A UUID (Universally Unique Identifier) is a 128-bit identifier standardized in RFC 4122, represented as 32 hexadecimal digits in the pattern 8-4-4-4-12, designed to be globally unique without a central authority.

What is the difference between UUID v4 and other versions (v1, v3, v5)?

v1 is based on MAC address and timestamp, v3/v5 are deterministic hashes of a namespace + name, and v4 is fully random. UUID v4 is the most widely used for general-purpose unique IDs.

Are UUID v4 values truly unique? Can they collide?

Collisions are theoretically possible but astronomically unlikely. Generating 1 billion UUIDs per second, it would take about 85 years to reach a 50% probability of collision.

Why shouldn't I use Math.random() to generate UUIDs?

Math.random() produces predictable pseudo-random numbers with guessable seeds. Always use the Web Crypto API (crypto.randomUUID() or crypto.getRandomValues()) for cryptographically secure UUID generation.

Are there performance issues using UUIDs as database primary keys?

UUID v4's random nature causes frequent B-tree index page splits, leading to worse write performance than sequential IDs. Consider UUID v7 (time-ordered) or ULID if write performance is critical.

What is the difference between UUID and ULID?

ULID combines a timestamp with randomness for sortable unique IDs. Unlike UUID v4, ULIDs are time-ordered, improving database index efficiency and making them sortable by creation time.

Can I remove the hyphens from a UUID?

Yes. A UUID without hyphens (32 hex characters) retains the same uniqueness. However, the RFC 4122 standard format includes hyphens, so verify your system accepts hyphen-free UUIDs.

Can I generate multiple UUIDs at once?

Yes. Enter the desired count to generate up to 100 UUIDs at once. Each is generated independently using crypto.randomUUID().

Is it safe to use a UUID v4 as a session token?

UUID v4 uses cryptographically secure randomness, making it suitable as a session ID. Always transmit session tokens over HTTPS and store them in HttpOnly cookies.

Why does a UUID v4 always have "4" as the first character of the third group?

RFC 4122 specifies that the third group's first character indicates the version (4), and the fourth group's first character encodes the variant bits.

Can I use UUIDs directly in URLs?

Yes. UUID characters (hex digits and hyphens) are all URL-safe. UUIDs work directly in RESTful URLs like /users/550e8400-e29b-41d4-a716-446655440000.

Are generated UUIDs sent to any server?

No. UUID generation uses your browser's Web Crypto API (crypto.randomUUID()). No generated values or data are transmitted to any server.

UUID Generator

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UUID v4는 122비트의 암호학적 난수로 생성됩니다. 충돌 확률은 약 1/2¹²²로 사실상 전 세계에서 고유합니다. 데이터베이스 기본 키, 세션 토큰, API 요청 ID 등에 활용됩니다.

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