"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, mostly in Microsoft systems. [1][2]
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 generally considered close enough to zero to be negligible.[3][4]
Thus, anyone can create a UUID and use it to identify something with near certainty that the identifier does not duplicatAe 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." - Wikipedia
Versions
The OSF DCE variant defines eight "versions" in the standard, and each version may be more appropriate than the others in specific use cases.
- Versions 1 and 6 (date-time and MAC address)
- Version 2 (date-time and MAC address, DCE security version)
- Versions 3 and 5 (namespace name-based)
- Version 4 (random)
- Version 7 (timestamp, counter and random)
- Version 8 (custom)
Collisions
For example, the number of random version-4 UUIDs which need to be generated in order to have a 50% probability of at least one collision is 2.71 quintillion, computed as follows:
This number is equivalent to generating 1 billion UUIDs per second for about 86 years. A file containing this many UUIDs, at 16 bytes per UUID, would be about 45 exabytes.
The smallest number of version-4 UUIDs which must be generated for the probability of finding a collision to be p is approximated by the formula
Thus, the probability to find a duplicate within 103 trillion version-4 UUIDs is one in a billion.
Issues
Collisions have occurred when manufacturers assign a default UUID to a product, such as a motherboard, and then fail to over-write the default UUID later in the manufacturing process. For example, UUID 03000200-0400-0500-0006-000700080009 occurs on many different units of Gigabyte-branded motherboards.
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