unique local address

An IPv6 address in fc00::/7, meant for use inside a site or between cooperating sites and not routed on the internet.

Unique local addresses (RFC 4193) are the nearest IPv6 counterpart to RFC 1918 space, with one difference that matters: each site picks a random 40-bit Global ID, so the prefix is meant to be globally unique and a leak should not collide with anyone. They give no security by themselves.

The address splits into the 7-bit fc00::/7 prefix, an L bit, a 40-bit Global ID, a 16-bit Subnet ID and the 64-bit interface identifier. Only L set to 1, locally assigned, is defined, so every ULA in use begins with fd. The Global ID must not be sequential or a well-known number: RFC 4193 derives it from a SHA-1 hash of the time of day and an EUI-64, keeping the low 40 bits, which makes fd00::/48 an invalid choice.

Their default scope is global, so only policy keeps them in. RFC 4193 asks exterior routing sessions and site borders to drop fc00::/7 by default, keeps their AAAA and PTR records out of the global DNS and expects them to reuse the global plan's subnet IDs when both run together; IANA marks the block not globally reachable.

Address selection treats them as a last choice: RFC 6724's default table gives fc00::/7 precedence 3 against 35 for IPv4, so by default a dual-stack host prefers an IPv4 destination to a ULA one. The IPv6 Toolkit labels these addresses unique local.

Also known as: ula, unique local ipv6 unicast address, fc00::/7, fd00::/8, rfc 4193

Sources

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