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];
}


##</code><code>##

my @sorted_keys = sort { ip_cmp($hash{$a}->ip_address, $hash{$b}->ip_address) } keys %hash;

##</code><code>##

use Sort::Key::IPv4 qw(ipv4keysort);
my @sorted_keys = ipv4keysort { $hash{$_}->ip_address } keys %hash;