Ghettocode

use Socket;
 
sub main {
      my @ips = qw( 192.168.1.1,
                    192.168.1.2,
                    10.15.15.1,
                    10.15.15.2,
                    172.129.4.1 );
      test(@ips);
}
 
sub test {
        my @ips = @_;
        my $netmask = ip2int("255.255.255.0");
        my %store;
        foreach my $ip (@ips) {
            my $int_ip = ip2int($ip);
            my $network_addr = network($int_ip, $netmask);
            $store{$network_addr}{$int_ip} = $ip;
        }
 
 
        while (my ($network_addr, $href) = each %store) {
            print int2ip($network_addr), "\n";
            # sort IPs by value to display nice
            foreach my $int_ip (sort {$a <=> $b} keys %$href) {
                print " "x4, $href->{$int_ip},"\n";
            }
        }
 
}
sub network {
    # logical AND of IP and netmask
    return $_[0] & $_[1]
}
 
sub ip2int {
    # this will only work for IPv4
    return unpack('N', Socket::inet_aton($_[0]));
}
sub int2ip {
    return Socket::inet_ntoa(pack("N", $_[0]));
}
 
main();