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I stopped memorising subnet tables
The table is a crutch that works until the question is phrased differently.
I had the /24-through-/30 table memorised cold. Block sizes, host counts, the lot. Then an interviewer asked me to split a /22 into eight pieces and I sat there, because my table did not go that way.
The table is a cache of answers to one shape of question. It is fast, and it fails the moment the question is shaped differently — which, outside an exam, it always is.
What replaced it was one sentence: the mask says which bits are the network. Everything else is counting. How many hosts? Two to the power of the zeros, minus two. Where does the next subnet start? At the last network bit plus one. How do I split a /22 eight ways? Eight is 2³, so borrow three bits: eight /25s.
It is slower than the table for the first week and faster for ever after, because you stop having to recognise the question before you can answer it.
The other thing that helped was building it rather than reading it. Put two hosts on a wire in the simulator with masks that disagree — /24 on one, /25 on the other — and watch one of them ARP for a destination it thinks is local while the other routes. You remember a thing you broke.
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