UUID v4 Explained: How Random UUIDs Are Generated and Used
Understand UUID v4 structure, how 122 bits of randomness are generated, collision probability, and when to use v4 over other UUID versions.
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Tags: developer-tools, uuid, explainer
UUID v4 Explained: How Random UUIDs Are Generated and Used UUID v4 is the most widely used unique identifier format in software today. It is simple, stateless, and requires no coordination across generators. But "random UUID" understates what is actually happening — understanding the structure, the randomness source, and the failure modes makes you a more deliberate engineer. This post covers all of it. |---|---| | | 32 | Random in v4 | | | 16 | Random in v4 | | | 16 | Top 4 bits = (version 4), rest random | | | 8 | Top 2 bits = (variant), rest random | | | 8 | Random | | | 48 | Random in v4 | The version bits ( = decimal 4) appear at bit positions 48–51. The variant bits () appear at bit positions 64–65. Everything else — 122 bits — is random. This means when you look at a v4 UUID:…
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