I find the human brain works well with /32’s, /24’s, /16’s and /8 network prefix sizes. Anyone with a basic understanding of subnetting can understand and feel why. They’re probably the first prefix sizes that people begin to build a basic mental model of, and therefore it can be useful when learning to subnet to work with these as if they are basic units.
In the metric system, we use prefixes such as deca, hecto, kilo, mega, giga, tera, peta, each a nice unit to work with for relative sizes. With computer storage, it’s much easier to say 639.54MB than 639540000 bytes. A similar intuition with relative sizes arises around each byte in a 32 bit mask for lengths of /8, /16, /24 and hosts /32.
You can quickly determine the quantity of each of these units in a mask with a fast mental trick.
If you have 255.255.255.252, you can take the last octet and subtract it from 256 to get the number of /32 addresses that fit.
256 - 252 = 4
There are 4 /32's that can fit in 255.255.255.252
You can do this, regardless of what number the last octet is to get the number of /32’s that fit in the mask. The same is true for blocks of /24’s.
Example: 255.255.224.0
256 - 224 = 32
There are 32 /24's that can fit in 255.255.224.0
And of course the same applies to units of /16’s and /8’s
Example: 255.192.0.0
256 - 192 = 64
There are 64 /16's that can fit in 255.192.0.0
While I have pretty well memorized the whole table, I still from time to time return to this trick to check my memory, especially if it’s been a few days since I’ve seen a certain prefix size.