UUID

Universally Unique Identifier (UUID) is a 128-bit long number that is used by Minecraft as a label to distinguish between separate instances.

Representation

• Hyphenated hexadecimal: The RFC 9562 standard string representation of UUID. A hexadecimal representation of the UUID, with hyphens separating the different sections into individual numbers.
  The hyphen is set to split the UUID into numbers of the format 8-4-4-4-12 with each number marking the number of hexadecimal digits fitting into the corresponding section. Further detail follows in the Technical aspects section.
  An example of this representation would be f81d4fae-7dec-11d0-a765-00a0c91e6bf6.
  • Because every part is evaluated as an individual number, empty digits at the start of a section can be ignored.
    For example, 00000001-0002-0003-0004-000000000005 can be evaluated as the same as 1-2-3-4-5.
• Hexadecimal: The same as the hyphenated hexadecimal representation, but without the separation of the different sections.
  An example for this representation would be 00000001000200030004000000000005, where it is impossible for the majority of empty digits to be removed as opposed to the hyphenated hexadecimal representation.
• Most/Least: A separation of the 64 most significant bits from the 64 least significant bits. Each of the two numbers is stored separately and uses the Long data type in the game.
  This format is deprecated as it was used before 1.16 and was entirely replaced by the Int-array format.
  An example of this representation would be UUIDMost:-568210367123287600 paired with UUIDLeast:-6384696206158828554.
• Int-array: A separation into four 32-bit numbers. Each part is stored in an integer array ordered from most significant to least significant.
  An example of this representation would be [I;-132296786,2112623056,-1486552928,-920753162]

Occurences

The following table represents the situations in which UUIDs are used in places accessible by a player:

Location (general)	Location (path)	Format	Use
All entities (NBT)	UUID	int-array	Own UUID
Tameable mobs (NBT)	Owner	int-array	Owner
Projectiles (NBT)	Owner	int-array	Entity that shot the projectile
Item entities (NBT)	Owner	int-array	Target player for /give command
Item entities (NBT)	Thrower	int-array	Player who dropped the item
Shulker bullets (NBT)	Target	int-array	Entity targeted for attack
Conduits (NBT)	Target	int-array	Entity targeted for attack
Player heads (NBT)	profile.	int-array	Owner of the skin
Player heads (NBT)	SkullOwner.Properties.textures[].Value.ProfileId	hexadecimal	Owner of the skin
Mobs (NBT)	Leash.UUID	int-array	Entity that leashed the mob
Breedable mobs (NBT)	LoveCause	int-array	Player who fed the mob
Wardens (NBT)	anger.suspects[].uuid	int-array	Target associated with the anger level
Zombie Villagers (NBT)	ConversionPlayer	int-array	Player who is converting the Zombie Villager
Most of the neutral mobs (NBT)	AngryAt	int-array	Entity that hurt the neutral mob
Villagers (NBT)	Gossips[].Target	int-array	Player who caused the gossip
Players (NBT)	RootVehicle.Attach	int-array	Identifier of the entity that the player is riding
Target selectors (Commands)	-	Hyphenated hexadecimal	Selected entity
server.properties	resource-pack-id	Hyphenated hexadecimal	Identifies the server resource pack with clients

Technical aspects

Value range

A UUID can be expressed as a 128-bit number, which means that it supports all integer values from -(2^127) to (2^127)-1. It is important to note that this evaluation does not refer to the range of values generated by the game but instead focuses on the capacity of the format.

Hyphenated hexadecimal format section names

Hyphenated hexadecimal format UUIDs follow the pattern xxxxxxxx-xxxx-Axxx-Bxxx-xxxxxxxxxxxx.

Section name	Bit range
	From	To	Size	Characters
time-low	96	127	32	8
time-mid	80	95	16	4
time-high-and-version	54	79	16	4
clock-seq-and-reserved	36	53	8	2
clock-seq-low	48	45	8	2
node	0	47	48	12

(The names make sense only for version 1 and 2 UUIDs, but it has been passed onto all forms of UUIDs used.)

Versions and variants

There are five versions of UUIDs. Versions 1 and 2 are based on current time and MAC address; versions 3 and 5 are based on hashing a string; version 4 is based on completely random generation. The "version" value occupies four bits, and is located in the UUID string format at the position indicated by an "A".

The "B" position is the variant (format) of the UUID. Depending on the specific variant, it may occupy the high 1 to 3 bits:

• 0xx is the "variant 0" of Apollo NCS 1.5, an old type of UUID.
• 10x is the "variant 1" of IETF RFC 4122 (Leach-Salz). It's referred to as "variant 2" by Java.
• 110 is the "variant 2" of old Microsoft products. It's referred to as "variant 6" by Java.
• 111 is the "reserved variant". It's referred to as "variant 7" by Java.

In Minecraft

Minecraft uses version 4, variant 1 (RFC) UUIDs, which means that the entire number with the exception of the bits used for metadata (version and variant) is randomly generated. Java's UUID.randomUUID() function performs this exact task.

A table with the locations and values of the metadata:

Bitwise version[show]

General			Hyphenated hexadecimal format		Most/Least format		int-array format
Bit number	Value	Use	Section	Bit number	Section	Bit number	Section	Bit number
62	0	UUID format	clock-seq-and-reserved	6	Least	62	Index 1	30
63	1			7		63		31
76	0	UUID version	time-high-and-version	12	Most	12	Index 2	12
77	0			13		13		13
78	1			14		14		14
79	0			15		15		15

Collapsed version[show]

General					Hyphenated hexadecimal format			Most/Least format			int-array format
Use	Bit range		Range size	Value	Section	Bit range		Section	Bit range		Section	Bit range
	From	To				From	To		From	To		From	To
UUID format	62	63	2	2	clock-seq-and-reserved	6	7	Least	62	63	Index 1	30	31
UUID version	76	79	4	4	time-high-and-version	12	15	Most	12	15	Index 2	12	15

External links

• UUID Java class documentation
• Generate UUIDs in hyphenated hexadecimal format
• Convert between and generate all different formats, with the exception that the non-hyphenated hexadecimal format is only accepted as input