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<pre>
!!! Overview
[{$pagename}] ()user identifier, often abbreviated [UID]) is used in [Unix Linux] like and [POSIX] [Operating Systems] identify a user by a value called a user identifier. [{$pagename}] or [UID], along with the group identifier ([GID] or [GidNumber]) and other access control criteria, is used to determine which system [resources] a [entity] can access. The [passwd] file maps textual user names to [{$pagename}]s. [UIDs] are stored in the inodes of the Unix [File System], running processes, tar archives, and the now-obsolete [Network Information Service] ([NIS]). In [POSIX]-compliant environments, the command-line command id gives the current user's [{$pagename}], as well as more information such as the user name, primary user group and group identifier ([GID]).[{$pagename}] the [UID] or [UserId]) which is a [number] assigned by [Linux] and [UNIX] to each user on the system. This number is used to identify the user to the system and to determine which system [resources] the user can access. [{$pagename}] is the [AttributeType] used in [LDAP] for the storing the number

[POSIX] requires the UID to be an [integer] type. Most Unix-like [Operating System] represent the UID as an unsigned [integer]. The size of [{$pagename}] values varies amongst different systems; some [UnixLinux] [Operating Systems] used 15-bit values, allowing values up to 32,767, while others such as [Linux] (before version 2.4) supported 16-bit [{$pagename}]s, making 65,536 [{$pagename}]s possible. The majority of modern Unix-like systems (e.g., Solaris-2.0 in 1990, Linux 2.4 in 2001) have switched to 32-bit [{$pagename}]s, allowing 4,294,967,296 (232) [{$pagename}]s.

!! [LDAP] [Attribute] Definition
The [{$pagename}] [AttributeTypes] is defined as:
* [OID] of [1.3.6.1.1.1.1.0] 
* [NAME|Attribute-Name]: [{$pagename}]
* [DESC]: 'An integer uniquely identifying a user in an administrative domain'
* [EQUALITY]: [integerMatch]
* [ORDERING]: [integerOrderingMatch]
* [SYNTAX]: [1.3.6.1.4.1.1466.115.121.1.27]
* [SINGLE-VALUE]
* [USAGE]: [UserApplications] 
* [Extended Flags]: 
** [X-ORIGIN]: [Draft-howard-rfc2307bis]
* Used as [MUST] in:
** [posixAccount]
** [sambaUnixIdPool]
* Used [MAY] in:
** [nisKeyObject]
** [sambaIdmapEntry]

!! Special values
* 0: The superuser normally has a [{$pagename}] of zero (0).[10]
* −1: The value (uid_t) -1 is reserved by POSIX to identify an omitted argument.[11]
* 65535: This value is still avoided because it was the [API] error return value when uid was 16 bits.
* [Nobody]: Historically, the user &quot;nobody&quot; was assigned UID -2 by several [Operating System], although other values such as 215−1 = 32,767 are also in use, such as by OpenBSD. \\ For compatibility between 16-bit and 32-bit UIDs, many Linux distributions now set it to be 216−2 = 65,534; the Linux kernel defaults to returning this value when a 32-bit UID does not fit into the return value of the 16-[bit] system calls.[13] Fedora [Linux] assigns the last [UID] of the range statically allocated for system use (0-99) to nobody: 99, and calls 65534 instead nfsnobody.

Even more complex is the situation around [nobody]. The user [nobody] and [group] nogroup came from the [NFS] software and was defined as being having the highest [{$pagename}], since the function was oposite to the [root]:
* 16-[bit] systems: 65,536 unique [{$pagename}]s
* 32-[bit] systems: 4,294,967,296 unique [{$pagename}]s

This resulted in some confusion. To this confusion was added the use of using -2 for the [nobody] ID, as was done by the software itself if nobody and nogroup where not defined. [GNU]/[Linux] distribution creators defined the account as 65534, however [Red Hat] supplied under that [{$pagename}] nfsnobody with another nobody having [{$pagename}] 99. And there is nogroup usage, but also [groups|PosixGroup] that are called [nobody].
All in all a rough overview of what is used where can be created like this:
%%zebra-table
%%sortable
%%table-filter
||IDs||Usage
|-2|[nobody] on [AIX] and [MacOS]
|0-99|Unix local [users|PosixAccount] and [groups|PosixGroup], statically assigned
|99|[Red Hat] based system [nobody] user and [group] [{$pagename}]
|100-499|[Unix] local users and [groups|PosixGroup], dynamic
|529|Used as [{$pagename}] for [nobody] on some systems (and not used by [Microsoft])
|32767|Historic reservation for [nobody] (have not find any use)
|60001|Nobody on [IRIX] and [SunOS]
|65530-65535|Unix [nobody] user and (no)group (Debian and nfsnobody [RHEL])
|4294967292|Group-owner on Isilon [BSD]
|4294967293|[Null] user on Isilon [BSD]
|4294967294|[Everyone|Everybody] on Isilon [BSD]
|4294967295|[Nobody] (32-[bit])
/%
/%
/%!! More Information
There might be more information for this subject on one of the following:
[{ReferringPagesPlugin before='*' after='\n' }]
----
* [#1] - [UID, GID, SID and RID|http://pig.made-it.com/uidgid.html|target='_blank'] - based on information obtained 2020-02-18 
* [#2] - [User_identifier|Wikipedia:User_identifier|target='_blank'] - based on information obtained 2020-12-14 
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