Class A subnetting is not performed any differently than subnetting with Classes B and C, but there are 24 bits to play with instead of the 16 in a Class B address and the 8 in a Class C address.
Let’s start by listing all the Class A masks:
That’s it. You must leave at least 2 bits for defining hosts. And I hope you can see the pattern by now. Remember, we’re going to do this the same way as a Class B or C subnet. It’s just that, again, we simply have more host bits, and we use the same subnet numbers we used with Class B and Class C, but we start using these numbers in the second octet.