Address “Classes” vs. Subnet Mask

Always remember the following three rules:

> The first octet on an address dictates the class of that address.

> The subnet mask dictates what portion of an address identifies the network, and what portion identifies the host.

> Each class has a default subnet mask.

Thus, the address 10.1.1.1 is a Class A address, and its default subnet mask is 255.0.0.0 (or in CIDR, /8).

However, it is possible to use subnet masks other  than the default, such as applying a Class B mask to a Class A address : 10.1.1.1 /16

However, this does not change the class of the above address. It remains a Class A address, which has been subnetted using a Class B mask. Only thing that determines the class of an IP address is the first octet of that address.

Likewise, the subnet mask is the only thing that determines what portion of an address is the network, and which portion is

the host.