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Https://discuss.educative.io/tag/designing-a-url-shortening-service-like-tinyurl__system-design-problems__grokking-the-system-design-interview

What does this mean ?
Using base64 encoding, a 6 letters long key would result in 64^6 = ~68.7 billion possible strings.
Using base64 encoding, an 8 letters long key would result in 64^8 = ~281 trillion possible strings.

that means you could have all those combinations using only the charactes in [A-Z][a-z][0-9] and /+.

i.e
AAAAAA
AAAAAB
.
.
.
999999

the 64 are the number of characters you can use in that set of characters, the exponent will be the length of the string you want to generate.