Traditionally subnets have been broken down into three size classes based on the 4 octets that make up an IP address. For example: 123.001.002.178.

Class A was any subnet that shared the first octet. Apple for example has a class A subnet. The first octet was 0017. All IP addresses starting with 17 are controlled by Apple. A class A subnet has over 16 Million possible IP addresses. Apple thus effectively controls 1/255th of the Internet.

Class B subnets share the first two octets. Class C subnets share the first three octets, effectively giving a class C subnet 254 possible IP addresses. (Remember that the first and last IP addresses are used as a network number and a broadcast address.)