rfc2373.txt

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   IPv6 unicast addresses are aggregatable with contiguous bit-wise
   masks similar to IPv4 addresses under Class-less Interdomain Routing
   [CIDR].

   There are several forms of unicast address assignment in IPv6,
   including the global aggregatable global unicast address, the NSAP
   address, the IPX hierarchical address, the site-local address, the
   link-local 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:



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   |                         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
   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.

2.5.1 Interface Identifiers

   Interface identifiers in IPv6 unicast addresses are used to identify
   interfaces on a link.  They are required to be unique on that link.
   They may also be unique over a broader scope.  In many cases an
   interface's identifier will be the same as that interface's link-
   layer address.  The same interface identifier may be used on multiple
   interfaces on a single node.

   Note that the use of the same interface identifier on multiple
   interfaces of a single node does not affect the interface
   identifier's global uniqueness or each IPv6 addresses global
   uniqueness created using that interface identifier.

   In a number of the format prefixes (see section 2.4) Interface IDs
   are required to be 64 bits long and to be constructed in IEEE EUI-64
   format [EUI64].  EUI-64 based Interface identifiers may have global
   scope when a global token is available (e.g., IEEE 48bit MAC) or may
   have local scope where a global token is not available (e.g., serial
   links, tunnel end-points, etc.).  It is required that the "u" bit
   (universal/local bit in IEEE EUI-64 terminology) be inverted when
   forming the interface identifier from the EUI-64.  The "u" bit is set
   to one (1) to indicate global scope, and it is set to zero (0) to
   indicate local scope.  The first three octets in binary of an EUI-64
   identifier are as follows:

       0       0 0       1 1       2
      |0       7 8       5 6       3|
      +----+----+----+----+----+----+
      |cccc|ccug|cccc|cccc|cccc|cccc|
      +----+----+----+----+----+----+






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   written in Internet standard bit-order , where "u" is the
   universal/local bit, "g" is the individual/group bit, and "c" are the
   bits of the company_id.  Appendix A: "Creating EUI-64 based Interface
   Identifiers" provides examples on the creation of different EUI-64
   based interface identifiers.

   The motivation for inverting the "u" bit when forming the interface
   identifier is to make it easy for system administrators to hand
   configure local scope identifiers when hardware tokens are not
   available.  This is expected to be case for serial links, tunnel end-
   points, etc.  The alternative would have been for these to be of the
   form 0200:0:0:1, 0200:0:0:2, etc., instead of the much simpler ::1,
   ::2, etc.

   The use of the universal/local bit in the IEEE EUI-64 identifier is
   to allow development of future technology that can take advantage of
   interface identifiers with global scope.

   The details of forming interface identifiers are defined in the
   appropriate "IPv6 over <link>" specification such as "IPv6 over
   Ethernet" [ETHER], "IPv6 over FDDI" [FDDI], etc.

2.5.2 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 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.

2.5.3 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 packet to itself.  It may
   never be assigned to any physical interface.  It may be thought of as
   being associated with a virtual interface (e.g., the loopback
   interface).

   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.






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2.5.4 IPv6 Addresses with Embedded IPv4 Addresses

   The IPv6 transition mechanisms [TRAN] 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:

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

2.5.5 NSAP Addresses

   This mapping of NSAP address into IPv6 addresses is defined in
   [NSAP].  This document recommends that network implementors who have
   planned or deployed an OSI NSAP addressing plan, and who wish to
   deploy or transition to IPv6, should redesign a native IPv6
   addressing plan to meet their needs.  However, it also defines a set
   of mechanisms for the support of OSI NSAP addressing in an IPv6
   network.  These mechanisms are the ones that must be used if such
   support is required.  This document also defines a mapping of IPv6
   addresses within the OSI address format, should this be required.

2.5.6 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.




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2.5.7 Aggregatable Global Unicast Addresses

   The global aggregatable global unicast address is defined in [AGGR].
   This address format is designed to support both the current provider
   based aggregation and a new type of aggregation called exchanges.
   The combination will allow efficient routing aggregation for both
   sites which connect directly to providers and who connect to
   exchanges.  Sites will have the choice to connect to either type of
   aggregation point.

   The IPv6 aggregatable global unicast address format is as follows:

   | 3|  13 | 8 |   24   |   16   |          64 bits               |
   +--+-----+---+--------+--------+--------------------------------+
   |FP| TLA |RES|  NLA   |  SLA   |         Interface ID           |
   |  | ID  |   |  ID    |  ID    |                                |
   +--+-----+---+--------+--------+--------------------------------+

   Where

      001          Format Prefix (3 bit) for Aggregatable Global
                   Unicast Addresses
      TLA ID       Top-Level Aggregation Identifier
      RES          Reserved for future use
      NLA ID       Next-Level Aggregation Identifier
      SLA ID       Site-Level Aggregation Identifier
      INTERFACE ID Interface Identifier

   The contents, field sizes, and assignment rules are defined in
   [AGGR].

2.5.8 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    |        54 bits          |          64 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.




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   Routers must not forward any packets with link-local source or
   destination addresses to other links.

   Site-Local addresses have the following format:

   |   10     |
   |  bits    |   38 bits   |  16 bits  |         64 bits            |
   +----------+-------------+-----------+----------------------------+
   |1111111011|    0        | subnet ID |       interface ID         |
   +----------+-------------+-----------+----------------------------+

   Site-Local addresses are designed to be used for addressing inside of
   a site without the need for a global prefix.

   Routers must not forward any packets with site-local source or
   destination addresses outside of the site.

2.6 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
   separate routing entry throughout the entire internet, which presents





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   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
   aggregation or sequence of aggregations.  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.

   There is little experience with widespread, arbitrary use of internet
   anycast addresses, and some known complications and hazards when
   using them in their full generality [ANYCST].  Until more experience
   has been gained and solutions agreed upon for those problems, the
   following restrictions are imposed on IPv6 anycast addresses:

      o An anycast address must not be used as the source address of an
        IPv6 packet.

      o An anycast address must not be assigned to an IPv6 host, that
        is, it may be assigned to an IPv6 router only.

2.6.1 Required Anycast Address

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

   |                         n bits                 |   128-n bits   |
   +------------------------------------------------+----------------+
   |                   subnet prefix                | 00000000000000 |
   +------------------------------------------------+----------------+

   The "subnet prefix" in an anycast address is the prefix which
   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 which they have
   interfaces.







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