Question How does one Subnet Class relate to another Subnet Class

[Modify_inc]

How can a subnet mask of /24, which would be a Class C, also have a listing for the # of subnets for Class A and Class B? Looking at one of many subnet charts, it shows /24 with the number of available subnets for both Class A and Class B, as 65,536 and 256 respectively. But if it’s a Class C or /24, how can it also be used as a Class A and Class B? In other words, how can you use the values listed above on a Class C network?

Is it for when you have to borrow bits from a Class A or Class B to get to /24? If so, I still don’t see how that is any different. Isn’t 24 bits just 24 bits, doesn’t matter how you got to 24 bits, right?

 

[hotaru.hino]

It's because you're masking off the number of bits from left to right. A Class A network, such as 10.0.0.0 to 10.255.255.255 masks off the first 8 bits. However, you can sub divide this further if you want something on say a 10.1.X.Y network or a 10.1.2.X network.

You can't arbitrarily pick and choose what to mask off in a subnet mask. So you can't say you want a subnet in say 10.X.1.Y where X and Y can be any arbitrary value.

 

[bill001g]

You are correct in many ways. The concept of classful networks is not really used much anymore is real life but it is still taught in networking classes. They have pretty much chopped up all the subnets into /24 blocks.

The concept of classful networks was used when routers did not have much memory or cpu power. Was used to decrease the time it took to recalculate the routing tables when some change occurred.

Mostly this concept is to make it easier to understand the boundries. You could have a network that runs from say 123.123.0.0-123.123.255.255 which would make it a class B network. But 123.123.128.0- 123.123.127.255 is not a valid network even though it contains exactly the same number of IP.