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rfc:rfc2373

Network Working Group R. Hinden Request for Comments: 2373 Nokia Obsoletes: 1884 S. Deering Category: Standards Track Cisco Systems

						     July 1998
                IP Version 6 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 (1998).  All Rights Reserved.

Abstract

 This specification defines the addressing architecture of the IP
 Version 6 protocol [IPV6].  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.

Table of Contents

 1. Introduction.................................................2
 2. IPv6 Addressing..............................................2
    2.1 Addressing Model.........................................3
    2.2 Text Representation of Addresses.........................3
    2.3 Text Representation of Address Prefixes..................5
    2.4 Address Type Representation..............................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 IPv6 Addresses with Embedded IPv4 Addresses.........10
      2.5.5 NSAP Addresses......................................10
      2.5.6 IPX Addresses.......................................10
      2.5.7 Aggregatable Global Unicast Addresses...............11
      2.5.8 Local-use IPv6 Unicast Addresses....................11
    2.6 Anycast Addresses.......................................12
      2.6.1 Required Anycast Address............................13
    2.7 Multicast Addresses.....................................14

Hinden & Deering Standards Track [Page 1] RFC 2373 IPv6 Addressing Architecture July 1998

      2.7.1 Pre-Defined Multicast Addresses.....................15
      2.7.2 Assignment of New IPv6 Multicast Addresses..........17
    2.8 A Node's Required Addresses.............................17
 3. Security Considerations.....................................18
 APPENDIX A: Creating EUI-64 based Interface Identifiers........19
 APPENDIX B: ABNF Description of Text Representations...........22
 APPENDIX C: CHANGES FROM RFC-1884..............................23
 REFERENCES.....................................................24
 AUTHORS' ADDRESSES.............................................25
 FULL COPYRIGHT STATEMENT.......................................26

1.0 INTRODUCTION

 This specification defines the addressing architecture of the IP
 Version 6 protocol.  It includes a detailed description of the
 currently defined address formats for IPv6 [IPV6].
 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,
 and Sue Thomson.
 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
 document are to be interpreted as described in [RFC 2119].

2.0 IPv6 ADDRESSING

 IPv6 addresses are 128-bit identifiers for interfaces and sets of
 interfaces.  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.

Hinden & Deering Standards Track [Page 2] RFC 2373 IPv6 Addressing Architecture July 1998

 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 "subscriber".  When this name is used with the term "ID" for
 identifier after the name (e.g., "subscriber ID"), it refers to the
 contents of the named field.  When it is used with the term "prefix"
 (e.g.  "subscriber prefix") it refers to all of the address 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
 zero-valued fields or end in zeros.

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 be assigned 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:
   An 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:

Hinden & Deering Standards Track [Page 3] RFC 2373 IPv6 Addressing Architecture July 1998

       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.
    The use of "::" indicates multiple groups of 16-bits of zeros.
    The "::" can only appear once in an address.  The "::" can also be
    used to compress the leading and/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

Hinden & Deering Standards Track [Page 4] RFC 2373 IPv6 Addressing Architecture July 1998

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.  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.
    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

Hinden & Deering Standards Track [Page 5] RFC 2373 IPv6 Addressing Architecture July 1998

2.4 Address Type Representation

 The specific type of an IPv6 address is indicated by the leading bits
 in the address.  The variable-length field comprising these leading
 bits is called the Format Prefix (FP).  The initial allocation of
 these prefixes is as follows:
  Allocation                            Prefix         Fraction of
                                        (binary)       Address Space
  -----------------------------------   --------       -------------
  Reserved                              0000 0000      1/256
  Unassigned                            0000 0001      1/256
  Reserved for NSAP Allocation          0000 001       1/128
  Reserved for IPX Allocation           0000 010       1/128
  Unassigned                            0000 011       1/128
  Unassigned                            0000 1         1/32
  Unassigned                            0001           1/16
  Aggregatable Global Unicast Addresses 001            1/8
  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" (see section 2.5.2), the loopback
        address (see section 2.5.3), and the IPv6 Addresses with
        Embedded IPv4 Addresses (see section 2.5.4), are assigned out
        of the 0000 0000 format prefix space.

Hinden & Deering Standards Track [Page 6] RFC 2373 IPv6 Addressing Architecture July 1998

    (2) The format prefixes 001 through 111, except for Multicast
        Addresses (1111 1111), are all required to have to have 64-bit
        interface identifiers in EUI-64 format.  See section 2.5.1 for
        definitions.
 This allocation supports the direct allocation of aggregation
 addresses, local use addresses, and multicast addresses.  Space is
 reserved for NSAP addresses and IPX addresses.  The remainder of the
 address space is unassigned for future use.  This can be used for
 expansion of existing use (e.g., additional aggregatable addresses,
 etc.) or new uses (e.g., separate locators and identifiers).  Fifteen
 percent of the address space is initially 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.

2.5 Unicast Addresses

 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:

Hinden & Deering Standards Track [Page 7] RFC 2373 IPv6 Addressing Architecture July 1998

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

Hinden & Deering Standards Track [Page 8] RFC 2373 IPv6 Addressing Architecture July 1998

 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.

Hinden & Deering Standards Track [Page 9] RFC 2373 IPv6 Addressing Architecture July 1998

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.

Hinden & Deering Standards Track [Page 10] RFC 2373 IPv6 Addressing Architecture July 1998

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.

Hinden & Deering Standards Track [Page 11] RFC 2373 IPv6 Addressing Architecture July 1998

 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

Hinden & Deering Standards Track [Page 12] RFC 2373 IPv6 Addressing Architecture July 1998

 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.

Hinden & Deering Standards Track [Page 13] RFC 2373 IPv6 Addressing Architecture July 1998

 The subnet-router anycast address is intended to be used for
 applications where a node needs to communicate with one of a set of
 routers on a remote subnet.  For example when a mobile host needs to
 communicate with one of the mobile agents on its "home" subnet.

2.7 Multicast Addresses

 An IPv6 multicast address is an identifier for a group of nodes.  A
 node may belong to any number of multicast groups.  Multicast
 addresses have the following format:
 |   8    |  4 |  4 |                  112 bits                   |
 +------ -+----+----+---------------------------------------------+
 |11111111|flgs|scop|                  group ID                   |
 +--------+----+----+---------------------------------------------+
    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|
                                  +-+-+-+-+
       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 global internet numbering authority.
       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  node-local scope
       2  link-local scope
       3  (unassigned)
       4  (unassigned)
       5  site-local scope
       6  (unassigned)
       7  (unassigned)
       8  organization-local scope
       9  (unassigned)
       A  (unassigned)
       B  (unassigned)
       C  (unassigned)

Hinden & Deering Standards Track [Page 14] RFC 2373 IPv6 Addressing Architecture July 1998

       D  (unassigned)
       E  global scope
       F  reserved
    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 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 at 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.

2.7.1 Pre-Defined Multicast Addresses

 The following well-known multicast addresses are pre-defined:
    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

Hinden & Deering Standards Track [Page 15] RFC 2373 IPv6 Addressing Architecture July 1998

                                    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 (node-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 (node-local), 2 (link-local), or 5 (site-local).
    Solicited-Node Address:  FF02:0:0:0:0:1:FFXX:XXXX
 The above multicast address is computed as a function of a node's
 unicast and anycast addresses.  The solicited-node multicast address
 is formed by taking the low-order 24 bits of the 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 the associated Solicited-Node
 multicast addresses for every unicast and anycast address it is
 assigned.

Hinden & Deering Standards Track [Page 16] RFC 2373 IPv6 Addressing Architecture July 1998

2.7.2 Assignment of New IPv6 Multicast Addresses

 The current approach [ETHER] to map IPv6 multicast addresses into
 IEEE 802 MAC addresses takes the low order 32 bits of the IPv6
 multicast address and uses it to create a MAC address.  Note that
 Token Ring networks are handled differently.  This is defined in
 [TOKEN].  Group ID's less than or equal to 32 bits will generate
 unique MAC addresses.  Due to this new IPv6 multicast addresses
 should be assigned so that the group identifier is always in the low
 order 32 bits as shown in the following:
 |   8    |  4 |  4 |          80 bits          |     32 bits     |
 +------ -+----+----+---------------------------+-----------------+
 |11111111|flgs|scop|   reserved must be zero   |    group ID     |
 +--------+----+----+---------------------------+-----------------+
 While this limits the number of permanent IPv6 multicast groups to
 2^32 this is unlikely to be a limitation in the future.  If it
 becomes necessary to exceed this limit in the future multicast will
 still work but the processing will be sightly slower.
 Additional IPv6 multicast addresses are defined and registered by the
 IANA [MASGN].

2.8 A Node's Required Addresses

 A host is required to recognize the following addresses as
 identifying itself:
    o Its Link-Local Address for each interface
    o Assigned Unicast Addresses
    o Loopback Address
    o All-Nodes Multicast Addresses
    o Solicited-Node Multicast Address for each of its assigned
      unicast and anycast addresses
    o Multicast Addresses of all other groups to which the host
      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 the interfaces it is
      configured to act as a router on.
    o All other Anycast addresses with which the router has been
      configured.
    o All-Routers Multicast Addresses

Hinden & Deering Standards Track [Page 17] RFC 2373 IPv6 Addressing Architecture July 1998

    o Multicast Addresses of all other groups to which the router
      belongs.
 The only address prefixes which should be predefined in an
 implementation are the:
    o Unspecified Address
    o Loopback Address
    o Multicast Prefix (FF)
    o Local-Use Prefixes (Link-Local and Site-Local)
    o Pre-Defined Multicast Addresses
    o IPv4-Compatible Prefixes
 Implementations should assume all other addresses are unicast unless
 specifically configured (e.g., anycast addresses).

3. Security Considerations

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

Hinden & Deering Standards Track [Page 18] RFC 2373 IPv6 Addressing Architecture July 1998

APPENDIX A : Creating EUI-64 based Interface Identifiers


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

Links or Nodes with EUI-64 Identifiers

 The only change needed to transform an EUI-64 identifier to an
 interface identifier is to invert the "u" (universal/local) bit.  For
 example, a globally unique 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 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 MAC with
 global scope:
 |0              1|1              3|3              4|
 |0              5|6              1|2              7|
 +----------------+----------------+----------------+
 |cccccc0gcccccccc|ccccccccmmmmmmmm|mmmmmmmmmmmmmmmm|
 +----------------+----------------+----------------+

Hinden & Deering Standards Track [Page 19] RFC 2373 IPv6 Addressing Architecture July 1998

 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 should use them to create interface
 identifiers due to their availability and uniqueness properties.

Links with Non-Global Identifiers

 There are a number of types of links that, while multi-access, do not
 have globally unique link identifiers.  Examples include LocalTalk
 and Arcnet.  The method to create an EUI-64 formatted 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 for
 the link.
 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.  To use this
 approach no other interface connecting the same node to the same link
 may use the same identifier.

Hinden & Deering Standards Track [Page 20] RFC 2373 IPv6 Addressing Architecture July 1998

 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 on the link.
 There are many possible approaches to select a link-unique interface
 identifier.  They include:
    Manual Configuration
    Generated Random Number
    Node Serial Number (or other node-specific token)
 The link-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.

Hinden & Deering Standards Track [Page 21] RFC 2373 IPv6 Addressing Architecture July 1998

APPENDIX B: ABNF Description of Text Representations


 This appendix defines the text representation of IPv6 addresses and
 prefixes in Augmented BNF [ABNF] for reference purposes.
    IPv6address = hexpart [ ":" IPv4address ]
    IPv4address = 1*3DIGIT "." 1*3DIGIT "." 1*3DIGIT "." 1*3DIGIT
    IPv6prefix  = hexpart "/" 1*2DIGIT
    hexpart = hexseq | hexseq "::" [ hexseq ] | "::" [ hexseq ]
    hexseq  = hex4 *( ":" hex4)
    hex4    = 1*4HEXDIG

Hinden & Deering Standards Track [Page 22] RFC 2373 IPv6 Addressing Architecture July 1998

APPENDIX C: CHANGES FROM RFC-1884


 The following changes were made from RFC-1884 "IP Version 6
 Addressing Architecture":
  1. Added an appendix providing a ABNF description of text

representations.

  1. Clarification that link unique identifiers not change after

reboot or other interface reconfigurations.

  1. Clarification of Address Model based on comments.
  2. Changed aggregation format terminology to be consistent with

aggregation draft.

  1. Added text to allow interface identifier to be used on more than

one interface on same node.

  1. Added rules for defining new multicast addresses.
  2. Added appendix describing procedures for creating EUI-64 based

interface ID's.

  1. Added notation for defining IPv6 prefixes.
  2. Changed solicited node multicast definition to use a longer

prefix.

  1. Added site scope all routers multicast address.
  2. Defined Aggregatable Global Unicast Addresses to use "001" Format

Prefix.

  1. Changed "010" (Provider-Based Unicast) and "100" (Reserved for

Geographic) Format Prefixes to Unassigned.

  1. Added section on Interface ID definition for unicast addresses.

Requires use of EUI-64 in range of format prefixes and rules for

      setting global/local scope bit in EUI-64.
    - Updated NSAP text to reflect working in RFC1888.
    - Removed protocol specific IPv6 multicast addresses (e.g., DHCP)
      and referenced the IANA definitions.
    - Removed section "Unicast Address Example".  Had become OBE.
    - Added new and updated references.
    - Minor text clarifications and improvements.

Hinden & Deering Standards Track [Page 23] RFC 2373 IPv6 Addressing Architecture July 1998

REFERENCES

 [ABNF]    Crocker, D., and P. Overell, "Augmented BNF for
           Syntax Specifications: ABNF", RFC 2234, November 1997.
 [AGGR]    Hinden, R., O'Dell, M., and S. Deering, "An
           Aggregatable Global Unicast Address Format", RFC 2374, July
           1998.
 [AUTH]    Atkinson, R., "IP Authentication Header", RFC 1826, August
           1995.
 [ANYCST]  Partridge, C., Mendez, T., and W. Milliken, "Host
           Anycasting Service", RFC 1546, November 1993.
 [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 Pacekts over Ethernet
           Networks", Work in Progress.
 [EUI64]   IEEE, "Guidelines for 64-bit Global Identifier (EUI-64)
           Registration Authority",
           http://standards.ieee.org/db/oui/tutorials/EUI64.html,
           March 1997.
 [FDDI]    Crawford, M., "Transmission of IPv6 Packets over FDDI
           Networks", Work in Progress.
 [IPV6]    Deering, S., and R. Hinden, Editors, "Internet Protocol,
           Version 6 (IPv6) Specification", RFC 1883, December 1995.
 [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.
 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
           Requirement Levels", BCP 14, RFC 2119, March 1997.
 [TOKEN]   Thomas, S., "Transmission of IPv6 Packets over Token Ring
           Networks", Work in Progress.
 [TRAN]    Gilligan, R., and E. Nordmark, "Transition Mechanisms for
           IPv6 Hosts and Routers", RFC 1993, April 1996.

Hinden & Deering Standards Track [Page 24] RFC 2373 IPv6 Addressing Architecture July 1998

AUTHORS' ADDRESSES

 Robert M. Hinden
 Nokia
 232 Java Drive
 Sunnyvale, CA 94089
 USA
 Phone: +1 408 990-2004
 Fax:   +1 408 743-5677
 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
 Fax:   +1 408 527-8254
 EMail: deering@cisco.com

Hinden & Deering Standards Track [Page 25] RFC 2373 IPv6 Addressing Architecture July 1998

Full Copyright Statement

 Copyright (C) The Internet Society (1998).  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.

Hinden & Deering Standards Track [Page 26]

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