Block sizes of classful masks?

johnnyqt25

Hello, this might be a very simple question, but I can't find the answer anywhere. What are the block sizes of these classful masks?

/8
/16
/24

If they are 0, then 256 - 0 = 256. So then the subnets are 0, 256, 512, 1024. This can't be right!?

aftereffector

Here's a chart that might be helpful:

http://www.tutorialspoint.com/ipv4/images/class_a_subnets.jpg

A /8 has roughly 16 million IPs in it. A /16 has roughly 65,000, and of course a /24 has 256, of which 254 are actually usable. By definition, there isn't any such thing as a number of subnets inside a classful netmask - a Class A is a /8, a Class B is a /16, and a Class C is a /24, with no further subdivisions at all. Subnetting goes along with classless networks which is a whole other ball of fun!

johnnyqt25

Thanks for the reply, but I'm still confused. Let's say we need to find to which subnet does 192.168.1.35/27 belong? And to do this, I find block size 256-224 = 32. So then the subnets are 0, 32, 64. And so this IP address belongs to the .32 subnet.

But how would we find the block size if the IP address was 192.168.1.35/24? Would it be 256-0?

Hope this makes sense.

GDaines

johnnyqt25 wrote: »

Thanks for the reply, but I'm still confused. Let's say we need to find to which subnet does 192.168.1.35/27 belong? And to do this, I find block size 256-224 = 32. So then the subnets are 0, 32, 64. And so this IP address belongs to the .32 subnet.

But how would we find the block size if the IP address was 192.168.1.35/24? Would it be 256-0?

Hope this makes sense.

Subnet mask for a /24 class C network (determined by the fact that it's a 192 address) is 255.255.255.0. 256-255 = block size of 1, therefore 1 network and 254 clients (1-254 as 0 is the network address and 255 the broadcast address). You're not working with the last octet until you start subnetting at the 25th bit.

/25 = 128 block size, 2 subnets, 126 hosts.
/26 = 64 block size, 4 subnets, 62 hosts
/27 = 32 block size, 8 subnets, 30 hosts
/28 = 16 block size, 16 subnets, 14 hosts.

james43026

I think you need a better understanding of subnetting and how it works. I think this video does a decent job explaining some concepts, but it doens't cover everything. Personally the magic number method is my favorite and fastest method for me.