Classful Subnetting Tricks

Here are some Classful Subnetting tricks that I learned while preparting for my CCNA Cert – I’m sure that brushing up on these can’t hurt anything.

Any time that you break these down you are subnetting  – so an example question would be if you have a subnet mask of 255.255.255.128 and you are subnetting 172.16.0.0

(A) How many subnets would you have? (B) What are the valid subnets? (C) How many hosts will be allowed?

The first part is easy – we know that 172.16.0.0/16 – has 16 whole bits free for subnetting. We need to know which bits were are going to use though.

How do we get this? – We derive this by looking at the amount of subnet bits we can use based on our mask so we count the extra bits by putting our subnet mask of 255.255.255.128 and convert it to a CIDR notation 255(8) . + 255(8). + 255(8). + 128(1) = 25

Now we subtract our subnet by the classful subnet and get:
25-16 = 9

So we use the following formula: 2^9 = 512

A. 512 valid subnets

Now the second part is a little harder. We use our magic formula of 256 – our decimal format. So for 255.255.255.128 we can subtract 256 by 255 for our third octect or we can just memorize that all bits and numbers can be used. What about the fourth octet? Same rules apply.  So 256-128 = 128 so we can use 172.16.0-255.0,128 – as our subnets.

B. 172.16.0 – 255.0, 128

Now the third part is easy again we take how many subnet bits we have, 25 and use this number to subtract against 32, as you can see 32 – 25 = 7

Again using our powers of 2 strategy 7^2 = 128

We are still not finished though, we need to account for the broadcast address and the network address. These are two addresses so we do this:

7^2 = 128 – 2 = 126

C) 126 Valid Hosts