Unique local address

From Wikipedia, the free encyclopedia

A unique local address (ULA) is an Internet Protocol version 6 (IPv6) address in the address range fc00::/7.[1] Its purpose in IPv6 is somewhat analogous to IPv4 private network addressing, but with significant differences. Unique local addresses may be used freely, without centralized registration, inside a single site or organization or spanning a limited number of sites or organizations. They are routable only within the scope of such private networks, but not in the global IPv6 Internet.


In December 1995, the IPv6 address block fec0::/10 was reserved for site-local addresses,[2] that could be used within a "site" for private IPv6 networks. However, insufficient definition of the term site led to confusion over the governing routing rules.

In September 2004, the Internet Engineering Task Force (IETF) deprecated the definition of this address range,[3] and postulated solutions to its problems. The special behaviour for this type of addresses as required at that time[4] was lifted in 2006 and the block returned to regular global unicast.[5]

In October 2005, the IETF reserved the address block fc00::/7 for use in private IPv6 networks and defined the associated term unique local addresses.[1]


Unique local addresses use prefix fc00::/7, extended with an 'L' bit which indicates that the address is locally assigned. This splits the address block in two equally sized halves, fc00::/8 and fd00::/8.

Unique local address format
bits 7 1 40 16 64
field prefix L random subnet id interface identifier

The prefix field contains the binary value 1111110. The L bit is one for locally assigned addresses; the address range with L set to zero is currently not defined. The random field is chosen randomly once, at the inception of the /48 routing prefix.

The block with L = 0, fc00::/8 is currently not defined.[a] It has been proposed that an allocation authority manage it, but this has not gained acceptance in the IETF.[6][7][8]

The block with L = 1, or locally assigned addresses, fd00::/8 is divided into /48 prefixes, formed by setting the forty bits following the prefix fd00/8 to a randomly generated bit string. This results in the format fdxx:xxxx:xxxx::/48 for a prefix in this range. RFC 4193 offers a suggestion for generating the random identifier to obtain a minimum-quality result if the user does not have access to a good source of random numbers.

RFC 4193 Block Prefix/L Global ID (random) Subnet ID Number of addresses in subnet
48 bits 16 bits 64 bits
fd00::/8 fd xx:xxxx:xxxx yyyy 18446744073709551616


A routing prefix in the range fd00::/8 may be constructed by generating a random 40-bit hexadecimal string, taken for this example to be 0x123456789a. The string is appended to the prefix fd00::/8, which forms the 48-bit routing prefix fd12:3456:789a::/48. With this prefix, 65536 subnets of size /64 are available for the private network: fd12:3456:789a::/64 to fd12:3456:789a:ffff::/64. For example Subnet ID 0x1 would be the subnet fd12:3456:789a:1::/64.

Prefix/L Global ID (random) Subnet ID Interface ID Address Subnet
fd xx:xxxx:xxxx yyyy zzzz:zzzz:zzzz:zzzz fdxx:xxxx:xxxx:yyyy:zzzz:zzzz:zzzz:zzzz fdxx:xxxx:xxxx:yyyy::/64
fd 12:3456:789a 0001 0000:0000:0000:0001 fd12:3456:789a:1::1 fd12:3456:789a:1::/64


Prefixes in the range fd00::/8 have some characteristics in common with the IPv4 private address ranges: They are not allocated by an address registry and may be used in networks by anyone without outside involvement. They are not mathematically guaranteed to be globally unique, but the probability of a collision is nevertheless extremely small. Reverse Domain Name System (DNS) entries (in ip6.arpa) for fd00::/8 ULAs cannot be delegated in the global DNS.

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 very small if the RFC 4193 selection algorithm was used.

Attempts of registration and allocation[edit]

SixXS attempted to maintain a voluntary registration database for fd00::/8 ULA prefixes to reduce the risk of different organisations using identical prefixes.[9] When the SixXS services were discontinued on 6 June 2017, the database became read-only.

On 6 December 2020, the Swiss based company 'ungleich' announced to revive the IPv6 ULA registry based on the original SixXS database, citing user demands for a ULA registry.[10]

For the range fc00::/8, different design decisions have been proposed and submitted to the IETF,[6][8] trading the risk of 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.[6][7][8]

See also[edit]


  1. ^ It is not defined at all. In particular: the standard does not say that prefixes in this block are 'globally assigned'.


  1. ^ a b R. Hinden; B. Haberman (October 2005). Unique Local IPv6 Unicast Addresses. Network Working Group. doi:10.17487/RFC4193. RFC 4193. Proposed Standard.
  2. ^ R. Hinden; S. Deering (December 1995). IP Version 6 Addressing Architecture. Network Working Group. doi:10.17487/RFC1884. RFC 1884. Obsolete. Obsoleted by RFC 2373.
  3. ^ C. Huitema; B. Carpenter (September 2004). Deprecating Site Local Addresses. Network Working Group. doi:10.17487/RFC3879. RFC 3879. Proposed Standard.
  4. ^ R. Hinden; S. Deering (April 2003). IP Version 6 Addressing Architecturen. Network Working Group. doi:10.17487/RFC3513. RFC 3513. Obsolete. Obsoletes RFC 2373. Obsoleted by RFC 4291.
  5. ^ R. Hinden; S. Deering (February 2006). IP Version 6 Addressing Architecture. Network Working Group. doi:10.17487/RFC4291. RFC 4291. Draft Standard. sec. 2.5.7. Obsoletes RFC 3513. Updated by RFC 5952, 6052, 7136, 7346, 7371 and 8064. [N]ew implementations must treat this prefix as Global Unicast.
  6. ^ a b c Internet Draft "Centrally Assigned Unique Local IPv6 Unicast Addresses". IETF. 2004–2007.
  7. ^ a b Internet Draft "An Analysis of Centrally Assigned Unique Local Addresses". IETF. 2007.
  8. ^ a b c Internet Draft "Centrally Assigned IPv6 Unicast Unique Local Address Prefixes". IETF. 2009–2010.
  9. ^ "IPv6 ULA (Unique Local Address) RFC4193 registration :: SixXS - IPv6 Deployment & Tunnel Broker". www.sixxs.net.
  10. ^ "Happy St Nicholas Day: Re-Launching the IPv6 ULA registry". mailarchive.ietf.org.

External links[edit]