 0xffffffff;
            m <<= 32 - pfx_len;
            mask->s6_addr32[i] = htonl(m);
        }
    }