Unique local address

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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 analogous to IPv4 private network addressing. 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.

As a result, the IETF reserved the address block fc00::/7 in October 2005 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, indicating whether or not 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 zero for globally assigned, and one for locally assigned addresses. The random field is chosen randomly once, at the inception of the /48 routing prefix.

The block with L = 0, or globally assigned addresses, fc00::/8 is currently undefined. It was proposed that the allocation authority manage it, but this has not gained acceptance in the IETF.[4][5][6] The block with L = 1, or locally assigned addresses, fd00::/8 is defined for /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 similar characteristics as those of 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 guaranteed to be globally unique. 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 small if the RFC 4193 selection algorithm was used.

Attempts of registration and allocation[edit]

SixXS attempted to maintain a voluntary registration database for ULA prefixes to reduce the risk of duplication.[7] 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.[8]

For the range fc00::/8, different design decisions have been proposed and submitted to the IETF,[4][6] 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.[4][5][6]

See also[edit]


  1. ^ a b R. Hinden; B. Haberman (October 2005). Unique Local IPv6 Unicast Addresses. IETF. doi:10.17487/RFC4193. RFC 4193.
  2. ^ R. Hinden; S.Deering (December 1995). IP Version 6 Addressing Architecture. IETF. doi:10.17487/RFC1884. RFC 1884.
  3. ^ C. Huitema; B. Carpenter (September 2004). Deprecating Site Local Addresses. IETF. doi:10.17487/RFC3879. RFC 3879.
  4. ^ a b c Internet Draft "Centrally Assigned Unique Local IPv6 Unicast Addresses". IETF. 2004–2007.
  5. ^ a b Internet Draft "An Analysis of Centrally Assigned Unique Local Addresses". IETF. 2007.
  6. ^ a b c Internet Draft "Centrally Assigned IPv6 Unicast Unique Local Address Prefixes". IETF. 2009–2010.
  7. ^ SIXXS voluntary database
  8. ^ Announcement from ungleich on the IETF mailing list.

External links[edit]