UUID Versions Explained

UUID Structure

xxxxxxxx-xxxx-Mxxx-Nxxx-xxxxxxxxxxxx

time_low (32 bits)time_mid (16 bits)M = version (4 bits) + time_hi (12)N = variant (2 bits) + clock (14)node / random (48 bits)

UUID v1Time-based (MAC)Sortable

Structure

timestamp (60-bit) + clock seq (14-bit) + MAC (48-bit)

Collision risk

Very low

6ba7b810-9dad-11d1-80b4-00c04fd430c8

Advantages

+Globally unique
+Sortable by time
+Embeds creation time

Disadvantages

−Exposes MAC address (privacy risk)
−Requires MAC or pseudo-random node
−Clock adjustments can cause duplicates

Best used for

Legacy systems. Avoid in new projects due to MAC address exposure.

UUID v3Name-based (MD5)Not sortable

Structure

MD5(namespace UUID + name) → UUID

Collision risk

MD5 collision risk (low for UUIDs)

6fa459ea-ee8a-3ca4-894e-db77e160355e

Advantages

+Deterministic — same input = same UUID
+Can recreate UUIDs from known inputs

Disadvantages

−MD5 is cryptographically weak
−Not random — attackers can predict given namespace+name
−Prefer v5 for new projects

Best used for

Generating UUIDs from URLs, email addresses, or other stable identifiers. Use v5 instead.

UUID v4RandomNot sortable

Structure

122 random bits + 4-bit version + 2-bit variant

Collision risk

Negligible (2^122 possibilities)

550e8400-e29b-41d4-a716-446655440000

Advantages

+Extremely low collision probability
+No timestamp/MAC leakage
+Simple generation — no state needed
+Most widely supported

Disadvantages

−Not sortable by generation time
−Random order degrades B-tree performance in databases
−Cannot verify when a UUID was created

Best used for

Default choice for most applications. IDs for records, sessions, tokens.

UUID v5Name-based (SHA-1)Not sortable

Structure

SHA-1(namespace UUID + name) → UUID (top 128 bits)

Collision risk

SHA-1 — extremely low for UUID purposes

886313e1-3b8a-5372-9b90-0c9aee199e5d

Advantages

+Deterministic like v3, but uses SHA-1
+Stronger hash than MD5 (v3)
+Standard namespaces: DNS, URL, OID, X.500

Disadvantages

−SHA-1 is fully deterministic — not suitable for secrets
−Truncates SHA-1 to 128 bits

Best used for

Deterministic IDs from a namespace + name. Example: UUID for 'https://example.com' is always the same.

UUID v7Time-ordered RandomSortable

Structure

Unix ms timestamp (48-bit) + version (4-bit) + random (74-bit)

Collision risk

Very low (74 random bits)

018f0b2d-4c9e-7abc-89de-f012345678ab

Advantages

+Sortable by time (great for database primary keys)
+Does NOT expose MAC address
+Near-zero collision probability
+RFC 9562 standard

Disadvantages

−Slightly less entropy than v4 (74 vs 122 bits)
−Requires a time source
−Newer — less library support than v4

Best used for

Database primary keys where sort order matters. Replaces v1 for time-ordered UUIDs.

UUID v8CustomSortable

Structure

128 bits, content defined by the implementation

Collision risk

Depends on implementation

2489e9ad-f195-8b67-82da-3291d8c9fc1b

Advantages

+Fully customizable
+Useful for embedding application-specific data

Disadvantages

−No standard semantics — interoperability must be documented
−Not widely supported

Best used for

Experimental or vendor-specific use cases where v1–v7 don't fit.

Quick Comparison

Version	Method	Sortable	Privacy-safe	Deterministic
v1	Time + MAC	✅	❌ (MAC leak)	❌
v3	MD5 hash	❌	✅	✅
v4	Random	❌	✅	❌
v5	SHA-1 hash	❌	✅	✅
v7	Time + Random	✅	✅	❌
v8	Custom	—	—	—

Generating UUIDs in Code

// JavaScript / Node.js (built-in, Node 15+)
import { randomUUID } from 'crypto';
const v4 = randomUUID();  // → "550e8400-e29b-41d4-a716-446655440000"

// Browser
const v4 = crypto.randomUUID();

// UUID v7 (npm: uuid package v10+)
import { v7 as uuidv7 } from 'uuid';
const v7 = uuidv7();  // → "018f0b2d-4c9e-7abc-89de-f012345678ab"

// Python
import uuid
v4 = str(uuid.uuid4())  # Random
v5 = str(uuid.uuid5(uuid.NAMESPACE_URL, 'https://example.com'))

# PostgreSQL
SELECT gen_random_uuid();        -- v4 (pg 13+)
SELECT uuid_generate_v4();       -- v4 (uuid-ossp extension)
SELECT gen_random_uuid()::text;  -- as text

UUID Guide: Versions, Formats, and Best Practices

UUID vs Other ID Formats

ULID (Universally Unique Lexicographically Sortable Identifier)

ULIDs are a compact alternative: 48-bit timestamp + 80-bit random, encoded as 26 Crockford Base32 characters. They sort lexicographically by creation time.

When to Use Auto-Increment IDs

Sequential IDs are simpler and more efficient for single-database applications. Use UUIDs when you need:

IDs generated by multiple systems independently
IDs that don't reveal insertion order to external users
Merge-friendly IDs for distributed databases

Database Performance Considerations

-- PostgreSQL: native UUID type (16 bytes, efficient)
CREATE TABLE users (
  id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
  name TEXT NOT NULL
);

-- MySQL: store as BINARY(16) for performance
CREATE TABLE users (
  id BINARY(16) PRIMARY KEY,
  name VARCHAR(255) NOT NULL
);

FAQ

Can UUIDs be used as security tokens?

UUID v4 uses 122 bits of randomness from a cryptographic source, which provides good entropy. However, for security tokens (API keys, session IDs), it is better to use purpose-built token formats with additional properties like checksums, expiration encoding, or prefix identification. UUIDs are for identity, not authentication.

UUID Versions FAQ

What is a UUID?

A UUID (Universally Unique Identifier) is a 128-bit identifier standardized in RFC 4122. It is formatted as 8-4-4-4-12 hexadecimal characters separated by hyphens, for example: 550e8400-e29b-41d4-a716-446655440000. They are designed to be unique across space and time without a central authority.

Which UUID version should I use?

Use v4 for most general purposes — it is randomly generated and has extremely low collision probability. Use v7 if you need time-ordered UUIDs for database primary keys (better index performance than v4). Use v5 if you need deterministic UUIDs from a namespace + name pair. Avoid v1 in new projects as it exposes the MAC address.

What is the difference between UUID v4 and v7?

UUID v4 is purely random (122 random bits). UUID v7 encodes a Unix timestamp in the first 48 bits followed by random bits. This makes v7 UUIDs sortable by creation time, which significantly improves B-tree index performance in databases compared to random v4 UUIDs.

Are UUID v4 IDs truly unique?

Practically yes. UUID v4 has 122 random bits, giving 2^122 ≈ 5.3 × 10^36 possible values. To have a 50% chance of a collision you would need to generate about 2.7 × 10^18 UUIDs — far more than any realistic application.

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Version

Method

Sortable

Privacy-safe

Deterministic

Time + MAC

✅

❌ (MAC leak)

❌

MD5 hash

❌

✅

Random

❌

✅

❌

SHA-1 hash

❌

✅

Time + Random

✅

❌

Custom

—

Generating UUIDs in Code

// JavaScript / Node.js (built-in, Node 15+)
import { randomUUID } from 'crypto';
const v4 = randomUUID();  // → "550e8400-e29b-41d4-a716-446655440000"

// Browser
const v4 = crypto.randomUUID();

// UUID v7 (npm: uuid package v10+)
import { v7 as uuidv7 } from 'uuid';
const v7 = uuidv7();  // → "018f0b2d-4c9e-7abc-89de-f012345678ab"

// Python
import uuid
v4 = str(uuid.uuid4())  # Random
v5 = str(uuid.uuid5(uuid.NAMESPACE_URL, 'https://example.com'))

# PostgreSQL
SELECT gen_random_uuid();        -- v4 (pg 13+)
SELECT uuid_generate_v4();       -- v4 (uuid-ossp extension)
SELECT gen_random_uuid()::text;  -- as text

UUID Guide: Versions, Formats, and Best Practices

UUID vs Other ID Formats

ULID (Universally Unique Lexicographically Sortable Identifier)

ULIDs are a compact alternative: 48-bit timestamp + 80-bit random, encoded as 26 Crockford Base32 characters. They sort lexicographically by creation time.

When to Use Auto-Increment IDs

Sequential IDs are simpler and more efficient for single-database applications. Use UUIDs when you need:

IDs generated by multiple systems independently
IDs that don't reveal insertion order to external users
Merge-friendly IDs for distributed databases

Database Performance Considerations

-- PostgreSQL: native UUID type (16 bytes, efficient)
CREATE TABLE users (
  id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
  name TEXT NOT NULL
);

-- MySQL: store as BINARY(16) for performance
CREATE TABLE users (
  id BINARY(16) PRIMARY KEY,
  name VARCHAR(255) NOT NULL
);

UUID Versions Explained

UUID Structure

Quick Comparison

Generating UUIDs in Code

UUID Guide: Versions, Formats, and Best Practices

UUID vs Other ID Formats

ULID (Universally Unique Lexicographically Sortable Identifier)

When to Use Auto-Increment IDs

Database Performance Considerations

FAQ

Can UUIDs be used as security tokens?

UUID Versions FAQ

What is a UUID?

Which UUID version should I use?

What is the difference between UUID v4 and v7?

Are UUID v4 IDs truly unique?

Related Tools

UUID Versions Explained

UUID Structure

Quick Comparison

Generating UUIDs in Code

UUID Guide: Versions, Formats, and Best Practices

UUID vs Other ID Formats

ULID (Universally Unique Lexicographically Sortable Identifier)

When to Use Auto-Increment IDs

Database Performance Considerations

FAQ

Can UUIDs be used as security tokens?

UUID Versions FAQ

What is a UUID?

Which UUID version should I use?

What is the difference between UUID v4 and v7?

Are UUID v4 IDs truly unique?

Related Tools