foreach (sort ipsort keys %ip) {
  # each sorted key will be in $_
}

sub ipsort {
  my @a = split /\./, $a;
  my @b = split /\./, $b;

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