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What is UUID?
A universally unique identifier (UUID) is a 128-bit label used for information in computer systems. The term globally unique identifier (GUID) is also used, often in software created by Microsoft. When generated according to the standard methods, UUIDs are, for practical purposes, unique. Their uniqueness does not depend on a central registration authority or coordination between the parties generating them, unlike most other numbering schemes. While the probability that a UUID will be duplicated is not zero, it is close enough to zero to be negligible. Thus, anyone can create a UUID and use it to identify something with near certainty that the identifier does not duplicate one that has already been, or will be, created to identify something else. Information labeled with UUIDs by independent parties can therefore be later combined into a single database or transmitted on the same channel, with a negligible probability of duplication. Adoption of UUIDs is widespread, with many computing platforms providing support for generating them and for parsing their textual representation.Versions
For both variants 1 and 2, five “versions” are defined in the standards, and each version may be more appropriate than the others in specific use cases. Version is indicated by the M in the string representation.
Version-1 UUIDs are generated from a time and a node ID (usually the MAC address); version-2 UUIDs are generated from an identifier (usually a group or user ID), time, and a node ID; versions 3 and 5 produce deterministic UUIDs generated by hashing a namespace identifier and name; and version-4 UUIDs are generated using a random or pseudo-random number.
Nil UUID
The “nil” UUID, a special case, is the UUID 00000000-0000-0000-0000-000000000000; that is, all bits set to zero.
Credit
The documentation uses material from the Wikipedia article “Universally unique identifier”, which are released under the Creative Commons Attribution-Share-Alike License 3.0.