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    Network Working Group                                          R. Hinden
    Request for Comments: 3513                                         Nokia
    Obsoletes: 2373                                               S. Deering
    Category: Standards Track                                  Cisco Systems
                                                                  April 2003

           Internet Protocol Version 6 (IPv6) Addressing Architecture

    Status of this Memo

       This document specifies an Internet standards track protocol for the
       Internet community, and requests discussion and suggestions for
       improvements.  Please refer to the current edition of the "Internet
       Official Protocol Standards" (STD 1) for the standardization state
       and status of this protocol.  Distribution of this memo is unlimited.

    Copyright Notice

       Copyright (C) The Internet Society (2003).  All Rights Reserved.

    Abstract

       This specification defines the addressing architecture of the IP
       Version 6 (IPv6) protocol.  The document includes the IPv6 addressing
       model, text representations of IPv6 addresses, definition of IPv6
       unicast addresses, anycast addresses, and multicast addresses, and an
       IPv6 node's required addresses.

    Hinden & Deering            Standards Track                     [Page 1]

    RFC 3513              IPv6 Addressing Architecture            April 2003

    Table of Contents

       1. Introduction.................................................3
       2. IPv6 Addressing..............................................3
          2.1 Addressing Model.........................................4
          2.2 Text Representation of Addresses.........................4
          2.3 Text Representation of Address Prefixes..................5
          2.4 Address Type Identification..............................6
          2.5 Unicast Addresses........................................7
              2.5.1 Interface Identifiers..............................8
              2.5.2 The Unspecified Address............................9
              2.5.3 The Loopback Address...............................9
              2.5.4 Global Unicast Addresses..........................10
              2.5.5 IPv6 Addresses with Embedded IPv4 Addresses.......10
              2.5.6 Local-use IPv6 Unicast Addresses..................11
          2.6 Anycast Addresses.......................................12
              2.6.1 Required Anycast Address..........................13
          2.7 Multicast Addresses.....................................13
              2.7.1 Pre-Defined Multicast Addresses...................15
          2.8 A Node's Required Addresses.............................17
       3. Security Considerations.....................................17
       4. IANA Considerations.........................................18
       5. References..................................................19
          5.1 Normative References....................................19
          5.2 Informative References..................................19
       APPENDIX A: Creating Modified EUI-64 format Interface IDs......21
       APPENDIX B: Changes from RFC-2373..............................24
       Authors' Addresses.............................................25
       Full Copyright Statement.......................................26

    Hinden & Deering            Standards Track                     [Page 2]

    RFC 3513              IPv6 Addressing Architecture            April 2003

    1.  Introduction

       This specification defines the addressing architecture of the IP
       Version 6 (IPv6) protocol.  It includes the basic formats for the
       various types of IPv6 addresses (unicast, anycast, and multicast).

       The authors would like to acknowledge the contributions of Paul
       Francis, Scott Bradner, Jim Bound, Brian Carpenter, Matt Crawford,
       Deborah Estrin, Roger Fajman, Bob Fink, Peter Ford, Bob Gilligan,
       Dimitry Haskin, Tom Harsch, Christian Huitema, Tony Li, Greg
       Minshall, Thomas Narten, Erik Nordmark, Yakov Rekhter, Bill Simpson,
       Sue Thomson, Markku Savela, and Larry Masinter.

    2. IPv6 Addressing

       IPv6 addresses are 128-bit identifiers for interfaces and sets of
       interfaces (where "interface" is as defined in section 2 of [IPV6]).
       There are three types of addresses:

       Unicast:   An identifier for a single interface.  A packet sent to a
                  unicast address is delivered to the interface identified
                  by that address.

       Anycast:   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).

       Multicast: 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.

    Hinden & Deering            Standards Track                     [Page 3]

    RFC 3513              IPv6 Addressing Architecture            April 2003

    2.1 Addressing Model

       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 section 2.8 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
       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 that a subnet prefix is
       associated with one link.  Multiple subnet prefixes may be assigned
       to the same link.

    2.2 Text Representation of Addresses

       There are three conventional forms for representing IPv6 addresses as
       text strings:

       1. The preferred form is x:x:x:x:x:x:x:x, where the 'x's are the
          hexadecimal values of the eight 16-bit pieces of the address.

          Examples:

             FEDC:BA98:7654:3210:FEDC:BA98:7654:3210

             1080:0:0:0:8:800:200C: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.).

       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.

    Hinden & Deering            Standards Track                     [Page 4]

    RFC 3513              IPv6 Addressing Architecture            April 2003

          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:

             1080:0:0:0:8:800:200C:417A  a unicast address
             FF01:0:0:0:0:0:0:101        a multicast address
             0:0:0:0:0:0:0:1             the loopback address
             0:0:0:0:0:0:0:0             the unspecified addresses

          may be represented as:

             1080::8:800:200C:417A       a unicast address
             FF01::101                   a multicast address
             ::1                         the loopback address
             ::                          the unspecified addresses

       3. 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

    2.3 Text Representation of Address Prefixes

       The text representation of IPv6 address prefixes is similar to the
       way IPv4 addresses prefixes are written in CIDR notation [CIDR].  An
       IPv6 address prefix is represented by the notation:

          ipv6-address/prefix-length

       where

          ipv6-address    is an IPv6 address in any of the notations listed
                          in section 2.2.

    Hinden & Deering            Standards Track                     [Page 5]

    RFC 3513              IPv6 Addressing Architecture            April 2003

          prefix-length   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 12AB00000000CD3 (hexadecimal):

          12AB:0000:0000:CD30:0000:0000:0000:0000/60
          12AB::CD30:0:0:0:0/60
          12AB:0:0:CD30::/60

       The following are NOT legal representations of the above prefix:

          12AB:0:0:CD3/60   may drop leading zeros, but not trailing zeros,
                            within any 16-bit chunk of the address

          12AB::CD30/60     address to left of "/" expands to
                            12AB:0000:0000:0000:0000:000:0000:CD30

          12AB::CD3/60      address to left of "/" expands to
                            12AB:0000:0000:0000:0000:000:0000:0CD3

       When writing both a node address and a prefix of that node address
       (e.g., the node's subnet prefix), the two can combined as follows:

          the node address      12AB:0:0:CD30:123:4567:89AB:CDEF
          and its subnet number 12AB:0:0:CD30::/60

          can be abbreviated as 12AB:0:0:CD30:123:4567:89AB:CDEF/60

    2.4 Address Type Identification

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

       Address type         Binary prefix        IPv6 notation   Section
       ------------         -------------        -------------   -------
       Unspecified          00...0  (128 bits)   ::/128          2.5.2
       Loopback             00...1  (128 bits)   ::1/128         2.5.3
       Multicast            11111111             FF00::/8        2.7
       Link-local unicast   1111111010           FE80::/10       2.5.6
       Site-local unicast   1111111011           FEC0::/10       2.5.6
       Global unicast       (everything else)

       Anycast addresses are taken from the unicast address spaces (of any
       scope) and are not syntactically distinguishable from unicast
       addresses.

    Hinden & Deering            Standards Track                     [Page 6]

    RFC 3513              IPv6 Addressing Architecture            April 2003

       The general format of global unicast addresses is described in
       section 2.5.4.  Some special-purpose subtypes of global unicast
       addresses which contain embedded IPv4 addresses (for the purposes of
       IPv4-IPv6 interoperation) are described in section 2.5.5.

       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.

    2.5 Unicast Addresses

       IPv6 unicast addresses are aggregable with prefixes of arbitrary
       bit-length similar to IPv4 addresses under Classless Interdomain
       Routing.

       There are several types of unicast addresses in IPv6, in particular
       global unicast, site-local unicast, and link-local unicast.  There
       are also some special-purpose subtypes of global unicast, such as
       IPv6 addresses with embedded IPv4 addresses or encoded NSAP
       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:

       |                           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   |
       +------------------------------------------------+----------------+

       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

    Hinden & Deering            Standards Track                     [Page 7]

    RFC 3513              IPv6 Addressing Architecture            April 2003

       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 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.  In some cases an interface's identifier will be
       derived directly from that interface's link-layer address.  The same
       interface identifier may be used on multiple interfaces on a single
       node, as long as they are attached to different subnets.

       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 a non-global scope interface identifier
       and a site-local address may be created with a global scope interface
       identifier.

       For all unicast addresses, except those that start with binary value
       000, Interface IDs are required to be 64 bits long and to be
       constructed in Modified EUI-64 format.

       Modified EUI-64 format based Interface identifiers may have global
       scope when derived from a global token (e.g., IEEE 802 48-bit MAC or
       IEEE EUI-64 identifiers [EUI64]) or may have local scope where a
       global token is not available (e.g., serial links, tunnel end-points,
       etc.) or where global tokens are undesirable (e.g., temporary tokens
       for privacy [PRIV]).

       Modified EUI-64 format interface identifiers are formed by inverting
       the "u" bit (universal/local bit in IEEE EUI-64 terminology) when
       forming the interface identifier from IEEE EUI-64 identifiers.  In
       the resulting Modified EUI-64 format 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 IEEE EUI-64 identifier
       are as follows:

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

       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 Modified EUI-64 format

    Hinden & Deering            Standards Track                     [Page 8]

    RFC 3513              IPv6 Addressing Architecture            April 2003

       Interface Identifiers" provides examples on the creation of Modified
       EUI-64 format based interface identifiers.

       The motivation for inverting the "u" bit when forming an interface
       identifier is to make it easy for system administrators to hand
       configure non-global 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 Modified EUI-64 format
       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.  An IPv6 packet with a
       source address of unspecified must never be forwarded by an IPv6
       router.

    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 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 destination address of loopback must be
       dropped.

    Hinden & Deering            Standards Track                     [Page 9]

    RFC 3513              IPv6 Addressing Architecture            April 2003

    2.5.4 Global Unicast Addresses

       The general format for IPv6 global unicast addresses is as follows:

       |         n bits         |   m bits  |       128-n-m bits         |
       +------------------------+-----------+----------------------------+
       | global routing prefix  | subnet ID |       interface ID         |
       +------------------------+-----------+----------------------------+

       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 section 2.5.1.

       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 section 2.5.1.  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 section
       2.5.5 and the IPv6 address containing encoded NSAP addresses
       specified in [NSAP].  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 [AGGR].

    2.5.5 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 use this technique are assigned
       special IPv6 unicast addresses that carry a global 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     |
       +--------------------------------------+----+---------------------+

       Note: The IPv4 address used in the "IPv4-compatible IPv6 address"
       must be a globally-unique IPv4 unicast address.

       A second type of IPv6 address which holds an embedded IPv4 address is
       also defined.  This address type is used to represent the addresses
       of IPv4 nodes as IPv6 addresses.  This type of address is termed an
       "IPv4-mapped IPv6 address" and has the format:

    Hinden & Deering            Standards Track                    [Page 10]

    RFC 3513              IPv6 Addressing Architecture            April 2003

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

    2.5.6 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 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.

       Site-Local addresses have the following format:

       |   10     |
       |  bits    |         54 bits         |         64 bits            |
       +----------+-------------------------+----------------------------+
       |1111111011|        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.  Although a subnet ID
       may be up to 54-bits long, it is expected that globally-connected
       sites will use the same subnet IDs for site-local and global
       prefixes.

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

    Hinden & Deering            Standards Track                    [Page 11]

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    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 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.

       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 are specified, the following
       restrictions are imposed on IPv6 anycast addresses:

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       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 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.

    2.7 Multicast Addresses

       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:

       |   8    |  4 |  4 |                  112 bits                   |
       +------ -+----+----+---------------------------------------------+
       |11111111|flgs|scop|                  group ID                   |
       +--------+----+----+---------------------------------------------+

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

                                           +-+-+-+-+
             flgs is a set of 4 flags:     |0|0|0|T|
                                           +-+-+-+-+

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                The high-order 3 flags are reserved, and must be initialized
                to 0.

                T = 0 indicates a permanently-assigned ("well-known")
                multicast address, assigned by the Internet Assigned Number
                Authority (IANA).

                T = 1 indicates a non-permanently-assigned ("transient")
                multicast address.

             scop is a 4-bit multicast scope value used to limit the scope
             of the multicast group.  The values are:

                0  reserved
                1  interface-local scope
                2  link-local scope
                3  reserved
                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.

                link-local and site-local multicast scopes span the same
                topological regions as the corresponding unicast scopes.

                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.

                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.

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    RFC 3513              IPv6 Addressing Architecture            April 2003

             group ID identifies the multicast group, either permanent or
             transient, within the given scope.

       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 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 of 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.

    2.7.1 Pre-Defined Multicast Addresses

       The following well-known multicast addresses are pre-defined.  The
       group ID's 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.

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          Reserved Multicast Addresses:   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.

          All Nodes Addresses:    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).

          All Routers Addresses:   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).

          Solicited-Node Address:  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

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       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 every unicast
       and anycast address it is assigned.

    2.8 A Node's Required Addresses

       A host is required to recognize the following addresses as
       identifying itself:

          o  Its required Link-Local Address for each interface.
          o  Any additional Unicast and Anycast Addresses that have been
             configured for the node's interfaces (manually or
             automatically).
          o  The loopback address.
          o  The All-Nodes Multicast Addresses defined in section 2.7.1.
          o  The Solicited-Node Multicast Address for each of its unicast
             and anycast addresses.
          o  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:

          o  The Subnet-Router Anycast Addresses for all interfaces for
             which it is configured to act as a router.
          o  All other Anycast Addresses with which the router has been
             configured.
          o  The All-Routers Multicast Addresses defined in section 2.7.1.

    3. Security Considerations

       IPv6 addressing documents do not have any direct impact on Internet
       infrastructure security.  Authentication of IPv6 packets is defined
       in [AUTH].

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    4. IANA Considerations

       The table and notes at http://www.isi.edu/in-
       notes/iana/assignments/ipv6-address-space.txt should be replaced with
       the following:

       INTERNET PROTOCOL VERSION 6 ADDRESS SPACE

       The initial assignment of IPv6 address space is as follows:

       Allocation                            Prefix         Fraction of
                                             (binary)       Address Space
       -----------------------------------   --------       -------------
       Unassigned (see Note 1 below)         0000 0000      1/256
       Unassigned                            0000 0001      1/256
       Reserved for NSAP Allocation          0000 001       1/128 [RFC1888]
       Unassigned                            0000 01        1/64
       Unassigned                            0000 1         1/32
       Unassigned                            0001           1/16
       Global Unicast                        001            1/8   [RFC2374]
       Unassigned                            010            1/8
       Unassigned                            011            1/8
       Unassigned                            100            1/8
       Unassigned                            101            1/8
       Unassigned                            110            1/8
       Unassigned                            1110           1/16
       Unassigned                            1111 0         1/32
       Unassigned                            1111 10        1/64
       Unassigned                            1111 110       1/128
       Unassigned                            1111 1110 0    1/512
       Link-Local Unicast Addresses          1111 1110 10   1/1024
       Site-Local Unicast Addresses          1111 1110 11   1/1024
       Multicast Addresses                   1111 1111      1/256

       Notes:

       1. The "unspecified address", the "loopback address", and the IPv6
          Addresses with Embedded IPv4 Addresses are assigned out of the
          0000 0000 binary prefix space.

       2. For now, IANA should limit its allocation of IPv6 unicast address
          space to the range of addresses that start with binary value 001.
          The rest of the global unicast address space (approximately 85% of
          the IPv6 address space) is reserved for future definition and use,
          and is not to be assigned by IANA at this time.

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    5.  References

    5.1  Normative References

       [IPV6]    Deering, S. and R. Hinden, "Internet Protocol, Version 6
                 (IPv6) Specification", RFC 2460, December 1998.

       [RFC2026] Bradner, S., "The Internet Standards Process -- Revision
                 3", BCP 9 , RFC 2026, October 1996.

    5.2  Informative References

       [ANYCST]  Partridge, C., Mendez, T. and W. Milliken, "Host Anycasting
                 Service", RFC 1546, November 1993.

       [AUTH]    Kent, S. and R. Atkinson, "IP Authentication Header", RFC
                 2402, November 1998.

       [AGGR]    Hinden, R., O'Dell, M. and S. Deering, "An Aggregatable
                 Global Unicast Address Format", RFC 2374, July 1998.

       [CIDR]    Fuller, V., Li, T., Yu, J. and K. Varadhan, "Classless
                 Inter-Domain Routing (CIDR): An Address Assignment and
                 Aggregation Strategy", RFC 1519, September 1993.

       [ETHER]   Crawford, M., "Transmission of IPv6 Packets over Ethernet
                 Networks", RFC 2464, December 1998.

       [EUI64]   IEEE, "Guidelines for 64-bit Global Identifier (EUI-64)
                 Registration Authority",
                 http://standards.ieee.org/regauth/oui/tutorials/EUI64.html,
                 March 1997.

       [FDDI]    Crawford, M., "Transmission of IPv6 Packets over FDDI
                 Networks", RFC 2467, December 1998.

       [MASGN]   Hinden, R. and S. Deering, "IPv6 Multicast Address
                 Assignments", RFC 2375, July 1998.

       [NSAP]    Bound, J., Carpenter, B., Harrington, D., Houldsworth, J.
                 and A. Lloyd, "OSI NSAPs and IPv6", RFC 1888, August 1996.

       [PRIV]    Narten, T. and R. Draves, "Privacy Extensions for Stateless
                 Address Autoconfiguration in IPv6", RFC 3041, January 2001.

       [TOKEN]   Crawford, M., Narten, T. and S. Thomas, "Transmission of
                 IPv6 Packets over Token Ring Networks", RFC 2470, December
                 1998.

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       [TRAN]    Gilligan, R. and E. Nordmark, "Transition Mechanisms for
                 IPv6 Hosts and Routers", RFC 2893, August 2000.

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    APPENDIX A: Creating Modified EUI-64 format Interface Identifiers

       Depending on the characteristics of a specific link or node there are
       a number of approaches for creating Modified EUI-64 format interface
       identifiers.  This appendix describes some of these approaches.

    Links or Nodes with IEEE EUI-64 Identifiers

       The only change needed to transform an IEEE EUI-64 identifier to an
       interface identifier is to invert the "u" (universal/local) bit.  For
       example, a globally unique IEEE EUI-64 identifier of the form:

       |0              1|1              3|3              4|4              6|
       |0              5|6              1|2              7|8              3|
       +----------------+----------------+----------------+----------------+
       |cccccc0gcccccccc|ccccccccmmmmmmmm|mmmmmmmmmmmmmmmm|mmmmmmmmmmmmmmmm|
       +----------------+----------------+----------------+----------------+

       where "c" are the bits of the assigned company_id, "0" is the value
       of the universal/local bit to indicate global scope, "g" is
       individual/group bit, and "m" are the bits of the manufacturer-
       selected extension identifier.  The IPv6 interface identifier would
       be of the form:

       |0              1|1              3|3              4|4              6|
       |0              5|6              1|2              7|8              3|
       +----------------+----------------+----------------+----------------+
       |cccccc1gcccccccc|ccccccccmmmmmmmm|mmmmmmmmmmmmmmmm|mmmmmmmmmmmmmmmm|
       +----------------+----------------+----------------+----------------+

       The only change is inverting the value of the universal/local bit.

    Links or Nodes with IEEE 802 48 bit MAC's

       [EUI64] defines a method to create a IEEE EUI-64 identifier from an
       IEEE 48bit MAC identifier.  This is to insert two octets, with
       hexadecimal values of 0xFF and 0xFE, in the middle of the 48 bit MAC
       (between the company_id and vendor supplied id).  For example, the 48
       bit IEEE MAC with global scope:

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       |0              1|1              3|3              4|
       |0              5|6              1|2              7|
       +----------------+----------------+----------------+
       |cccccc0gcccccccc|ccccccccmmmmmmmm|mmmmmmmmmmmmmmmm|
       +----------------+----------------+----------------+

       where "c" are the bits of the assigned company_id, "0" is the value
       of the universal/local bit to indicate global scope, "g" is
       individual/group bit, and "m" are the bits of the manufacturer-
       selected extension identifier.  The interface identifier would be of
       the form:

       |0              1|1              3|3              4|4              6|
       |0              5|6              1|2              7|8              3|
       +----------------+----------------+----------------+----------------+
       |cccccc1gcccccccc|cccccccc11111111|11111110mmmmmmmm|mmmmmmmmmmmmmmmm|
       +----------------+----------------+----------------+----------------+

       When IEEE 802 48bit MAC addresses are available (on an interface or a
       node), an implementation may use them to create interface identifiers
       due to their availability and uniqueness properties.

    Links with Other Kinds of Identifiers

       There are a number of types of links that have link-layer interface
       identifiers other than IEEE EIU-64 or IEEE 802 48-bit MACs.  Examples
       include LocalTalk and Arcnet.  The method to create an Modified EUI-
       64 format identifier is to take the link identifier (e.g., the
       LocalTalk 8 bit node identifier) and zero fill it to the left.  For
       example, a LocalTalk 8 bit node identifier of hexadecimal value 0x4F
       results in the following interface identifier:

       |0              1|1              3|3              4|4              6|
       |0              5|6              1|2              7|8              3|
       +----------------+----------------+----------------+----------------+
       |0000000000000000|0000000000000000|0000000000000000|0000000001001111|
       +----------------+----------------+----------------+----------------+

       Note that this results in the universal/local bit set to "0" to
       indicate local scope.

    Links without Identifiers

       There are a number of links that do not have any type of built-in
       identifier.  The most common of these are serial links and configured
       tunnels.  Interface identifiers must be chosen that are unique within
       a subnet-prefix.

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       When no built-in identifier is available on a link the preferred
       approach is to use a global interface identifier from another
       interface or one which is assigned to the node itself.  When using
       this approach no other interface connecting the same node to the same
       subnet-prefix may use the same identifier.

       If there is no global interface identifier available for use on the
       link the implementation needs to create a local-scope interface
       identifier.  The only requirement is that it be unique within a
       subnet prefix.  There are many possible approaches to select a
       subnet-prefix-unique interface identifier.  These include:

          Manual Configuration
          Node Serial Number
          Other node-specific token

       The subnet-prefix-unique interface identifier should be generated in
       a manner that it does not change after a reboot of a node or if
       interfaces are added or deleted from the node.

       The selection of the appropriate algorithm is link and implementation
       dependent.  The details on forming interface identifiers are defined
       in the appropriate "IPv6 over <link>" specification.  It is strongly
       recommended that a collision detection algorithm be implemented as
       part of any automatic algorithm.

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    APPENDIX B: Changes from RFC-2373

       The following changes were made from RFC-2373 "IP Version 6
       Addressing Architecture":

       -  Clarified text in section 2.2 to allow "::" to represent one or
          more groups of 16 bits of zeros.
       -  Changed uniqueness requirement of Interface Identifiers from
          unique on a link to unique within a subnet prefix.  Also added a
          recommendation that the same interface identifier not be assigned
          to different machines on a link.
       -  Change site-local format to make the subnet ID field 54-bit long
          and remove the 38-bit zero's field.
       -  Added description of multicast scop values and rules to handle the
          reserved scop value 0.
       -  Revised sections 2.4 and 2.5.6 to simplify and clarify how
          different address types  are identified.  This was done to insure
          that implementations do not build in any knowledge about global
          unicast format prefixes.  Changes include:
             o  Removed Format Prefix (FP) terminology
             o  Revised list of address types to only include exceptions to
                global unicast and a singe entry that identifies everything
                else as Global Unicast.
             o  Removed list of defined prefix exceptions from section 2.5.6
                as it is now the main part of section 2.4.
       -  Clarified text relating to EUI-64 identifiers to distinguish
          between IPv6's "Modified EUI-64 format" identifiers and IEEE EUI-
          64 identifiers.
       -  Combined the sections on the Global Unicast Addresses and NSAP
          Addresses into a single section on Global Unicast Addresses,
          generalized the Global Unicast format, and cited [AGGR] and [NSAP]
          as examples.
       -  Reordered sections 2.5.4 and 2.5.5.
       -  Removed section 2.7.2 Assignment of New IPv6 Multicast Addresses
          because this is being redefined elsewhere.
       -  Added an IANA considerations section that updates the IANA IPv6
          address allocations and documents the NSAP and AGGR allocations.
       -  Added clarification that the "IPv4-compatible IPv6 address" must
          use global IPv4 unicast addresses.
       -  Divided references in to normative and non-normative sections.
       -  Added reference to [PRIV] in section 2.5.1
       -  Added clarification that routers must not forward multicast
          packets outside of the scope indicated in the multicast address.
       -  Added clarification that routers must not forward packets with
           source address of the unspecified address.
       -  Added clarification that routers must drop packets received on an
          interface with destination address of loopback.
       -  Clarified the definition of IPv4-mapped addresses.

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       -  Removed the ABNF Description of Text Representations Appendix.
       -  Removed the address block reserved for IPX addresses.
       -  Multicast scope changes:
             o  Changed name of scope value 1 from "node-local" to
                "interface-local"
             o  Defined scope value 4 as "admin-local"
       -  Corrected reference to RFC1933 and updated references.
       -  Many small changes to clarify and make the text more consistent.

    Authors' Addresses

       Robert M. Hinden
       Nokia
       313 Fairchild Drive
       Mountain View, CA 94043
       USA

       Phone: +1 650 625-2004
       EMail: hinden@iprg.nokia.com

       Stephen E. Deering
       Cisco Systems, Inc.
       170 West Tasman Drive
       San Jose, CA 95134-1706
       USA

       Phone: +1 408 527-8213
       EMail: deering@cisco.com

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    RFC 3513              IPv6 Addressing Architecture            April 2003

    Full Copyright Statement

       Copyright (C) The Internet Society (2003).  All Rights Reserved.

       This document and translations of it may be copied and furnished to
       others, and derivative works that comment on or otherwise explain it
       or assist in its implementation may be prepared, copied, published
       and distributed, in whole or in part, without restriction of any
       kind, provided that the above copyright notice and this paragraph are
       included on all such copies and derivative works.  However, this
       document itself may not be modified in any way, such as by removing
       the copyright notice or references to the Internet Society or other
       Internet organizations, except as needed for the purpose of
       developing Internet standards in which case the procedures for
       copyrights defined in the Internet Standards process must be
       followed, or as required to translate it into languages other than
       English.

       The limited permissions granted above are perpetual and will not be
       revoked by the Internet Society or its successors or assigns.

       This document and the information contained herein is provided on an
       "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
       TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
       BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
       HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
       MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

    Acknowledgement

       Funding for the RFC Editor function is currently provided by the
       Internet Society.

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