Editing Universally unique identifier (section)

== Collisions ==

[[Hash collision|Collision]] occurs when the same UUID is generated more than once and assigned to different referents. In the case of standard version-1 and version-2 UUIDs using unique MAC addresses from network cards, collisions are unlikely to occur, with an increased possibility only when an implementation varies from the standards, either inadvertently or intentionally.

In contrast to version-1 and version-2 UUIDs generated using MAC addresses, with version-1 and -2 UUIDs which use randomly generated node ids, hash-based version-3 and version-5 UUIDs, and random version-4 UUIDs, collisions can occur even without implementation problems, albeit with a probability so small that it can normally be ignored. This probability can be computed precisely based on analysis of the [[birthday problem]].<ref>{{cite web |url=http://repositorium.sdum.uminho.pt/bitstream/1822/36065/1/1159.pdf |first1=Paulo |last1=Jesus |first2=Carlos |last2=Baquero |first3=Paulo |last3=Almaeida |title=ID Generation in Mobile Environments |website=Repositorium.Sdum.Uminho.pt }}</ref>

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&nbsp;quintillion, computed as follows:<ref>{{ cite journal | last = Mathis | first = Frank H. |date= June 1991 | title = A Generalized Birthday Problem | journal = SIAM Review | volume = 33 | issue = 2 | pages = 265–270 | issn = 0036-1445 | doi = 10.1137/1033051 | oclc = 37699182 | jstor = 2031144 | citeseerx = 10.1.1.5.5851 }}</ref>

{{block indent|
<math>n \approx \frac{1}{2} + \sqrt{\frac{1}{4} + \ln(2) \times 2^{123}} \approx 2.71 \times 10^{18}.</math>
}}

This number would be equivalent to generating 1&nbsp;billion UUIDs per second for about 86&nbsp;years. A file containing this many UUIDs, at 16 bytes per UUID, would be about 43.4&nbsp;[[exabyte]]s (37.7&nbsp;[[exbibyte|EiB]]).

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

{{block indent|
<math>\sqrt{2 \times 2^{122} \times \ln\frac{1}{1 - p}}.</math>
}}

Thus, the probability to find a duplicate within 103&nbsp;trillion version-4 UUIDs is one in a billion.

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 Technology|Gigabyte]]-branded motherboards.{{Citation needed|date=February 2025}}