They are meant for generating UUIDs from names that are drawn from, and unique within, some name space. Versions 3 and 5 are generated by hashing a name or namespace identifier and using the resultant hash, MD5 or SHA-1 respectively, as the source of uniqueness instead of the time-based sources like in versions 1 and 2. Version 1 is the most commonly used of the UUID versions. Most implementations, because of this and other reasons, omit version 2. Versions 1 and 2 are effectively the same, except in the latter version the least significant bits of the of the clock sequence are replaced with an identifier specific to the system. Version 1 and 2 use time-based sources (a 60 bit timestamp sourced from the system clock) for its randomness. There are 5 different versions of UUIDs, excluding the Nil UUID version, which is a special case UUID where all its bytes are set to 0, and most contain some variants that allow for special cases specific to vendors like Microsoft. Apple, Microsoft, Samsung, and others use UUIDs, either defined by the IETF spec or a proprietary variant, to identify and track hardware both internally and sold to consumers. or colliding, UUID has been generated elsewhere. One of the benefits to UUIDs is that they don’t need to be issued by a central authority, but can be generated independently and then used across a given system without suspicion that a duplicate. Their uniqueness and low probability in being repeated makes them useful for being associative keys in databases and identifiers for physical hardware within an organization.

UUIDs are generally used for identifying information that needs to be unique within a system or network thereof. This specification was originally created by Microsoft and standardized by both the IETF and ITU. Universally Unique Identifiers, or UUIDS, are 128 bit numbers, composed of 16 octets and represented as 32 base-16 characters, that can be used to identify information across a computer system.

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