Subnet


 * Class A addresses have their first octet in the range 1 to 126 (binary address begins with 0).
 * Class B addresses have their first octet in the range 128 to 191 (binary address begins with 10).
 * Class C addresses have their first octet in the range 192 to 223 (binary address begins with 110).

http://www.subnet-calculator.com/subnet.php