Nano ID is a compact, URL-safe random string identifier: by default 21 characters drawn uniformly from a 64-symbol alphabet 'A-Za-z0-9_-', giving 126 bits of entropy (collision odds comparable to UUIDv4). It has no internal structure, timestamp, or fixed binary width — it is purely random — so it is not time-sortable, but it is shorter than a UUID and safe in URLs and filenames.
No fixed binary layout: every character is independent uniform randomness from a 64-symbol alphabet, so there is no bit_layout, timestamp, or version field.
Default size 21 chars * log2(64) = 126 bits of entropy, slightly more than UUIDv4's 122; the alphabet and length are configurable.
URL- and filename-safe ('_' and '-' instead of base64's '+'/'/'); not sortable and not deterministic.
Example V1StGXR8_Z5jdHi6B-myT is the README sample.