IP Version 6 Addressing Architecture

IPv6 addresses are 128-bit identifiers for interfaces and sets of interfaces (where "interface" is as defined in Section 2 of ). There are three types of addresses: An identifier for a single interface. A packet sent to a unicast address is delivered to the interface identified by that address. An identifier for a set of interfaces (typically belonging to different nodes). A packet sent to an anycast address is delivered to one of the interfaces identified by that address (the "nearest" one, according to the routing protocols' measure of distance). An identifier for a set of interfaces (typically belonging to different nodes). A packet sent to a multicast address is delivered to all interfaces identified by that address. There are no broadcast addresses in IPv6, their function being superseded by multicast addresses. In this document, fields in addresses are given a specific name, for example, "subnet". When this name is used with the term "ID" for identifier after the name (e.g., "subnet ID"), it refers to the contents of the named field. When it is used with the term "prefix" (e.g., "subnet prefix"), it refers to all of the address from the left up to and including this field. In IPv6, all zeros and all ones are legal values for any field, unless specifically excluded. Specifically, prefixes may contain, or end with, zero-valued fields.

IPv6 addresses of all types are assigned to interfaces, not nodes. An IPv6 unicast address refers to a single interface. Since each interface belongs to a single node, any of that node's interfaces' unicast addresses may be used as an identifier for the node. All interfaces are required to have at least one Link-Local unicast address (see for additional required addresses). A single interface may also have multiple IPv6 addresses of any type (unicast, anycast, and multicast) or scope. Unicast addresses with a scope greater than link-scope are not needed for interfaces that are not used as the origin or destination of any IPv6 packets to or from non-neighbors. This is sometimes convenient for point-to-point interfaces. There is one exception to this addressing model: A unicast address or a set of unicast addresses may be assigned to multiple physical interfaces if the implementation treats the multiple physical interfaces as one interface when presenting it to the internet layer. This is useful for load-sharing over multiple physical interfaces. Currently, IPv6 continues the IPv4 model in that a subnet prefix is associated with one link. Multiple subnet prefixes may be assigned to the same link.

There are three conventional forms for representing IPv6 addresses as text strings: The preferred form is x:x:x:x:x:x:x:x, where the 'x's are one to four hexadecimal digits of the eight 16-bit pieces of the address. Examples: abcd:ef01:2345:6789:abcd:ef01:2345:6789 2001:db8:0:0:8:800:200c:417a Note that it is not necessary to write the leading zeros in an individual field, but there must be at least one numeral in every field (except for the case described in 2.). Due to some methods of allocating certain styles of IPv6 addresses, it will be common for addresses to contain long strings of zero bits. In order to make writing addresses containing zero bits easier, a special syntax is available to compress the zeros. The use of "::" indicates one or more groups of 16 bits of zeros. The "::" can only appear once in an address. The "::" can also be used to compress leading or trailing zeros in an address. For example, the following addresses a unicast address a multicast address the loopback address the unspecified address may be represented as a unicast address a multicast address the loopback address the unspecified address An alternative form that is sometimes more convenient when dealing with a mixed environment of IPv4 and IPv6 nodes is x:x:x:x:x:x:d.d.d.d, where the 'x's are the hexadecimal values of the six high-order 16-bit pieces of the address, and the 'd's are the decimal values of the four low-order 8-bit pieces of the address (standard IPv4 representation). Examples: 0:0:0:0:0:0:13.1.68.3 0:0:0:0:0:ffff:129.144.52.38 or in compressed form: ::13.1.68.3 ::ffff:129.144.52.38

The text representation of IPv6 address prefixes is similar to the way IPv4 address prefixes are written in Classless Inter-Domain Routing (CIDR) notation . An IPv6 address prefix is represented by the notation: ipv6-address/prefix-length where is an IPv6 address in any of the notations listed in . is a decimal value specifying how many of the leftmost contiguous bits of the address comprise the prefix. For example, the following are legal representations of the 60-bit prefix 20010db80000cd3 (hexadecimal): 2001:0db8:0000:cd30:0000:0000:0000:0000/60 2001:0db8::cd30:0:0:0:0/60 2001:0db8:0:cd30::/60 The following are NOT legal representations of the above prefix: may drop leading zeros, but not trailing zeros, within any 16-bit chunk of the address address to left of "/" expands to 2001:0db8:0000:0000:0000:0000:0000:cd30 address to left of "/" expands to 2001:0db8:0000:0000:0000:0000:0000:0cd3 When writing both a node address and a prefix of that node address (e.g., the node's subnet prefix), the two can be combined as follows: 2001:0db8:0:cd30:123:4567:89ab:cdef 2001:0db8:0:cd30::/60 2001:0db8:0:cd30:123:4567:89ab:cdef/60

This section provides a recommendation for systems generating and outputting IPv6 addresses as text. Note, all implementations must accept and process all addresses in the formats defined in the previous two sections of this document. The recommendations are as follows: The hexadecimal digits "a", "b", "c", "d", "e", and "f" in an IPv6 address must be represented in lowercase. Leading zeros in a 16-Bit Field must be suppressed. For example, 2001:0db8::0001 is not correct and must be represented as 2001:db8::1 A single 16-bit 0000 field must be represented as 0. The use of the symbol "::" must be used to its maximum capability. For example: 2001:db8:0:0:0:0:2:1 must be shortened to 2001:db8::2:1 Likewise, 2001:db8::0:1 is not correct, because the symbol "::" could have been used to produce a shorter representation 2001:db8::1. When there is an alternative choice in the placement of a "::", the longest run of consecutive 16-bit 0 fields must be shortened, that is, in 2001:0:0:1:0:0:0:1 the sequence with three consecutive zero fields is shortened to 2001:0:0:1::1 When the length of the consecutive 16-bit 0 fields are equal, for example 2001:db8:0:0:1:0:0:1 the first sequence of zero bits must be shortened. For example 2001:db8::1:0:0:1 is the correct representation. The symbol "::" must not be used to shorten just one 16-bit 0 field. For example, the representation 2001:db8:0:1:1:1:1:1 is correct, but 2001:db8::1:1:1:1:1 is not correct. The text representation method describe in this section should also be use for text Representation of IPv6 Address Prefixes. For example 0:0:0:0:0:ffff:192.0.2.1 should be shown as ::ffff:192.0.2.1 The text representation method describe in this section should be applied for IPv6 addresses with embedded IPv4 address. For example 2001:0db8:0000:cd30:0000:0000:0000:0000/60 should be shown as 2001:0db8:0:cd30::/60

The type of an IPv6 address is identified by the high-order bits of the address, as follows:

Anycast addresses are taken from the unicast address spaces (of any scope) and are not syntactically distinguishable from unicast addresses. The general format of Global Unicast addresses is described in . Some special-purpose subtypes of Global Unicast addresses that contain embedded IPv4 addresses (for the purposes of IPv4-IPv6 interoperation) are described in . Future specifications may redefine one or more sub-ranges of the Global Unicast space for other purposes, but unless and until that happens, implementations must treat all addresses that do not start with any of the above-listed prefixes as Global Unicast addresses. The current assigned IPv6 prefixes and references to their usage can be found in the IANA Internet Protocol Version 6 Address Space registry and the IANA IPv6 Special-Purpose Address Registry .

IPv6 unicast addresses are aggregatable with prefixes of arbitrary bit-length, similar to IPv4 addresses under Classless Inter-Domain Routing. There are several types of unicast addresses in IPv6, in particular, Global Unicast, Local unicast, and Link-Local unicast. There are also some special-purpose subtypes of Global Unicast, such as IPv6 addresses with embedded IPv4 addresses. Additional address types or subtypes can be defined in the future. IPv6 nodes may have considerable or little knowledge of the internal structure of the IPv6 address, depending on the role the node plays (for instance, host versus router). At a minimum, a node may consider that unicast addresses (including its own) have no internal structure:

A slightly sophisticated host (but still rather simple) may additionally be aware of subnet prefix(es) for the link(s) it is attached to, where different addresses may have different values for n:

Though a very simple router may have no knowledge of the internal structure of IPv6 unicast addresses, routers will more generally have knowledge of one or more of the hierarchical boundaries for the operation of routing protocols. The known boundaries will differ from router to router, depending on what positions the router holds in the routing hierarchy. Except for the knowledge of the subnet boundary discussed in the previous paragraphs, nodes should not make any assumptions about the structure of an IPv6 address.

Interface identifiers in IPv6 unicast addresses are used to identify interfaces on a link. They are required to be unique within a subnet prefix. It is recommended that the same interface identifier not be assigned to different nodes on a link. They may also be unique over a broader scope. The same interface identifier may be used on multiple interfaces on a single node, as long as they are attached to different subnets. Interface IDs must be viewed outside of the node that created Interface ID as an opaque bit string without any internal structure. Note that the uniqueness of interface identifiers is independent of the uniqueness of IPv6 addresses. For example, a Global Unicast address may be created with an interface identifier that is only unique on a single subnet, and a Link-Local address may be created with interface identifier that is unique over multiple subnets. For all unicast addresses, except those that start with the binary value 000, Interface IDs are required to be 64 bits long. Background on the 64 bit boundary in IPv6 addresses can be found in . The details of forming interface identifiers are defined in other specifications, such as "Privacy Extensions for Stateless Address Autoconfiguration in IPv6" or "A Method for Generating Semantically Opaque Interface Identifiers with IPv6 Stateless Address Autoconfiguration (SLAAC)". Specific cases are described in appropriate "IPv6 over <link>" specifications, such as "IPv6 over Ethernet" and "Transmission of IPv6 Packets over ITU-T G.9959 Networks" . The security and privacy considerations for IPv6 address generation is described in . Earlier versions of this document described a method of forming interface identifiers derived from IEEE MAC-layer addresses call Modified EUI-64 format. These are described in Appendix A and are no longer recommended.

The address 0:0:0:0:0:0:0:0 is called the unspecified address. It must never be assigned to any node. It indicates the absence of an address. One example of its use is in the Source Address field of any IPv6 packets sent by an initializing host before it has learned its own address. The unspecified address must not be used as the destination address of IPv6 packets or in IPv6 Routing headers. An IPv6 packet with a source address of unspecified must never be forwarded by an IPv6 router.

The unicast address 0:0:0:0:0:0:0:1 is called the loopback address. It may be used by a node to send an IPv6 packet to itself. It must not be assigned to any physical interface. It is treated as having Link-Local scope, and may be thought of as the Link-Local unicast address of a virtual interface (typically called the "loopback interface") to an imaginary link that goes nowhere. The loopback address must not be used as the source address in IPv6 packets that are sent outside of a single node. An IPv6 packet with a destination address of loopback must never be sent outside of a single node and must never be forwarded by an IPv6 router. A packet received on an interface with a destination address of loopback must be dropped.

The general format for IPv6 Global Unicast addresses is as follows:

where the global routing prefix is a (typically hierarchically- structured) value assigned to a site (a cluster of subnets/links), the subnet ID is an identifier of a link within the site, and the interface ID is as defined in . All Global Unicast addresses other than those that start with binary 000 have a 64-bit interface ID field (i.e., n + m = 64), formatted as described in . Global Unicast addresses that start with binary 000 have no such constraint on the size or structure of the interface ID field. Examples of Global Unicast addresses that start with binary 000 are the IPv6 address with embedded IPv4 addresses described in . An example of global addresses starting with a binary value other than 000 (and therefore having a 64-bit interface ID field) can be found in .

Two types of IPv6 addresses are defined that carry an IPv4 address in the low-order 32 bits of the address. These are the "IPv4-Compatible IPv6 address" and the "IPv4-mapped IPv6 address".

The "IPv4-Compatible IPv6 address" was defined to assist in the IPv6 transition. The format of the "IPv4-Compatible IPv6 address" is as follows:

Note: The IPv4 address used in the "IPv4-Compatible IPv6 address" must be a globally-unique IPv4 unicast address. The "IPv4-Compatible IPv6 address" is now deprecated because the current IPv6 transition mechanisms no longer use these addresses. New or updated implementations are not required to support this address type.

A second type of IPv6 address that holds an embedded IPv4 address is defined. This address type is used to represent the addresses of IPv4 nodes as IPv6 addresses. The format of the "IPv4-mapped IPv6 address" is as follows:

See for background on the usage of the "IPv4-mapped IPv6 address".

Link-Local addresses are for use on a single link. Link-Local addresses have the following format:

Link-Local addresses are designed to be used for addressing on a single link for purposes such as automatic address configuration, neighbor discovery, or when no routers are present. Routers must not forward any packets with Link-Local source or destination addresses to other links.

Unique Local Addresses (ULA) , the current form of Local IPv6 Addresses, are intended to be used for local communications, have global unicast scope, and are not expected to be routable on the global Internet. Site-Local addresses, deprecated by , the previous form of Local IPv6 Addresses, were originally designed to be used for addressing inside of a site without the need for a global prefix. The special behavior of Site-Local defined in must no longer be supported in new implementations (i.e., new implementations must treat this prefix as Global Unicast). Existing implementations and deployments may continue to use this prefix.

An IPv6 anycast address is an address that is assigned to more than one interface (typically belonging to different nodes), with the property that a packet sent to an anycast address is routed to the "nearest" interface having that address, according to the routing protocols' measure of distance. Anycast addresses are allocated from the unicast address space, using any of the defined unicast address formats. Thus, anycast addresses are syntactically indistinguishable from unicast addresses. When a unicast address is assigned to more than one interface, thus turning it into an anycast address, the nodes to which the address is assigned must be explicitly configured to know that it is an anycast address. For any assigned anycast address, there is a longest prefix P of that address that identifies the topological region in which all interfaces belonging to that anycast address reside. Within the region identified by P, the anycast address must be maintained as a separate entry in the routing system (commonly referred to as a "host route"); outside the region identified by P, the anycast address may be aggregated into the routing entry for prefix P. Note that in the worst case, the prefix P of an anycast set may be the null prefix, i.e., the members of the set may have no topological locality. In that case, the anycast address must be maintained as a separate routing entry throughout the entire Internet, which presents a severe scaling limit on how many such "global" anycast sets may be supported. Therefore, it is expected that support for global anycast sets may be unavailable or very restricted. One expected use of anycast addresses is to identify the set of routers belonging to an organization providing Internet service. Such addresses could be used as intermediate addresses in an IPv6 Routing header, to cause a packet to be delivered via a particular service provider or sequence of service providers. Some other possible uses are to identify the set of routers attached to a particular subnet, or the set of routers providing entry into a particular routing domain.

The Subnet-Router anycast address is predefined. Its format is as follows:

The "subnet prefix" in an anycast address is the prefix that identifies a specific link. This anycast address is syntactically the same as a unicast address for an interface on the link with the interface identifier set to zero. Packets sent to the Subnet-Router anycast address will be delivered to one router on the subnet. All routers are required to support the Subnet-Router anycast addresses for the subnets to which they have interfaces. The Subnet-Router anycast address is intended to be used for applications where a node needs to communicate with any one of the set of routers.

An IPv6 multicast address is an identifier for a group of interfaces (typically on different nodes). An interface may belong to any number of multicast groups. Multicast addresses have the following format:

binary 11111111 at the start of the address identifies the address as being a multicast address.

The high-order flag is reserved, and must be initialized to 0. T = 0 indicates a permanently-assigned ("well-known") multicast address, assigned by the Internet Assigned Numbers Authority (IANA). T = 1 indicates a non-permanently-assigned ("transient" or "dynamically" assigned) multicast address. The P flag's definition and usage can be found in . The R flag's definition and usage can be found in . scop is a 4-bit multicast scope value used to limit the scope of the multicast group. The values are as follows: 0 reserved 1 Interface-Local scope 2 Link-Local scope 3 Realm-Local scope 4 Admin-Local scope 5 Site-Local scope 6 (unassigned) 7 (unassigned) 8 Organization-Local scope 9 (unassigned) A (unassigned) B (unassigned) C (unassigned) D (unassigned) E Global scope F reserved Interface-Local scope spans only a single interface on a node and is useful only for loopback transmission of multicast. Packets with interface-local scope received from another node must be discarded. Link-Local multicast scope spans the same topological region as the corresponding unicast scope. Interface-Local, Link-Local, and Realm-Local scope boundaries are automatically derived from physical connectivity or other non-multicast-related configurations. Global scope has no boundary. The boundaries of all other non-reserved scopes of Admin-Local or larger are administratively configured. For reserved scopes, the way of configuring their boundaries will be defined when the semantics of the scope are defined. According to , the zone of a Realm-Local scope must fall within zones of larger scope. Because the zone of a Realm-Local scope is configured automatically while the zones of larger scopes are configured manually, care must be taken in the definition of those larger scopes to ensure that the inclusion constraint is met. Realm-Local scopes created by different network technologies are considered to be independent and will have different zone indices (see Section 6 of ). A router with interfaces on links using different network technologies does not forward traffic between the Realm-Local multicast scopes defined by those technologies. Site-Local scope is intended to span a single site. Organization-Local scope is intended to span multiple sites belonging to a single organization. scopes labeled "(unassigned)" are available for administrators to define additional multicast regions. group ID identifies the multicast group, either permanent or transient, within the given scope. Additional definitions of the multicast group ID field structure are provided in . The "meaning" of a permanently-assigned multicast address is independent of the scope value. For example, if the "NTP servers group" is assigned a permanent multicast address with a group ID of 101 (hex), then ff01:0:0:0:0:0:0:101 means all NTP servers on the same interface (i.e., the same node) as the sender. ff02:0:0:0:0:0:0:101 means all NTP servers on the same link as the sender. ff05:0:0:0:0:0:0:101 means all NTP servers in the same site as the sender. ff0e:0:0:0:0:0:0:101 means all NTP servers in the Internet. Non-permanently-assigned multicast addresses are meaningful only within a given scope. For example, a group identified by the non- permanent, site-local multicast address ff15:0:0:0:0:0:0:101 at one site bears no relationship to a group using the same address at a different site, nor to a non-permanent group using the same group ID with a different scope, nor to a permanent group with the same group ID. Multicast addresses must not be used as source addresses in IPv6 packets or appear in any Routing header. Routers must not forward any multicast packets beyond the scope indicated by the scop field in the destination multicast address. Nodes must not originate a packet to a multicast address whose scop field contains the reserved value 0; if such a packet is received, it must be silently dropped. Nodes should not originate a packet to a multicast address whose scop field contains the reserved value F; if such a packet is sent or received, it must be treated the same as packets destined to a global (scop E) multicast address.

The following well-known multicast addresses are pre-defined. The group IDs defined in this section are defined for explicit scope values. Use of these group IDs for any other scope values, with the T flag equal to 0, is not allowed. ff00:0:0:0:0:0:0:0 ff01:0:0:0:0:0:0:0 ff02:0:0:0:0:0:0:0 ff03:0:0:0:0:0:0:0 ff04:0:0:0:0:0:0:0 ff05:0:0:0:0:0:0:0 ff06:0:0:0:0:0:0:0 ff07:0:0:0:0:0:0:0 ff08:0:0:0:0:0:0:0 ff09:0:0:0:0:0:0:0 ff0a:0:0:0:0:0:0:0 ff0b:0:0:0:0:0:0:0 ff0c:0:0:0:0:0:0:0 ff0d:0:0:0:0:0:0:0 ff0e:0:0:0:0:0:0:0 ff0f:0:0:0:0:0:0:0 The above multicast addresses are reserved and shall never be assigned to any multicast group. ff01:0:0:0:0:0:0:1 ff02:0:0:0:0:0:0:1 The above multicast addresses identify the group of all IPv6 nodes, within scope 1 (interface-local) or 2 (link-local). ff01:0:0:0:0:0:0:2 ff02:0:0:0:0:0:0:2 ff05:0:0:0:0:0:0:2 The above multicast addresses identify the group of all IPv6 routers, within scope 1 (interface-local), 2 (link-local), or 5 (site-local). ff02:0:0:0:0:1:ffxx:xxxx Solicited-Node multicast address are computed as a function of a node's unicast and anycast addresses. A Solicited-Node multicast address is formed by taking the low-order 24 bits of an address (unicast or anycast) and appending those bits to the prefix FF02:0:0:0:0:1:FF00::/104 resulting in a multicast address in the range ff02:0:0:0:0:1:ff00:0000 to ff02:0:0:0:0:1:ffff:ffff For example, the Solicited-Node multicast address corresponding to the IPv6 address 4037::01:800:200e:8c6c is ff02::1:ff0e:8c6c. IPv6 addresses that differ only in the high-order bits (e.g., due to multiple high-order prefixes associated with different aggregations) will map to the same Solicited-Node address, thereby reducing the number of multicast addresses a node must join. A node is required to compute and join (on the appropriate interface) the associated Solicited-Node multicast addresses for all unicast and anycast addresses that have been configured for the node's interfaces (manually or automatically).

A host is required to recognize the following addresses as identifying itself: Its required Link-Local address for each interface. Any additional Unicast and Anycast addresses that have been configured for the node's interfaces (manually or automatically). The loopback address. The All-Nodes multicast addresses defined in . The Solicited-Node multicast address for each of its unicast and anycast addresses. Multicast addresses of all other groups to which the node belongs. A router is required to recognize all addresses that a host is required to recognize, plus the following addresses as identifying itself: The Subnet-Router Anycast addresses for all interfaces for which it is configured to act as a router. All other Anycast addresses with which the router has been configured. The All-Routers multicast addresses defined in .