Unique local address
Encyclopedia
A unique local address is an IPv6
address in the block fc00::/7, defined in RFC 4193. It is the 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.
) string fedcba9876 to the fd00::/8 prefix. This forms the 48-bit routing prefix fdfe:dcba:9876::/48. Using this prefix, 65536 subnets of size /64 are available for use in the private network: fdfe:dcba:9876::/64 to fdfe:dcba:9876:ffff::/64.
It should be noted that, as the 40-bit string from this example was (obviously) not randomly chosen, the above is in fact not a validly generated ULA routing prefix according to the standard. The standard requires that the 40-bit string be (pseudo-)randomly generated, e.g. by using the algorithm specified in the standard. This is required in order to minimize the risk of address collisions.
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.
For the fc00:/8 range, different design decisions have been proposed, 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
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IPv6
Internet Protocol version 6 is a version of the Internet Protocol . It is designed to succeed the Internet Protocol version 4...
address in the block fc00::/7, defined in RFC 4193. It is the 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.
History
In 1995, RFC 1884 reserved the block fec0::/10 for site-local addresses, that could be used within a "site" for private IPv6 networks. However, insufficient definition of the term site lead to confusion over the resulting routing rules. RFC 3879 (September 2004) deprecated this address range, and postulated solutions to its problems. In October 2005, RFC 4193 was published, reserving the address block fc00::/7 for use in private IPv6 networks, and defining the associated term unique local addresses.Definition
The address block fc00::/7 is divided into two /8 groups:- The block fc00::/8 has not been defined yet. It has been proposed to be managed by an allocation authority, but this has not gained acceptance in the IETF.
- The block fd00::/8 is defined for /48 prefixes, formed by setting the 40 least-significant bits of the prefix to a randomly-generated bit string. This results in the format fdxx:xxxx:xxxx:: 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.
Example
As an example, a routing prefix in the fd00::/8 range would be constructed by appending the example 40-bit (hexadecimalHexadecimal
In mathematics and computer science, hexadecimal is a positional numeral system with a radix, or base, of 16. It uses sixteen distinct symbols, most often the symbols 0–9 to represent values zero to nine, and A, B, C, D, E, F to represent values ten to fifteen...
) string fedcba9876 to the fd00::/8 prefix. This forms the 48-bit routing prefix fdfe:dcba:9876::/48. Using this prefix, 65536 subnets of size /64 are available for use in the private network: fdfe:dcba:9876::/64 to fdfe:dcba:9876:ffff::/64.
It should be noted that, as the 40-bit string from this example was (obviously) not randomly chosen, the above is in fact not a validly generated ULA routing prefix according to the standard. The standard requires that the 40-bit string be (pseudo-)randomly generated, e.g. by using the algorithm specified in the standard. This is required in order to minimize the risk of address collisions.
Properties
Prefixes in the fd00::/8 range have similar properties 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 SystemDomain name systemThe Domain Name System is a hierarchical distributed naming system for computers, services, or any resource connected to the Internet or a private network. It associates various information with domain names assigned to each of the participating entities...
(DNS) entries (under 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 extremely small if the RFC 4193 selection algorithm was used.
Attempts of registration and allocation
Attempts have been made to create registration databases for ULA prefixes to reduce the risk of duplicate prefixes.For the fc00:/8 range, different design decisions have been proposed, 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
Request for Comments
In computer network engineering, a Request for Comments is a memorandum published by the Internet Engineering Task Force describing methods, behaviors, research, or innovations applicable to the working of the Internet and Internet-connected systems.Through the Internet Society, engineers and...
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