ASCII
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... 128 bit integer, encode that in base 85 notation, then encode that
using 85 ASCII characters.
...
... IPv6 addresses, so 20 characters would be needed
even with base 94 encoding. As there are just 94 ASCII characters
(excluding control characters, space, and del) base 94 is the largest
...
... Thus, any value between 85 and 94 inclusive could reasonably be
chosen. Selecting 85 allows the use of the smallest possible subset
of the ASCII characters, enabling more characters to be retained for
other uses, eg, to delimit the address.
...
...
This set has been chosen with considerable care. From the 94
printable ASCII characters, the following nine were omitted:
...