what is a unique local address in IPv6?

A Unique Local Address (RFC 4193) is a private IPv6 prefix under fd00::/8, built from a random 40-bit Global ID, intended for addressing inside a site or organization without being routable on the public internet.

how is the fd00::/8 prefix different from fc00::/8?

fd00::/8 is the locally-assigned half of the ULA space, where the Global ID is generated randomly as this tool does. fc00::/8 was reserved for a centrally-assigned scheme that was never standardized, so fd00::/8 is what's actually used.

can two ULA prefixes generated separately collide?

It's possible but unlikely: the Global ID is 40 random bits, giving over a trillion possible prefixes, so two independently generated ULAs landing on the same value is a low-probability event rather than an impossible one.

is a ULA the same as an IPv6 link-local address?

No, link-local addresses (fe80::/10) are auto-configured and only valid on a single network segment, while a ULA is a routable private prefix you assign yourself, usable across your internal network like RFC 1918 space in IPv4.

IPv6 ULA Generator

Generate a random RFC 4193 Unique Local Address prefix (fd00::/8) with subnet, locally.

Local, Runs entirely in your browser, your data never leaves your device.

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A random private /48 prefix under fd00::/8

This tool draws 5 random bytes from the Web Crypto API for the 40-bit Global ID, combines them with the fd prefix to build a /48 Unique Local Address, and lets you set a 16-bit Subnet ID to derive a /64 network and its first host address. It's the calculation you need when numbering a lab, home or internal network with IPv6 addresses that won't collide with anyone else's and aren't routable on the public internet. The random bytes come straight from your browser, with no registry or coordination service involved.

How to use the IPv6 ULA Generator

Load the page to generate a random /48 ULA prefix under fd00::/8.
Set the Subnet ID field, 0 to 65535, to pick which /64 network to derive.
Click Generate new for a fresh random Global ID if you want a different prefix.
Copy the /48 prefix, /64 network or first address for the field you need.

The Global ID is 40 random bits, so two independently generated prefixes colliding is unlikely, though RFC 4193 doesn't guarantee uniqueness the way IANA-assigned global unicast space does. ULAs are not routable on the public internet and are meant for internal use only.

Frequently asked questions

what is a unique local address in IPv6?: A Unique Local Address (RFC 4193) is a private IPv6 prefix under fd00::/8, built from a random 40-bit Global ID, intended for addressing inside a site or organization without being routable on the public internet.
how is the fd00::/8 prefix different from fc00::/8?: fd00::/8 is the locally-assigned half of the ULA space, where the Global ID is generated randomly as this tool does. fc00::/8 was reserved for a centrally-assigned scheme that was never standardized, so fd00::/8 is what's actually used.
can two ULA prefixes generated separately collide?: It's possible but unlikely: the Global ID is 40 random bits, giving over a trillion possible prefixes, so two independently generated ULAs landing on the same value is a low-probability event rather than an impossible one.
is a ULA the same as an IPv6 link-local address?: No, link-local addresses (fe80::/10) are auto-configured and only valid on a single network segment, while a ULA is a routable private prefix you assign yourself, usable across your internal network like RFC 1918 space in IPv4.