UUID Family

UUID (Universally Unique Identifier) is a 128-bit identifier standardized by RFC 4122 and updated by RFC 9562. UUIDs are the most widely used identifier format.

Format

All UUIDs share the same canonical format:

xxxxxxxx-xxxx-Mxxx-Nxxx-xxxxxxxxxxxx

Where:

• M indicates the version (1-8)
• N indicates the variant (usually 8, 9, a, or b for RFC 4122)

Example: 550e8400-e29b-41d4-a716-446655440000

Versions

UUIDv1 - Timestamp + MAC Address

• Specification: RFC 4122
• Timestamp: Yes (100-nanosecond intervals since October 15, 1582)
• Sortable: No (timestamp is split across the ID)
• Privacy: Contains MAC address

idt gen uuidv1
# Example: 6ba7b810-9dad-11d1-80b4-00c04fd430c8

Structure:

• Time-low (32 bits)
• Time-mid (16 bits)
• Version + Time-high (16 bits)
• Variant + Clock sequence (16 bits)
• Node/MAC address (48 bits)

UUIDv3 - MD5 Namespace Hash

• Specification: RFC 4122
• Timestamp: No
• Sortable: No
• Deterministic: Yes (same namespace + name = same UUID)

# Currently generation is not supported
# Useful for inspecting existing UUIDv3s
idt inspect 5df41881-3aed-3515-88a7-2f4a814cf09e

UUIDv4 - Random

• Specification: RFC 4122
• Timestamp: No
• Sortable: No
• Random bits: 122

The most commonly used UUID version. Purely random except for version and variant bits.

idt gen uuid
# or
idt gen uuidv4
# Example: 550e8400-e29b-41d4-a716-446655440000

Collision probability: With 122 random bits, you’d need to generate about 2.71 quintillion UUIDs to have a 50% chance of collision.

UUIDv5 - SHA-1 Namespace Hash

• Specification: RFC 4122
• Timestamp: No
• Sortable: No
• Deterministic: Yes (same namespace + name = same UUID)

Similar to UUIDv3 but uses SHA-1 instead of MD5.

# Currently generation is not supported
# Useful for inspecting existing UUIDv5s
idt inspect 21f7f8de-8051-5b89-8680-0195ef798b6a

UUIDv6 - Reordered Timestamp

• Specification: RFC 9562
• Timestamp: Yes (same resolution as v1)
• Sortable: Yes
• Privacy: Contains MAC address (like v1)

Reorders v1’s timestamp bits for lexicographic sorting.

idt gen uuidv6
# Example: 1ec9414c-232a-6b00-b3c8-9e6bdeced846

When to use: When you need v1 compatibility but want sortable IDs.

UUIDv7 - Unix Timestamp + Random

• Specification: RFC 9562
• Timestamp: Yes (Unix milliseconds)
• Sortable: Yes
• Random bits: 62
• Recommended: Yes, for new applications

The recommended choice for new applications needing sortable UUIDs.

idt gen uuidv7
# Example: 019c04e5-6118-7b22-95cb-a10e84dad469

Structure:

• Unix timestamp in milliseconds (48 bits)
• Version (4 bits)
• Random (12 bits)
• Variant (2 bits)
• Random (62 bits)

Advantages:

• Naturally sorted by creation time
• Compatible with existing UUID infrastructure
• No MAC address (privacy-friendly)
• Sufficient randomness to avoid collisions

UUID-nil - All Zeros

The nil UUID is all zeros, often used as a placeholder or “no value” indicator.

idt gen uuid-nil
# Output: 00000000-0000-0000-0000-000000000000

UUID-max - All Ones

The max UUID is all ones, sometimes used as a sentinel value.

idt gen uuid-max
# Output: ffffffff-ffff-ffff-ffff-ffffffffffff

Inspecting UUIDs

$ idt inspect 019c04e5-6118-7b22-95cb-a10e84dad469
UUIDV7
  019c04e5-6118-7b22-95cb-a10e84dad469

  Time (UTC)          2026-01-28T13:57:47.416Z
  Local Time (+09:00) 2026-01-28T22:57:47.416+09:00
  Version             7
  Variant             RFC4122
  Random              62 bits

  Hex                 019c04e561187b2295cba10e84dad469
  Base64              AZwE5WEYeyKVy6EOhNrUaQ==
  Int                 2139325608653621017571381452845274217

Converting UUIDs

# Remove dashes
idt convert 550e8400-e29b-41d4-a716-446655440000 -f hex

# To Base64
idt convert 550e8400-e29b-41d4-a716-446655440000 -f base64

# To integer
idt convert 550e8400-e29b-41d4-a716-446655440000 -f int

Choosing a UUID Version

Specifications

• RFC 4122: Original UUID specification (v1-v5)
  • https://datatracker.ietf.org/doc/html/rfc4122
• RFC 9562: New UUID formats (v6-v8)
  • https://datatracker.ietf.org/doc/html/rfc9562