To understand CIDR and VLSM, you'll want to also understand their "opposite" terms: Classful assignments and FLSM:

Classful and CIDR — these have to do with the size of networks as they are assigned from IANA.
FLSM and VLSM — these have to do with how you allocate your IP space within your networks.
To define each term individually:

Classful addressing is IANA/RIRs assigning IP space from Class A, B, or C blocks (legacy).
Classless or CIDR is IANA/RIRs assigning IP space in any size block, as required (modern standard).

FLSM, or Fixed Length Subnet Masking, mandates that every IP subnet within your deployment be the same size (legacy).

VLSM, or Variable Length Subnet Masking, allows any IP subnet within your deployment to be any size (modern standard).
More details here:

https://www.practicalnetworking.net/stand-alone/classful-cidr-flsm-vlsm/

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