IPv6 address

An Internet Protocol Version 6 address (IPv6 address) is a numerical label that is used to identify a network interface of a computer or other network node participating in an IPv6 computer network.

An IP address serves the purpose of uniquely identifying an individual network interface of a host, locating it on the network, and thus permitting the routing of IP packets between hosts. For routing, IP addresses are present in fields of the packet header where they indicate source and destination of the packet.

IPv6 is the successor to the Internet's first addressing infrastructure, Internet Protocol version 4 (IPv4). In contrast to IPv4, which defined an IP address as a 32-bit value, IPv6 addresses have a size of 128 bits. Therefore, IPv6 has a vastly enlarged address space compared to IPv4.

IPv6 address classes

IPv6 addresses are classified by the primary addressing and routing methodologies common in networking: unicast addressing, anycast addressing, and multicast addressing.^[1]

• A unicast address identifies a single network interface. The Internet Protocol delivers packets sent to a unicast address to that specific interface.
• An anycast address is assigned to a group of interfaces, usually belonging to different nodes. A packet sent to an anycast address is delivered to just one of the member interfaces, typically the nearest host, according to the routing protocol’s definition of distance. Anycast addresses cannot be identified easily, they have the same format as unicast addresses, and differ only by their presence in the network at multiple points. Almost any unicast address can be employed as an anycast address.
• A multicast address is also used by multiple hosts, which acquire the multicast address destination by participating in the multicast distribution protocol among the network routers. A packet that is sent to a multicast address is delivered to all interfaces that have joined the corresponding multicast group.

IPv6 does not implement broadcast addressing. Broadcast's traditional role is subsumed by multicast addressing to the all-nodes link-local multicast group ff02::1. However, the use of the all-nodes group is not recommended, and most IPv6 protocols use a dedicated link-local multicast group to avoid disturbing every interface in the network.

Address formats

An IPv6 address consists of 128 bits.^[1] Addresses are classified into various types for applications in the major addressing and routing methodologies: unicast, multicast, and anycast networking. In each of these, various address formats are recognized by logically dividing the 128 address bits into bit groups and establishing rules for associating the values of these bit groups with special addressing features.

Unicast and anycast address format

Unicast and anycast addresses are typically composed of two logical parts: a 64-bit network prefix used for routing, and a 64-bit interface identifier used to identify a host's network interface.

General unicast address format (routing prefix size varies)

bits	48 (or more)	16 (or fewer)	64
field	routing prefix	subnet id	interface identifier

The network prefix (the routing prefix combined with the subnet id) is contained in the most significant 64 bits of the address. The size of the routing prefix may vary; a larger prefix size means a smaller subnet id size. The bits of the subnet id(entifier) field are available to the network administrator to define subnets within the given network. The 64-bit interface identifier is either automatically generated from the interface's MAC address using the modified EUI-64 format, obtained from a DHCPv6 server, automatically established randomly, or assigned manually.

A link-local address is also based on the interface identifier, but uses a different format for the network prefix.

Link-local address format

bits	10	54	64
field	prefix	zeroes	interface identifier

The prefix field contains the binary value 1111111010. The 54 zeroes that follow make the total network prefix the same for all link-local addresses (fe80::/64 link-local address prefix), rendering them non-routable.

Multicast address format

Multicast addresses are formed according to several specific formatting rules, depending on the application.

General multicast address format

bits	8	4	4	112
field	prefix	flg	sc	group ID

The prefix holds the binary value 11111111 for any multicast address.

Currently, 3 of the 4 flag bits in the flg field are defined;^[1] the most-significant flag bit is reserved for future use.

Multicast address flags^[2]

bit	flag	Meaning when 0	Meaning when 1
8	reserved	reserved	reserved
9	R (Rendezvous)[3]	Rendezvous point not embedded	Rendezvous point embedded
10	P (Prefix)[4]	Without prefix information	Address based on network prefix
11	T (Transient)[1]	Well-known multicast address	Dynamically assigned multicast address

The 4-bit scope field (sc) is used to indicate where the address is valid and unique.

There are special multicast addresses, like Solicited Node.

Solicited-Node multicast address format

bits	8	4	4	79	9	24
field	prefix	flg	sc	zeroes	ones	unicast address

The sc(ope) field holds the binary value 0010 (link-local). Solicited-node multicast addresses are computed as a function of a node's unicast or anycast addresses. A solicited-node multicast address is created by copying the last 24 bits of a unicast or anycast address to the last 24 bits of the multicast address.

Unicast-prefix-based multicast address format^[3][4]

bits	8	4	4	4	4	8	64	32
field	prefix	flg	sc	res	riid	plen	network prefix	group ID

Link-scoped multicast addresses use a comparable format.^[5]

Presentation

An IPv6 address is represented as eight groups of four hexadecimal digits, each group representing 16 bits (two octets). The groups are separated by colons (:). An example of an IPv6 address is:

2001:0db8:85a3:0000:0000:8a2e:0370:7334

The hexadecimal digits are case-insensitive, but IETF recommendations suggest the use of lower case letters. The full representation of eight 4-digit groups may be simplified by several techniques, eliminating parts of the representation.

Leading zeroes

Leading zeroes in a group may be omitted.^[1] Thus, the example address may be written as:

2001:db8:85a3:0:0:8a2e:370:7334

Groups of zeroes

One or more consecutive groups of zero value may be replaced with a single empty group using two consecutive colons (::).^[1] Thus, the example address can be further simplified:

2001:db8:85a3::8a2e:370:7334

The localhost (loopback) address, 0:0:0:0:0:0:0:1, and the IPv6 unspecified address, 0:0:0:0:0:0:0:0, are reduced to ::1 and ::, respectively. This two-colon replacement may only be applied once in an address, because multiple occurrences would create an ambiguous representation.

Dotted-quad notation

During the transition of the Internet from IPv4 to IPv6 it is typical to operate in a mixed addressing environment, and for this purpose a special notation has been introduced to express IPv4-mapped and IPv4-compatible IPv6 addresses by writing the final 32 bits of an address in the familiar IPv4 dotted-quad notation. For example, the IPv4-mapped IPv6 address ::ffff:c000:0280 is usually written as ::ffff:192.0.2.128, thus expressing clearly the original IPv4 address that was mapped to IPv6.

Recommended representation as text

In an attempt to simplify IPv6 addresses, the standards provides flexibility in their representation. However, this also complicates several common operations: searching for a specific address in a text file or stream, and comparing two addresses to determine their equivalence. To mitigate these problems, the IETF has proposed a standard in RFC 5952 for a canonical format for rendering IPv6 addresses in text. Its specific recommendations are:

• Leading zeros in each 16-bit field are suppressed. For example, 2001:0db8::0001 is rendered as 2001:db8::1, though any all-zero field that is explicitly presented is rendered as 0.
• Representations are shortened as much as possible. The longest sequence of consecutive all-zero fields is replaced by double-colon. For example, 2001:db8:0:0:0:0:2:1 is shortened to 2001:db8::2:1, but 2001:db8:0000:1:1:1:1:1 is rendered as 2001:db8:0:1:1:1:1:1. If there are multiple longest runs of all-zero fields, then it is the leftmost that is compressed. E.g., 2001:db8:0:0:1:0:0:1 is rendered as 2001:db8::1:0:0:1 rather than as 2001:db8:0:0:1::1.
• Hexadecimal digits are expressed as lower-case letters. For example, 2001:db8::1 is preferred over 2001:DB8::1.

Networks

An IPv6 network uses an address block that is a contiguous group of IPv6 addresses of a size that is a power of two. The leading set of bits of the addresses are identical for all hosts in a given network, and are called the network's address or routing prefix.

Network address ranges are written in CIDR notation. A network is denoted by the first address in the block (ending in all zeroes), a slash (/), and a decimal value equal to the size in bits of the prefix. For example, the network written as 2001:db8:1234::/48 starts at address 2001:db8:1234:0000:0000:0000:0000:0000 and ends at 2001:db8:1234:ffff:ffff:ffff:ffff:ffff.

The routing prefix of an interface address may be directly indicated with the address by CIDR notation. For example, the configuration of an interface with address 2001:db8:a::123 connected to subnet 2001:db8:a::/64 is written as 2001:db8:a::123/64.

Address block sizes

The size of a block of addresses is specified by writing a slash (/) followed by a number in decimal whose value is the length of the network prefix in bits, rather than by explicitly specifying which addresses are in the block. For example, an address block with 48 bits in the prefix is indicated by /48. Such a block contains 2^{128 − 48} = 2⁸⁰ addresses. The smaller the value of the network prefix, the larger the block: a /21 block is 8 times larger than a /24 block.

Literal IPv6 addresses in network resource identifiers

Colon (:) characters in IPv6 addresses may conflict with the established syntax of resource identifiers, such as URIs and URLs. The colon has traditionally been used to terminate the host path before a port number.^[6] To alleviate this conflict, literal IPv6 addresses are enclosed in square brackets in such resource identifiers, for example:

http://[2001:db8:85a3:8d3:1319:8a2e:370:7348]/

When the URL also contains a port number the notation is:

https://[2001:db8:85a3:8d3:1319:8a2e:370:7348]:443/

Literal IPv6 addresses in UNC path names

In Microsoft Windows operating systems, IPv4 addresses are valid location identifiers in Uniform Naming Convention (UNC) path names. However, the colon is an illegal character in a UNC path name. Thus, the use of IPv6 addresses is also illegal in UNC names. For this reason, Microsoft implemented a transcription algorithm to represent an IPv6 address in the form of a domain name that can be used in UNC paths. For this purpose, Microsoft registered and reserved the second-level domain ipv6-literal.net on the Internet. IPv6 addresses are transcribed as a hostname or subdomain name within this name space, in the following fashion:

2001:db8:85a3:8d3:1319:8a2e:370:7348

is written as

2001-db8-85a3-8d3-1319-8a2e-370-7348.ipv6-literal.net

This notation is automatically resolved by Microsoft software without any queries to DNS name servers. If the IPv6 address contains a zone index, it is appended to the address portion after an 's' character:

fe80--1s4.ipv6-literal.net

IPv6 address scopes

Every IPv6 address, except the unspecified address (::), has a "scope",^[7] which specifies in which part of the network it is valid.

In the unicast addressing class, link-local addresses and the loopback address have link-local scope, which means they are to be used in the directly attached network (link). All other addresses (except Unique local addresses) have global (or universal) scope, which means they are globally routable, and can be used to connect to addresses with global scope anywhere, or addresses with link-local scope on the directly attached network.

Unique local addresses are not globally routable, so their scope is limited to the extent of the network(s) in which they are used. These addresses will only be routed by routers or tunnels whose routing tables have been specifically configured to allow it.

The scope of an anycast address is defined identically to that of a unicast address.

For multicasting, the four least-significant bits of the second address octet of a multicast address (ff0s::) identify the address scope, i.e. the span over which the multicast address is propagated. Currently defined scopes^[1] are:

Scope values

Value	Scope name	Notes
0x0	reserved
0x1	interface-local	Interface-local scope spans only a single interface on a node, and is useful only for loopback transmission of multicast.
0x2	link-local	Link-local and site-local multicast scopes span the same topological regions as the corresponding unicast scopes.
0x4	admin-local	Admin-local scope is the smallest scope that must be administratively configured, i.e., not automatically derived from physical connectivity or other, non- multicast-related configuration.
0x5	site-local	Link-local and site-local multicast scopes span the same topological regions as the corresponding unicast scopes.
0x8	organization-local	Organization-local scope is intended to span multiple sites belonging to a single organization.
0xe	global
0xf	reserved

IPv6 address space

General allocation

The management of IPv6 address allocation process is delegated to the Internet Assigned Numbers Authority (IANA)^[8] by the Internet Architecture Board and the Internet Engineering Steering Group. Its main function is the assignment of large address blocks to the regional Internet registries (RIRs), which have the delegated task of allocation to network service providers and other local registries. The IANA has maintained the official list of allocations of the IPv6 address space since December 1995.^[9]

Only one eighth of the total address space is currently allocated for use on the Internet, 2000::/3, in order to provide efficient route aggregation, thereby reducing the size of the Internet routing tables; the rest of the IPv6 address space is reserved for future use or for special purposes. The address space is assigned to the RIRs in large blocks of /23 up to /12.^[10]

The RIRs assign smaller blocks to local Internet registries that distributes them to users. These are typically in sizes from /19 to /32.^[11][12][13] The addresses are typically distributed in /48 to /56 sized blocks to the end users.^[14]

Global unicast assignment records can be found at the various RIRs or other websites.^[15]

IPv6 addresses are assigned to organizations in much larger blocks as compared to IPv4 address assignments—the recommended allocation is a /48 block which contains 2⁸⁰ addresses, being 2⁴⁸ or about 2.8×10¹⁴ times larger than the entire IPv4 address space of 2³² addresses and about 7.2×10¹⁶ times larger than the /8 blocks of IPv4 addresses, which are the largest allocations of IPv4 addresses. The total pool, however, is sufficient for the foreseeable future, because there are 2¹²⁸ or about 3.4×10³⁸ (340 trillion trillion trillion) unique IPv6 addresses.

Each RIR can divide each of its multiple /23 blocks into 512 /32 blocks, typically one for each ISP; an ISP can divide its /32 block into 65536 /48 blocks, typically one for each customer;^[16] customers can create 65536 /64 networks from their assigned /48 block, each having 2⁶⁴ addresses. In contrast, the entire IPv4 address space has only 2³² (about 4.3×10⁹) addresses.

By design, only a very small fraction of the address space will actually be used. The large address space ensures that addresses are almost always available, which makes the use of network address translation (NAT) for the purposes of address conservation completely unnecessary. NAT has been increasingly used for IPv4 networks to help alleviate IPv4 address exhaustion.

Special allocation

To allow for provider changes without renumbering, provider-independent address space – assigned directly to the end user by the RIRs – is taken from the special range 2001:678::/29.

Internet Exchange Points (IXPs) are assigned special addresses from the range 2001:7f8::/29 for communication with their connected ISPs.^[17] Root name servers have been assigned addresses from the same range.

Reserved anycast addresses

The lowest address within each subnet prefix (the interface identifier set to all zeroes) is reserved as the "subnet-router" anycast address.^[1] Applications may use this address when talking to any one of the available routers, as packets sent to this address are delivered to just one router.

The 128 highest addresses within each /64 subnet prefix are reserved to be used as anycast addresses.^[18] These addresses usually have the 57 first bits of the interface identifier set to 1, followed by the 7-bit anycast ID. Prefixes for the network, including subnets, are required to have a length of 64 bits, in which case the universal/local bit must be set to 0 to indicate the address is not globally unique. The address with value 0x7e in the 7 least-significant bits is defined as a mobile IPv6 home agents anycast address. The address with value 0x7f (all bits 1) is reserved and may not be used. No more assignments from this range are made, so values 0x00 through 0x7d are reserved as well.

Special addresses

There are a number of addresses with special meaning in IPv6:^[19]

Unicast Addresses

Unspecified address

•  ::/128 — The address with all zero bits is called the unspecified address (corresponding to 0.0.0.0/32 in IPv4).
  This address must never be assigned to an interface and is to be used only in software before the application has learned its host's source address appropriate for a pending connection. Routers must not forward packets with the unspecified address.
  Applications may be listening on one or more specific interfaces for incoming connections, which are shown in listings of active internet connections by a specific IP address (and a port number, separated by a colon). When the unspecified address is shown it means that an application is listening for incoming connections on all available interfaces.

Default route

•  ::/0 — The default unicast route address (corresponding to 0.0.0.0/0 in IPv4).

Local addresses

•  ::1/128 — The loopback address is a unicast localhost address. If an application in a host sends packets to this address, the IPv6 stack will loop these packets back on the same virtual interface (corresponding to 127.0.0.1/8 in IPv4).
• fe80::/10 — Addresses in the link-local prefix are only valid and unique on a single link. Within this prefix only one subnet is allocated (54 zero bits), yielding an effective format of fe80::/64. The least significant 64 bits are usually chosen as the interface hardware address constructed in modified EUI-64 format. A link-local address is required on every IPv6-enabled interface—in other words, applications may rely on the existence of a link-local address even when there is no IPv6 routing. These addresses are comparable to the auto-configuration addresses 169.254.0.0/16 of IPv4.

Unique local addresses

• fc00::/7 — Unique local addresses (ULAs) are intended for local communication. They are routable only within a set of cooperating sites.^[20] The block is split into two halves, the upper half (fd00::/8) is used for "probabilistically unique" addresses in which a 40-bit pseudorandom number is used to obtain a /48 allocation. This means that there is only a small chance that two sites that wish to merge or communicate with each other will have conflicting addresses. No allocation method for the lower half of the block is currently defined.

Transition from IPv4

•  ::ffff:0:0/96 — This prefix is designated as an IPv4-mapped IPv6 address. With a few exceptions, this address type allows the transparent use of the Transport Layer protocols over IPv4 through the IPv6 networking application programming interface. Server applications only need to open a single listening socket to handle connections from clients using IPv6 or IPv4 protocols. IPv6 clients will be handled natively by default, and IPv4 clients appear as IPv6 clients at their IPv4-mapped IPv6 address. Transmission is handled similarly; established sockets may be used to transmit IPv4 or IPv6 datagram, based on the binding to an IPv6 address, or an IPv4-mapped address. (See also Transition mechanisms.)
•  ::ffff:0:0:0/96 — A prefix used for IPv4-translated addresses which are used by the Stateless IP/ICMP Translation (SIIT) protocol.
• 64:ff9b::/96 — The "Well-Known" Prefix. Addresses with this prefix are used for automatic IPv4/IPv6 translation.^[21]

• 2002::/16 — This prefix is used for 6to4 addressing. Here, an address from the IPv4 network 192.88.99.0/24 is also used.

Special-purpose addresses

IANA has reserved a so-called 'Sub-TLA ID' address block for special assignments^[22][23] which consists of 64 network prefixes in the range 2001:0000::/29 through 2001:01f8::/29. Three assignments from this block have been made:

• 2001::/32 — Used for Teredo tunneling (which also falls into the category of IPv6 transition mechanisms).
• 2001:2::/48 — Assigned to the Benchmarking Methodology Working Group (BMWG)^[24] for benchmarking IPv6 (corresponding to 198.18.0.0/15 for benchmarking IPv4). Note: RFC 5180 specified the prefix 2001:0200::/48, which is not part of the RFC 4773 pool.^[25]
• 2001:10::/28 — ORCHID (Overlay Routable Cryptographic Hash Identifiers).^[26] These are non-routed IPv6 addresses used for Cryptographic Hash Identifiers.

Documentation

• 2001:db8::/32 — This prefix is used in documentation.^[27] The addresses should be used anywhere an example IPv6 address is given or model networking scenarios are described (corresponding to 192.0.2.0/24, 198.51.100.0/24, and 203.0.113.0/24 in IPv4.)^[28]

Deprecated and obsolete addresses

Multicast addresses

The multicast addresses ff00::0/8 are reserved^[1] and should not be assigned to any multicast group. The Internet Assigned Numbers Authority (IANA) manages address reservations.^[29]

Some common IPv6 multicast addresses are the following:

Address	Description	Available Scopes
ff0X::1	All nodes address, identify the group of all IPv6 nodes	Available in scope 1 (interface-local) and 2 (link-local): ff01::1 → All nodes in the interface-local ff02::1 → All nodes in the link-local
ff0X::2	All routers	Available in scope 1 (interface-local), 2 (link-local) and 5 (site-local): ff01::2 → All routers in the interface-local ff02::2 → All routers in the link-local ff05::2 → All routers in the site-local
ff02::5	OSPFIGP	2 (link-local)
ff02::6	OSPFIGP Designated Routers	2 (link-local)
ff02::9	RIP Routers	2 (link-local)
ff02::a	EIGRP Routers	2 (link-local)
ff02::d	All PIM Routers	2 (link-local)
ff02::1a	All RPL Routers	2 (link-local)
ff0X::fb	mDNSv6	Available in all scopes
ff0X::101	All Network Time Protocol (NTP) servers	Available in all scopes
ff02::1:1	Link Name	2 (link-local)
ff02::1:2	All-dhcp-agents	2 (link-local)
ff02::1:3	Link-local Multicast Name Resolution	2 (link-local)
ff05::1:3	All-dhcp-servers	5 (site-local)
ff02::1:ff00:0/104	Solicited-node multicast address. See below	2 (link-local)
ff02::2:ff00:0/104	Node Information Queries	2 (link-local)

Solicited-node multicast address

The least significant 24 bits of the solicited-node multicast address group ID are filled with the least significant 24 bits of the interface's unicast or anycast address. These addresses allow link-layer address resolution via Neighbor Discovery Protocol (NDP) on the link without disturbing all nodes on the local network. A host is required to join a Solicited-Node multicast group for each of its configured unicast or anycast addresses.

Stateless address autoconfiguration

On system startup, a node automatically creates a link-local address on each IPv6-enabled interface, even if globally routable addresses are manually configured or obtained through "configuration protocols" (see below). It does so independently and without any prior configuration by stateless address autoconfiguration (SLAAC),^[30] using a component of the Neighbor Discovery Protocol. This address is selected with the prefix fe80::/64.

In IPv4, typical "configuration protocols" include DHCP or PPP. Although DHCPv6 exists, IPv6 hosts normally use the Neighbor Discovery Protocol to create a globally routable unicast address: the host sends router solicitation requests and an IPv6 router responds with a prefix assignment.^[31]

The lower 64 bits of these addresses are populated with a 64-bit interface identifier in modified EUI-64 format. This identifier is usually shared by all automatically configured addresses of that interface, which has the advantage that only one multicast group needs to be joined for neighbor discovery. For this, a multicast address is used, formed from the network prefix ff02::1:ff00:0/104 and the 24 least significant bits of the address.

Modified EUI-64

A 64-bit interface identifier is most commonly derived from its 48-bit MAC address. A MAC address 00:0C:29:0C:47:D5 is turned into a 64-bit EUI-64 by inserting FF:FE in the middle: 00:0C:29:FF:FE:0C:47:D5. When this EUI-64 is used to form an IPv6 address it is modified:^[1] the meaning of the Universal/Local bit (the 7th most significant bit of the EUI-64, starting from 1) is inverted, so that a 1 now means Universal. To create an IPv6 address with the network prefix 2001:db8:1:2::/64 it yields the address 2001:db8:1:2:020c:29ff:fe0c:47d5 (with the underlined U/L bit inverted to a 1, because the MAC address is universally unique).

Duplicate address detection

The assignment of a unicast IPv6 address to an interface involves an internal test for the uniqueness of that address using Neighbor Solicitation and Neighbor Advertisement (ICMPv6 type 135 and 136) messages. While in the process of establishing uniqueness an address has a tentative state.

The node joins the solicited-node multicast address for the tentative address (if not already done so) and sends neighbor solicitations, with the tentative address as target address and the unspecified address (::/128) as source address. The node also joins the all-hosts multicast address ff02::1, so it will be able to receive Neighbor Advertisements.

If a node receives a neighbor solicitation with its own tentative address as the target address, then that address is not unique. The same is true if the node receives a neighbor advertisement with the tentative address as the source of the advertisement. Only after having successfully established that an address is unique may it be assigned and used by an interface.

Address lifetime

Each IPv6 address that is bound to an interface has a fixed lifetime. Lifetimes are infinite, unless configured to a shorter period. There are two lifetimes that govern the state of an address: the preferred lifetime and the valid lifetime.^[32] Lifetimes can be configured in routers that provide the values used for autoconfiguration, or specified when manually configuring addresses on interfaces.

When an address is assigned to an interface it gets the status "preferred", which it holds during its preferred-lifetime. After that lifetime expires the status becomes "deprecated" and no new connections should be made using this address. The address becomes "invalid" after its valid-lifetime also expires; the address is removed from the interface and may be assigned somewhere else on the Internet.

Note: In most cases, the lifetime does not expire because new RAs refresh the timers. But if there are no more RAs, eventually the preferred lifetime elapses and the address becomes “deprecated.

Temporary addresses

The globally unique and static MAC addresses, used by stateless address autoconfiguration to create interface identifiers, offer an opportunity to track user equipment—across time and IPv6 network prefix changes—and so users.^[33] To reduce the prospect of a user identity being permanently tied to an IPv6 address portion, a node may create temporary addresses with interface identifiers based on time-varying random bit strings^[34] and relatively short lifetimes (hours to days), after which they are replaced with new addresses.

Temporary addresses may be used as source address for originating connections, while external hosts use a public address by querying the Domain Name System.

Network interfaces configured for IPv6 use temporary addresses by default in OS X Lion or later Apple systems, and in Windows Vista, Windows 2008 Server or later Microsoft systems.

Default address selection

IPv6-enabled network interfaces usually have more than one IPv6 address, for example, a link-local and a global address, and permanent versus temporary addresses. IPv6 introduces the concepts of address scope and selection preference, yielding multiple choices for source and destination address selections in communication with another host.

The preference selection algorithm,^[35] which selects the most appropriate address to use in communications with a particular destination (including the use of IPv4-mapped addresses in dual-stack implementations), is based on a user-customizable preference table that associates each routing prefix with a precedence level. The default table is as follows:^[35]

Prefix	Precedence	Label
::1/128	50	0
::/0	40	1
::ffff:0:0/96	35	4
2002::/16	30	2
2001::/32	5	5
fc00::/7	3	13
::/96	1	3
fec0::/10	1	11
3ffe::/16	1	12

The default configuration places preference on IPv6, rather than IPv4, and on destination addresses within the smallest possible scope, so that link-local communication is preferred over globally routed paths when otherwise equally suitable. The prefix policy table is similar to a routing table, with the precedence value serving as the role of a link cost, where higher preference is expressed as a larger value. Source addresses are preferred to have the same label value as the destination address. Addresses are matched to prefixes based on the longest matching most-significant bit-sequence. Candidate source addresses are obtained from the operating system and candidate destination addresses may be queried via the Domain Name System (DNS).

Link-local addresses and zone indices

Because all link-local addresses in a host have a common prefix, normal routing procedures cannot be used to choose the outgoing interface when sending packets to a link-local destination. A special identifier, known as a zone index,^[7] is needed to provide the additional routing information; in the case of link-local addresses, zone indices correspond to interface identifiers.

When an address is written textually, the zone index is appended to the address, separated by a percent sign (%). The actual syntax of zone indices depends on the operating system:

• the Microsoft Windows IPv6 stack uses numeric zone indexes, e.g., fe80::3%1. The index is determined by the interface number;
• most Unix-like systems (e.g., BSD, Linux, OS X) use the interface name as a zone index: fe80::3%eth0.

Zone index notations cause syntax conflicts when used in uniform resource identifiers (URI), so the '%' character must be escaped via percent-encoding:^[36] http://[fe80::3%25eth0]

IPv6 addresses in the Domain Name System

In the Domain Name System hostnames are mapped to IPv6 addresses by AAAA resource records, so-called quad-A records. For reverse lookup the IETF reserved the domain ip6.arpa, where the name space is hierarchically divided by the 1-digit hexadecimal representation of nibble units (4 bits) of the IPv6 address. This scheme is defined in RFC 3596.

As in IPv4, each host is represented in the DNS by two DNS records, an address record and a reverse mapping pointer record. For example, a host computer named justin in zone example.com has the Unique Local Address fdda:5cc1:23:4::1f. Its quad-A address record is

justin.example.com.  IN  AAAA  fdda:5cc1:23:4::1f

and its IPv6 pointer record is

f.1.0.0.0.0.0.0.0.0.0.0.0.0.0.0.4.0.0.0.3.2.0.0.1.c.c.5.a.d.d.f.ip6.arpa.  IN  PTR   justin.example.com.

This pointer record may be defined in a number of zones, depending on the chain of delegation of authority in the zone d.f.ip6.arpa.

The DNS protocol is independent of its Transport Layer protocol. Queries and replies may be transmitted over IPv6 or IPv4 transports regardless of the address family of the data requested.

AAAA record fields

NAME	Domain name
TYPE	AAAA (28)
CLASS	Internet (1)
TTL	Time to live in seconds
RDLENGTH	Length of RDATA field
RDATA	128-bit IPv6 address, network byte order

Transition challenges

As of 2009, many DNS resolvers in home-networking NAT devices and routers still handle AAAA records improperly.^[37] Some of these simply drop DNS requests for such records, instead of properly returning the appropriate negative DNS response. Because the request is dropped, the host sending the request has to wait for a timeout to trigger. This often causes a slow-down when connecting to dual-stack IPv6/IPv4 hosts, as the client software will wait for the IPv6 connection to fail before trying IPv4. Use of the Happy Eyeballs algorithm by client software can mitigate this problem, by trying both IPv6 and IPv4 connections simultaneously, then using whichever connects first.

Historical notes

Deprecated and obsolete addresses

• The site-local prefix fec0::/10 specifies that the address is valid only within the site network of an organization. It was part of the original addressing architecture^[38] in December 1995, but its use was deprecated in September 2004^[39] because the definition of the term site was ambiguous, which led to confusing routing rules. New networks must not support this special type of address. In October 2005, a new specification^[40] replaced this address type with unique local addresses.

• The address block 0200::/7 was defined as an OSI NSAP-mapped prefix set in August 1996,^[41][42] but was deprecated in December 2004.^[43]

• The 96-bit zero-value prefix ::/96, originally known as IPv4-compatible addresses, was mentioned in 1995^[38] but first described in 1998.^[44] This class of addresses was used to represent IPv4 addresses within an IPv6 transition technology. Such an IPv6 address has its first (most significant) 96 bits set to zero, while its last 32 bits are the IPv4 address that is represented. In February 2006, the Internet Engineering Task Force (IETF) has deprecated the use of IPv4-compatible addresses.^[1] The only remaining use of this address format is to represent an IPv4 address in a table or database with fixed size members that must also be able to store an IPv6 address.

• Address block 3ffe::/16 was allocated for test purposes for the 6bone network in December 1998.^[44] Prior to that, the address block 5F00::/8 was used for this purpose. Both address blocks were returned to the address pool in June 2006.^[45]

Miscellany

• IPv6 addresses were originally registered in the Domain Name System (DNS) in the ip6 zone under the int top-level domain for reverse lookups. In 2000, the Internet Architecture Board (IAB) reverted their intentions to retire arpa, and decided in 2001 that the arpa top-level domain should retain its original function. Domains in ip6.int should be moved to ip6.arpa.^[46] The ip6.int zone was officially removed on 6 June 2006.

• In March 2011, the IETF refined their recommendations for allocation of address blocks to end sites.^[14] Instead of assigning either a /48, /64, or /128 (according to IAB's and IESG's views of 2001),^[47] Internet service providers should consider assigning smaller blocks (for example a /56) to end users. The ARIN, RIPE & APNIC regional registries' policies encourage /56 assignments where appropriate.^[14]

References

External links

• IP Version 6 multicast addresses
• Beijnum, van, Iljitsch (2005). Running IPv6. ISBN 1-59059-527-0. 
• Elz, Robert (1996-04-01). "A Compact Representation of IPv6 Addresses (RFC1924)". IETF. "Represent any IPv6 address in 20 octets."  This humorous RFC specifies an alternative way of representing IPv6 addresses, using a base-85 encoding.

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