Unique local address

A unique local address (ULA) is an IPv6 address in the block fc00::/7, defined in RFC 4193. It is the approximate IPv6 counterpart of the IPv4 private address. Unique local addresses are available for use in private networks, e.g. inside a single site or organization or spanning a limited number of sites or organizations. They are not routable in the global IPv6 Internet.

Definition

The address block fc00::/7 is divided into two /8 groups:

Example

As an example, a routing prefix in the fd00::/8 range would be constructed by generating a random 40-bit hexadecimal string, taken to be e48dba82e1 in this example. The string is appended to the fd00::/8 prefix. This forms the 48-bit routing prefix fde4:8dba:82e1::/48. With this prefix, 65536 subnets of size /64 are available for the private network: fde4:8dba:82e1::/64 to fde4:8dba:82e1:ffff::/64.

Properties

Prefixes in the fd00::/8 range have similar properties as those of the IPv4 private address ranges:

As fd00::/8 ULAs are not meant to be routed outside their administrative domain (site or organization), administrators of interconnecting networks normally do not need to worry about the uniqueness of ULA prefixes. However, if networks require routing ULAs between each other in the event of a merger, for example, the risk of address collision is extremely small if the RFC 4193 selection algorithm was used.

Attempts of registration and allocation

For the fc00::/8 range, different design decisions have been proposed,[1][3] trading the non-uniqueness for the requirement that the range be managed by a central allocation authority. However, such attempts at standardizing this range have not resulted in a Request for Comments.[1][2][3]

See also

References

  1. 1 2 3 Internet Draft "Centrally Assigned Unique Local IPv6 Unicast Addresses". IETF. 2004–2007.
  2. 1 2 Internet Draft "An Analysis of Centrally Assigned Unique Local Addresses". IETF. 2007.
  3. 1 2 3 Internet Draft "Centrally Assigned IPv6 Unicast Unique Local Address Prefixes". IETF. 2009–2010.
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