Print Autonomous System Number in ASDOT notation

The ASN (Autonomous System Number, and you can think it as port number) will be outputted in ASPLAIN format by default. However, if -b option is specified (code is here):

    ......
    case 'b':
        ++ndo->ndo_bflag;
        break;
    ......

The ASN will be displayed in ASDOT notation when the number is bigger than 65535 (code is here):

static char *
as_printf(netdissect_options *ndo,
          char *str, int size, u_int asnum)
{
    if (!ndo->ndo_bflag || asnum <= 0xFFFF) {
        snprintf(str, size, "%u", asnum);
    } else {
        snprintf(str, size, "%u.%u", asnum >> 16, asnum & 0xFFFF);
    }
    return str;
}