IPv6 ULA prefix generator.

How it's calculated

A unique local address (ULA) is the IPv6 counterpart of private address space. RFC 4193 section 3.1 lays it out as a 7-bit prefix fc00::/7, an L bit, a 40-bit Global ID, a 16-bit Subnet ID and a 64-bit Interface ID. L is set to 1 for a prefix you generate yourself, so every locally generated ULA starts with fd and your site gets a /48. The 16-bit Subnet ID gives you 65,536 subnets of /64 each.

The Global ID must be pseudo-random, never sequential or a well-known number (section 3.2). The sample algorithm in section 3.2.2 is: take the time of day in 64-bit NTP format, add an EUI-64 or other identifier unique to the machine, join them into a key, compute the SHA-1 digest (160 bits), and use the least significant 40 bits as the Global ID. This tool does exactly that with your browser's WebCrypto SHA-1. A web page cannot read a hardware EUI-64, so the identifier is 8 random bytes, which is the "suitably unique identifier" the RFC allows. The key, digest and Global ID are shown in the result so you can check the work. The second method skips the hash and uses 40 random bits from crypto.getRandomValues, which the RFC also permits because it asks only for a pseudo-random Global ID. If your browser has no WebCrypto SHA-1 (some non-HTTPS contexts), the tool falls back to random bits and says so.

Addresses are written in the RFC 5952 section 4 form: no leading zeros, lowercase, and the longest run of zero groups replaced by :: (the first run if two tie, and never a single zero group).

Collisions. Two sites can pick the same Global ID, because nobody coordinates. RFC 4193 section 3.2.3 gives the chance as P = 1 - exp(-N² / 2⁴¹) for N interconnected Global IDs: about 1.81 in 10¹² for 2, 4.54 in 10⁹ for 100 and 4.54 in 10⁵ for 10,000. That is unlikely, not impossible. RFC sources read 2026-09-30.