IPv6 address
An Internet Protocol Version 6 address (IPv6 address) is a numeric label that is used to identify and locate a network interface of a computer or a network node participating in an computer network using IPv6. IP addresses are included in the packet header to indicate the source and the destination of each packet. The IP address of the destination is used to make decisions about routing IP packets to other networks. IPv6 is the successor to the first addressing infrastructure of the Internet, 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, in comparison, IPv6 has a vastly enlarged address space. Addressing methods. IPv6 addresses are classified by the primary addressing and routing methodologies common in networking: unicast addressing, anycast addressing, and multicast addressing. 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 that 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 . 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. For each of the major addressing and routing methodologies, various address formats are recognized by dividing the 128 address bits into bit groups and using established 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. 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" 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. Unique local addresses are addresses analogous to IPv4 private network addresses. 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 routing prefix. A link-local address is also based on the interface identifier, but uses a different format  for the network prefix. 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 ( 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. For all multicast addresses, the "prefix" field holds the binary value 11111111. Currently, three of the four flag bits in the "flg" field are defined; the most-significant flag bit is reserved for future use. The four-bit scope field ("sc") is used to indicate where  the address is valid and unique. In addition, the scope field is used to identify special multicast addresses, like solicited node. 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. Link-scoped multicast addresses use a comparable format. Representation. An IPv6 address is represented as eight groups of four hexadecimal digits, each group representing  16 bits The groups are separated by colons (:). An example of an IPv6 address is: The standards provide flexibility in the representation of IPv6 addresses. The full representation of eight four-digit groups may be simplified by several techniques, eliminating parts of the representation. In general, representations are shortened as much as possible. However, this practice complicates several common operations, namely searching for a specific address or an address pattern in text documents or streams, and comparing addresses to determine equivalence. For mitigation of these complications, the IETF has defined a canonical format for rendering IPv6 addresses in text: These  methods can lead to very short representations for IPv6 addresses. For example, the localhost (loopback) address, , and the IPv6 unspecified address, , are reduced to and , respectively. During the transition of the Internet from IPv4 to IPv6, it is typical to operate in a mixed addressing environment. For such use cases, a special notation has been introduced, which expresses IPv4-mapped and IPv4-compatible IPv6 addresses by writing the least-significant 32 bits of an address in the familiar IPv4 dot-decimal notation, whereas the 96 most-significant bits are written in IPv6 format. For example, the IPv4-mapped IPv6 address  is written as , thus expressing clearly the original IPv4 address that was mapped to IPv6. 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 starts at address and ends at . The routing prefix of an interface address may be directly indicated with the address using CIDR notation. For example, the configuration of an interface with address connected to subnet is written as . 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. For example, an address block with 48 bits in the prefix is indicated by . Such a block contains  2 = 2 addresses. The smaller the value of the network prefix, the larger the block: a block is 8 times larger than a 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 is conventionally used to terminate the host path before a port number. To alleviate this conflict, literal IPv6 addresses are enclosed in square brackets in such resource identifiers, for example: When the URL also contains a port number the notation is: where the trailing 443 is the  example's port number. Scoped literal IPv6 addresses (with zone index). For addresses with other than global scope (as described below), and in particular for link-local addresses, the choice of the network interface for sending a packet may depend on which zone the address belongs to: the same address may be valid in different zones, and be in use by a different host in each of those zones. Even if a single address is not in use in different zones, the address prefixes for addresses in those zones may still be identical, which makes the operating system unable to  select an outgoing interface based on the information in the routing table (which is prefix-based). In order to resolve the ambiguity in textual addresses, a "" must be appended to the address, the two separated by a percent sign (%). The syntax of zone indices is an implementation-dependent string, although numeric zone indices must be universally supported as well. The link-local address could be expressed by or: The former (using an interface name) is customary on most Unix-like operating systems (e.g., BSD, Linux, macOS). The latter (using an interface number) is the standard syntax on Microsoft Windows, but  as support for this syntax is mandatory, it is also available on other operating systems. BSD-based operating systems (including macOS) also support an alternative, non-standard syntax, where a numeric zone index is encoded in the second 16-bit word of the address. E.g.: In all operating systems mentioned above, the zone index for link-local addresses actually refers to an interface, not to a zone. As multiple interfaces may belong to the same zone (e.g. when connected to the same switch), in practice two addresses with different zone identifiers may actually be equivalent, and refer to the same  host on the same link. Use of zone indices in URIs. When used in uniform resource identifiers (URI), the use of the percent sign causes a syntax conflict, therefore it must be escaped via percent-encoding, e.g.: 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 (although they gave up the domain in January 2014). IPv6 addresses are transcribed as a hostname or subdomain name within this name space, in the following fashion: is written as This notation is automatically resolved locally 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: is written as Address scopes. Every IPv6  address, except the unspecified address (), has a "scope", which specifies in which part of the network it is valid. Unicast. For unicast addresses, two scopes are defined: link-local and global. Link-local addresses and the loopback address have "link-local" scope, which means they can only be used on a single directly attached network (link). All other addresses (including Unique local addresses) have "global" (or "universal") scope, which means they are (or could be) globally routable, and can be used to connect to addresses with "global" scope anywhere, or to addresses with "link-local" scope on the directly attached  network. Packets with a source or destination in one scope cannot be routed to a different scope. Unique local addresses have global scope, but they are not globally administered. As a result, only other hosts in the same administrative domain (e.g., an organization), or within a cooperating administrative domain are able to reach such addresses, if properly routed. As their scope is global, these addresses are valid as a source address when communicating with any other global-scope address, even though it may be impossible to route packets from the destination back to the source. Anycast. Anycast addresses are  syntactically identical to and indistinguishable from unicast addresses. Their only difference is administrative. Scopes for anycast addresses are therefore the same as for unicast addresses. Multicast. For multicast addresses, the four least-significant bits of the second address octet () identify the address scope, i.e. the domain in which the multicast packet should be propagated. Predefined and reserved scopes are: All other scopes are unassigned, and available to administrators for defining additional regions. Address space. General allocation. The management of IPv6 address allocation process is delegated to the Internet Assigned Numbers Authority (IANA) 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. Only one eighth of the total address space is currently allocated for use on the Internet, , 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 up to . The RIRs assign smaller blocks to local Internet registries that distributes them to users. These are typically in sizes from to . The addresses are typically distributed in to sized blocks to the end users. Global unicast assignment records can be found at the various RIRs or other websites. IPv6 addresses are assigned to organizations in much larger blocks as compared to IPv4 address assignments—the recommended allocation is a block which contains 2 addresses, being 2 or about times larger than the entire IPv4 address space of  2 addresses and about times larger than the 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 2 (exactly 340,282,366,920,938,463,463,374,607,431,768,211,456) or about (340 trillion trillion trillion) unique IPv6 addresses. Each RIR can divide each of its multiple blocks into 512 blocks, typically one for each ISP; an ISP can divide its block into blocks, typically one for each customer; customers can create networks from their assigned block, each having 2 (18,446,744,073,709,551,616) addresses. In contrast, the entire IPv4 address space has only 2 (exactly 4,294,967,296  or about ) 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 . Internet Exchange Points (IXPs) are assigned special addresses from the ranges , , and  for communication with their connected ISPs. Root name servers have been assigned addresses from the 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. 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 subnet prefix are reserved to be used as anycast addresses. These addresses usually have the first 57 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. They represent less than 2% of the entire address space: Multicast addresses. The multicast addresses where is any hexadecimal value are reserved and should not be assigned to any multicast group. The Internet Assigned Numbers Authority (IANA) manages address reservations. Some common IPv6 multicast addresses are the following: 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), using a component of the Neighbor Discovery Protocol. This address is selected with the prefix . 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. 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 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 is turned into a 64-bit EUI-64 by inserting in the middle: . When this EUI-64 is used to form an IPv6 address, it is modified: 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 it yields the address (with the "Universal/Local" bit, the second-least-significant bit of  the underlined quartet, inverted to 1 in this case 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 () as source address. The  node also joins the all-hosts multicast address , 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". 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 Router Advertisements (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. 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 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 and later Apple systems as well as in Windows Vista, Windows 2008 Server and later Microsoft systems. Cryptographically generated addresses. As a means to enhance security for Neighbor Discovery Protocol "cryptographically generated addresses" (or CGAs) were introduced in 2005 as  part of the Secure Neighbor Discovery (SEND) Protocol. Such an address is generated using two hash functions that take several inputs. The first uses a public key and a random modifier; the latter being incremented repeatedly until a specific amount of zero bits of the resulting hash is acquired. The second hash function takes the network prefix and the previous hash value. The least significant 64 bits of the second hash result is appended to the 64-bit network prefix to form a 128-bit address. The hash functions can also be used to verify if a specific IPv6 address  satisfies the requirement of being a valid CGA. This way, communication can be set up between trusted addresses exclusively. Stable privacy addresses. The use of stateless autoconfigured addresses has serious implications for security and privacy concerns, because the underlying hardware address (most typically the MAC address) is exposed beyond the local network, permitting the tracking of user activities and correlation of user accounts to other information. It also permits vendor-specific attack strategies, and reduces the size of the address space for searching for attack targets. Stable privacy addresses were introduced to remedy these shortcomings. They are stable within a  specific network but change when moving to another, to improve privacy. They are chosen deterministically, but randomly, in the entire address space of the network. Generation of a stable privacy address is based on a hash function that uses several stable parameters. It is implementation specific, but it is recommended to use at least the network prefix, the name of the network interface, a duplicate address counter, and a secret key. The resulting hash value is used to construct the final address: Typically the 64 least significant bits are concatenated to the 64-bit network prefix, to yield a 128 -bit address. If the network prefix is smaller than 64 bits, more bits of the hash are used. If the resulting address does not conflict with existing or reserved addresses, it is assigned to the interface. Default address selection. IPv6-enabled network interfaces usually have more than one IPv6 address, for example, a link-local and a global address. They may also have temporary addresses that change after a certain lifetime has expired. 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 published in RFC 6724 selects the most appropriate address to use in communications with a particular destination, including the use of IPv4-mapped addresses in dual-stack implementations. It uses a configurable preference table that associates each routing prefix with a precedence level. The default table has the following content: The default configuration places preference on IPv6 usage, and selects 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). For minimizing the time of establishing connections when multiple addresses are available for communication, the Happy Eyeballs algorithm was devised. It queries the Domain Name System for IPv6 and IPv4 addresses of the target host, sorts  candidate addresses using the default address selection table, and tries to establish connections in parallel. The first connection that is established aborts current and future attempts to connect to other addresses. 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. 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 "derrick" in zone "example.com" has the Unique Local Address . Its quad-A address record is and its IPv6 pointer record is 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.