use Socket; 

$net = "192.168.0.0"; 
$mask = "28"; # /28 is 255.255.255.240
$ip = "192.168.0.5"; 

print "$ip belongs to $net/$mask\n" if ((net2long($ip) & ~((1 << (32 - $mask)) - 1 )) == net2long($net)); 


sub net2long { return unpack("N", inet_aton(shift)); }