RFC 4193 / START HERE
Unique local IPv6, before the details
ULA means Unique Local Address. IPv6 is Internet Protocol version6; unicast means an address for one destination interface.
RFC4193 defines a local IPv6 address format. Sites generate their own prefix, a shared starting part of an address, and divide it into subnets. These addresses can cross internal routers and selected links between sites.
The idea in one picture
Generate a prefix
Route deliberately
Keep local policy
A simplified picture of the idea; the chapters explain the actual mechanisms.
Why it exists
Local communication can keep working when an Internet provider changes. Pseudo-random Global IDs make independent sites unlikely to choose the same prefix. ULA has global scope, but is not expected to be routed on the public Internet. It is different from automatically created link-local addresses.
A familiar example
A fictional lab connects two offices with distinct local prefixes and explicitly configured routes. Its local addressing survives a provider-prefix change. The prefix itself supplies no encryption, authentication or automatic border filter, and uniqueness is highly probable rather than guaranteed.
Helpful background
You can start here without reading another RFC. If these ideas are unfamiliar, these short introductions may help.
What you’ll learn next
Begin with chapter one, or choose a topic.
- Read the local IPv6 address fieldsA Unique Local Address (ULA) is a 128-bit IPv6 unicast address: one destination interface. Its prefix is intended for local communication, not ordinary public Internet routing.→
- Generate a prefix without a central allocatorA site creates its own 40-bit Global ID. Independent pseudo-random choices make an overlap unlikely when sites later connect; they do not prove uniqueness.→
- Separate internal routes from border policyULA addresses have global scope but limited routability. Scope is not a promise of Internet reachability, and a default route is not an inter-site agreement.→
- Keep local addresses when sites or providers changeIndependent local prefixes can survive an Internet Service Provider (ISP) change. When sites join, distinct prefixes help avoid renumbering, but routes still need configuration.→
- Keep reverse DNS and access controls explicitDomain Name System (DNS) lookups and service authorization are separate from choosing an IPv6 prefix. A local address is neither a private tunnel nor proof that a peer is trusted.→
This introduction explains RFC 4193 (2005). Read the original RFC ↗