Subnetting in your head- The soundian easy-peasy lemon squeezy way

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You can pretty much figure out anything you want by knowing one of two things:
1) Block Size
2) Number of host bits

One of these is pretty much handed to you on a plate (if you're given subnet mask a simple subtraction gives you the block size, if you're given it in CIDR format then the number of host bits is a simple subtraction away also)

Subnet masks
Block size=256-<number in significant octet>
e.g. 255.255.240.0 would give a block size of 256-240=16
To calculate the number of host bits in the significant octet simply count up from 2, multiplying by 2 each time, until you reach the block size.
In the above example there are 2,4,8,16=4 host bits in the significant octet, and 8 in the 4th octet=12 host bits.

CIDR
Number of host bits=32-CIDR number
the above example would be /20. 32-20=12. That means there are 8 bits in the 4th octet so it leaves 4 host bits in the significant octet.
That means the block size is 2,4,8,16 (stop at the 4th number)

Once you have the block size you can work out all the valid hosts, network addresses and broadcast addresses that you could possibly want.
One tip to remember here is that every block size (2,4,8,16,32,64,128 ) is a multiple of the ones before it. In other words 128 will be a network address in every single block size so you can just jump straight there if you are working with host numbers >128

Once you know the number of host bits it's easy to find out the number of subnets and number of hosts.
number of available hosts per subnet=(number of host bits)^2-2
number of subnets is simply subtracting the number of host bits from the number of available bits depending on class of address, i.e. Class A will have 24 available for subnetting and hosts, Class B=16, class C=8 . Number of subnets=(number of network bits)^2

 

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I too use the subtraction method while dealing with subnet masks.

Repped u for your effort!