sub ip_cmp ($$) {
  my @a = split /\./, $_[0];
  my @b = split /\./, $_[1];

  return $a[0] <=> $b[0]
      || $a[1] <=> $b[1]
      || $a[2] <=> $b[2]
      || $a[3] <=> $b[3];
}