Editing Universally unique identifier (section)

==== Versions 1 and 6 (date-time and MAC address) ====

Version 1 concatenates the 48-bit [[MAC address]] of the "node" (that is, the computer generating the UUID), with a 60-bit timestamp, being the number of 100-[[nanosecond]] intervals since midnight 15 October 1582 [[Coordinated Universal Time]] (UTC), the date on which the [[Gregorian calendar]] was first adopted by the bulk of Europe. <nowiki>RFC 4122</nowiki> states that the time value rolls over around 3400 AD,<ref name="RFC 4122" />{{rp|3|q="it is possible for values to rollover (around A.D. 3400, depending on the specific algorithm used)"}} depending on the algorithm used, which implies that the 60-bit timestamp is a signed quantity. However some software, such as the libuuid library, treats the timestamp as unsigned, putting the rollover time in 5623 AD.<ref name="e2fsprogs" /> The rollover time as defined by ITU-T Rec. X.667 is 3603 AD.<ref name="X667">{{cite web |date=October 2012 |title=Recommendation ITU-T X.667 |url=https://www.itu.int/rec/dologin_pub.asp?lang=e&id=T-REC-X.667-201210-I!!PDF-E&type=items |access-date=19 December 2020 |website=[[ITU-T|www.itu.int]]}}</ref>{{rp|v|q="a UUID is&nbsp;[...] guaranteed to be different from all other UUIDs generated before 3603 A.D."}}

A 13-bit or 14-bit "uniquifying" clock sequence extends the timestamp in order to handle cases where the processor clock does not advance fast enough, or where there are multiple processors and UUID generators per node. When UUIDs are generated faster than the system clock could advance, the lower bits of the timestamp fields can be generated by incrementing it every time a UUID is being generated, to simulate a high-resolution timestamp.  With each version 1 UUID corresponding to a single point in space (the node) and time (intervals and clock sequence), the chance of two properly generated version-1 UUIDs being unintentionally the same is practically nil.  Since the time and clock sequence total 74 bits, 2<sup>74</sup> (1.8{{e|22}}, or 18 sextillion) version-1 UUIDs can be generated per node ID, at a maximal average rate of 163 billion per second per node ID.<ref name="RFC 4122" />

In contrast to other UUID versions, version-1 and -2 UUIDs based on MAC addresses from [[network card]]s rely for their uniqueness in part on an identifier issued by a central registration authority, namely the [[Organizationally unique identifier|Organizationally Unique Identifier]] (OUI) part of the MAC address, which is issued by the [[IEEE]] to manufacturers of networking equipment.<ref name="IEEE-RA">
{{cite web |title=Registration Authority |url=http://standards.ieee.org/develop/regauth/ |archive-url=https://web.archive.org/web/20110404225440/http://standards.ieee.org/develop/regauth/ |url-status=dead |archive-date=4 April 2011 |website=[[IEEE Standards Association]]}}</ref> The uniqueness of version-1 and version-2 UUIDs based on network-card MAC addresses also depends on network-card manufacturers properly assigning unique MAC addresses to their cards, which like other manufacturing processes is subject to error. Virtual machines receive a MAC address in a range that is configurable in the hypervisor.<ref>{{cite web |title=MAC addresses for virtual machines |url=https://superuser.com/questions/932651/mac-addresses-for-virtual-machines}}</ref> Additionally some operating systems permit the end user to customise the MAC address, notably [[OpenWrt|OpenWRT]].<ref>{{Cite web |date=15 September 2021 |title=MAC Address Setup |url=https://openwrt.org/docs/guide-developer/mac.address |website=OpenWRT}}</ref>

Usage of the node's network card MAC address for the node ID means that a version-1 UUID can be tracked back to the computer that created it. Documents can sometimes be traced to the computers where they were created or edited through UUIDs embedded into them by [[word processing]] software. This [[privacy]] hole was used when locating the creator of the [[Melissa (computer virus)|Melissa virus]].<ref>{{cite web |last=Reiter |first=Luke |date=1999-04-02 |title=Tracking Melissa's Alter Egos |url=https://www.zdnet.com/article/tracking-melissas-alter-egos/ |access-date=2017-01-16 |publisher=[[ZDNet]]}}</ref>

RFC 9562{{r|RFC 9562}} does allow the MAC address in a version-1 (or 2) UUID to be replaced by a random 48-bit node ID, either because the node does not have a MAC address, or because it is not desirable to expose it. In that case, the RFC requires that the least significant bit of the first octet of the node ID should be set to 1.<ref name="RFC 4122" /> This corresponds to the [[multicast]] bit in MAC addresses, and setting it serves to differentiate UUIDs where the node ID is randomly generated from UUIDs based on MAC addresses from network cards, which typically have [[unicast]] MAC addresses.<ref name="RFC 4122" />

Version 6 is the same as version 1 except all timestamp bits are ordered from most significant to least significant. This allows systems to sort UUIDs in order of creation simply by sorting them lexically, whereas this is not possible with version 1.