rfc1884.txt

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RFC 1884              IPv6 Addressing Architecture         December 1995


   allocated.  The remaining 85% is reserved for future use.

   Unicast addresses are distinguished from multicast addresses by the
   value of the high-order octet of the addresses: a value of FF
   (11111111) identifies an address as a multicast address; any other
   value identifies an address as a unicast address.  Anycast addresses
   are taken from the unicast address space, and are not syntactically
   distinguishable from unicast addresses.


   2.4 Unicast Addresses

   The IPv6 unicast address is contiguous bit-wise maskable, similar to
   IPv4 addresses under Class-less Interdomain Routing [CIDR].

   There are several forms of unicast address assignment in IPv6,
   including the global provider based unicast address, the geographic
   based unicast address, the NSAP address, the IPX hierarchical
   address, the site-local-use address, the link-local-use address, and
   the IPv4-capable host address.  Additional address types 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:

    |                           128 bits                              |
    +-----------------------------------------------------------------+
    |                          node address                           |
    +-----------------------------------------------------------------+


   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:

    |                         n bits                 |   128-n bits   |
    +------------------------------------------------+----------------+
    |                   subnet prefix                | interface ID   |
    +------------------------------------------------+----------------+


   Still more sophisticated hosts may be aware of other hierarchical
   boundaries in the unicast address.  Though a very simple router may
   have no knowledge of the internal structure of IPv6 unicast



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   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.


   2.4.1 Unicast Address Examples

   An example of a Unicast address format which will likely be common on
   LANs and other environments where IEEE 802 MAC addresses are
   available is:


    |              n bits            | 80-n bits |     48 bits        |
    +--------------------------------+-----------+--------------------+
    |        subscriber prefix       | subnet ID |   interface ID     |
    +--------------------------------+-----------+--------------------+

   Where the 48-bit Interface ID is an IEEE-802 MAC address.  The use of
   IEEE 802 MAC addresses as a interface ID is expected to be very
   common in environments where nodes have an IEEE 802 MAC address.  In
   other environments, where IEEE 802 MAC addresses are not available,
   other types of link layer addresses can be used, such as E.164
   addresses, for the interface ID.

   The inclusion of a unique global interface identifier, such as an
   IEEE MAC address, makes possible a very simple form of auto-
   configuration of addresses.  A node may discover a subnet ID by
   listening to Router Advertisement messages sent by a router on its
   attached link(s), and then fabricating an IPv6 address for itself by
   using its IEEE MAC address as the interface ID on that subnet.

   Another unicast address format example is where a site or
   organization requires additional layers of internal hierarchy.  In
   this example the subnet ID is divided into an area ID and a subnet
   ID.  Its format is:

    |         s bits       | n bits  |   m bits     | 128-s-n-m bits  |
    +----------------------+---------+--------------+-----------------+
    |   subscriber prefix  | area ID |  subnet ID   |  interface ID   |
    +----------------------+---------+--------------+-----------------+

   This technique can be continued to allow a site or organization to
   add additional layers of internal hierarchy.  It may be desirable to
   use an interface ID smaller than a 48-bit IEEE 802 MAC address to
   allow more space for the additional layers of internal hierarchy.
   These could be interface IDs which are administratively created by



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   the site or organization.


   2.4.2 The Unspecified Address

   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 datagrams 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 datagrams or in IPv6 Routing Headers.


   2.4.3 The Loopback Address

   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 datagram to itself.  It may
   never be assigned to any interface.

   The loopback address must not be used as the source address in IPv6
   datagrams that are sent outside of a single node.  An IPv6 datagram
   with a destination address of loopback must never be sent outside of
   a single node.


   2.4.4 IPv6 Addresses with Embedded IPv4 Addresses

   The IPv6 transition mechanisms include a technique for hosts and
   routers to dynamically tunnel IPv6 packets over IPv4 routing
   infrastructure.  IPv6 nodes that utilize this technique are assigned
   special IPv6 unicast addresses that carry an IPv4 address in the
   low-order 32-bits.  This type of address is termed an "IPv4-
   compatible IPv6 address" and has the format:


    |                80 bits               | 16 |      32 bits        |
    +--------------------------------------+--------------------------+
    |0000..............................0000|0000|    IPv4 address     |
    +--------------------------------------+----+---------------------+


   A second type of IPv6 address which holds an embedded IPv4 address is
   also defined.  This address is used to represent the addresses of
   IPv4-only nodes (those that *do not* support IPv6) as IPv6 addresses.
   This type of address is termed an "IPv4-mapped IPv6 address" and has
   the format:



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    |                80 bits               | 16 |      32 bits        |
    +--------------------------------------+--------------------------+
    |0000..............................0000|FFFF|    IPv4 address     |
    +--------------------------------------+----+---------------------+



   2.4.5 NSAP Addresses

   This mapping of NSAP address into IPv6 addresses is as follows:


    |   7   |                   121 bits                              |
    +-------+---------------------------------------------------------+
    |0000001|                 to be defined                           |
    +-------+---------------------------------------------------------+

   The draft definition, motivation, and usage are under study [NSAP].


   2.4.6 IPX Addresses

   This mapping of IPX address into IPv6 addresses is as follows:


    |   7   |                   121 bits                              |
    +-------+---------------------------------------------------------+
    |0000010|                 to be defined                           |
    +-------+---------------------------------------------------------+

   The draft definition, motivation, and usage are under study.


   2.4.7 Provider-Based Global Unicast Addresses

   The global provider-based unicast address is assigned as described in
   [ALLOC].  This initial assignment plan for these unicast addresses is
   similar to assignment of IPv4 addresses under the CIDR scheme [CIDR].
   The IPv6 global provider-based unicast address format is as follows:


    | 3 |  n bits   |  m bits   |   o bits    |   125-n-m-o bits   |
    +---+-----------+-----------+-------------+--------------------+
    |010|registry ID|provider ID|subscriber ID|  intra-subscriber  |
    +---+-----------+-----------+-------------+--------------------+





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   The high-order part of the address is assigned to registries, who
   then assign portions of the address space to providers, who then
   assign portions of the address space to subscribers, etc.

   The registry ID identifies the registry which assigns the provider
   portion of the address.  The term "registry prefix" refers to the
   high-order part of the address up to and including the registry ID.

   The provider ID identifies a specific provider which assigns the
   subscriber portion of the address.  The term "provider prefix" refers
   to the high-order part of the address up to and including the
   provider ID.

   The subscriber ID distinguishes among multiple subscribers attached
   to the provider identified by the provider ID.  The term "subscriber
   prefix" refers to the high-order part of the address up to and
   including the subscriber ID.

   The intra-subscriber portion of the address is defined by an
   individual subscriber and is organized according to the subscribers
   local internet topology.  It is likely that many subscribers will
   choose to divide the intra-subscriber portion of the address into a
   subnet ID and an interface ID.  In this case the subnet ID identifies
   a specific physical link and the interface ID identifies a single
   interface on that subnet.


   2.4.8 Local-use IPv6 Unicast Addresses

   There are two types of local-use unicast addresses defined.  These
   are Link-Local and Site-Local.  The Link-Local is for use on a single
   link and the Site-Local is for use in a single site.  Link-Local
   addresses have the following format:

    |   10     |
    |  bits    |        n bits           |       118-n bits           |
    +----------+-------------------------+----------------------------+
    |1111111010|           0             |       interface ID         |
    +----------+-------------------------+----------------------------+

   Link-Local addresses are designed to be used for addressing on a
   single link for purposes such as auto-address configuration, neighbor
   discovery, or when no routers are present.

   Routers MUST not forward any packets with link-local source
   addresses.





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   Site-Local addresses have the following format:

    |   10     |
    |  bits    | n bits  |    m bits     |       118-n-m bits         |
    +----------+---------+---------------+----------------------------+
    |1111111011|    0    |   subnet ID   |       interface ID         |
    +----------+---------+---------------+----------------------------+


   Site-Local addresses may be used for sites or organizations that are
   not (yet) connected to the global Internet.  They do not need to
   request or "steal" an address prefix from the global Internet address
   space.  IPv6 site-local addresses can be used instead.  When the
   organization connects to the global Internet, it can then form global
   addresses by replacing the site-local prefix with a subscriber
   prefix.

   Routers MUST not forward any packets with site-local source addresses
   outside of the site.

   2.5 Anycast Addresses

   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 address prefix P
   that identifies the topological region in which all interfaces
   belonging to that anycast address reside.  Within the region
   identified by P, each member of the anycast set must be advertised 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 advertisement 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 advertised as a



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RFC 1884              IPv6 Addressing Architecture         December 1995