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Versions: 00 01 02 rfc1884                                              
                                                      R. Hinden, Editor
                                                 Ipsilon Networks, Inc.
                                                          March 9, 1995

                  IP Version 6 Addressing Architecture

Status of this Memo

   This document is an Internet-Draft.  Internet-Drafts are working
   documents of the Internet Engineering Task Force (IETF), its areas,
   and its working groups.  Note that other groups may also distribute
   working documents as Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet- Drafts as reference
   material or to cite them other than as ``work in progress.''

   To learn the current status of any Internet-Draft, please check the
   ``1id-abstracts.txt'' listing contained in the Internet- Drafts
   Shadow Directories on ds.internic.net (US East Coast), nic.nordu.net
   (Europe), ftp.isi.edu (US West Coast), or munnari.oz.au (Pacific

   This Internet Draft expires October 1, 1995.

draft-ietf-ipngwg-ipv6-addr-arch-00.txt                         [Page 1]

INTERNET-DRAFT        IPv6 Addressing Architecture            March 1995


This specification defines the addressing architecture of the IP Version
6 protocol.  It includes a detailed description of the address formats
for IPv6 [IPV6].

The document editor would like to acknowledge the contributions of Paul
Francis, Steve Deering, Jim Bound, Brian Carpenter, Bob Gilligan,
Christian Huitema, Greg Minshall, Erik Nordmark, Bill Simpson, and Sue
Thomson.  Special mention is also given to Yakov Rekhtor, Tony Li,
Deborah Estrin, and Peter Ford for the current definition of region


IPv6 addresses are 128-bit identifiers for interfaces and sets of
interfaces.  There are three types of addresses:

    Unicast:   Identifier for a single interface.  Packets sent to a
               unicast address are delivered to the interface identified
               by that address.

    Region:    Identifier for a set of interfaces on the border of a
               region.  Packets sent to a region address are delivered
               to one interface in that region.

    Multicast: Identifier for a set of interfaces (typically belonging
               to different nodes).  Packets sent to a multicast address
               are delivered to all interfaces identified by that
               multicast 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 "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

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fields or end in zeros.

2.1 Addressing Model

IPv6 Addresses of all types are assigned to interfaces, not nodes.
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.

An IPv6 unicast address refers to a single interface.  A single
interface may be assigned multiple IPv6 addresses of any type (unicast,
region, and multicast).  There are two exceptions to this model.  These

  1) A single address 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.

  2) Routers may have unnumbered interfaces (i.e., no IPv6 address
     assigned to the interface) on point-to-point links to eliminate the
     necessity to manually configure and advertise the addresses.
     Addresses are not need for point-to-point interfaces on routers if
     those interfaces are not to be used as the origins or destinations
     of any IPv6 datagrams.

IPv6 continues the IPv4 model that a subnet is associated with one link.
Multiple subnets 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.



     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 the method of allocating certain styles of IPv6 addresses,

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     it will be common for addresses to contain long strings of zero
     bits.  In order to make writing address containing zero bits easier
     a special syntax is available to compress the zeros.  The use of
     two "::" indicate multiple groups of 16-bits of zeros.  For example
     the multicast address:


     may be represented as:


     The "::" can only appear once in an address.  The "::" can also be
     used to compress the leading or trailing zeros in an address.

  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:



     or in compressed form:



2.3 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:

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     Allocation                         Prefix         Fraction of
                                        (binary)       Address Space
     -------------------------------    --------       -------------
     Reserved                           0000 0000      1/256
     Reserved                           0000 0001      1/256

     NSAP Allocation                    0000 001       1/128
     IPX Allocation                     0000 010       1/128

     Reserved                           0000 011       1/128
     Reserved                           0000 1         1/32
     Reserved                           0001           1/16
     Reserved                           001            1/8

     Provider-Based Unicast Address     010            1/8

     Reserved                           011            1/8

     Reserved for Neutral-Interconnect-
     Based Unicast Addresses            100            1/8

     Reserved                           101            1/8
     Reserved                           110            1/8
     Reserved                           1110           1/16
     Reserved                           1111 0         1/32
     Reserved                           1111 10        1/64
     Reserved                           1111 110       1/128

     Reserved                           1111 1110 0    1/512

     Link Local Use Addresses           1111 1110 10   1/1024
     Site Local Use Addresses           1111 1110 11   1/1024

     Multicast Addresses                1111 1111      1/256

     Note:  IPv6 Addresses with Embedded IPv4 Addresses (see section
     2.4.7), the "unspecified address" (see section 2.4.5), and the
     loopback address (see section 2.4.6), are assigned out of the 0000
     0000 format prefix space.

This allocation supports the direct allocation of provider addresses,
NSAP addresses, IPX addresses, local use addresses, and multicast
addresses.  Space is reserved for neutral-interconnect addresses.  The
remainder of the address space is reserved for future use.  This can be
used for expansion of existing use (e.g., additional provider addresses,
IPX addresses, etc.) or new uses (e.g., separate locators and
identifiers).  Fifteen percent of the address space is initially

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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.  Region addresses are taken from the
unicast address space, and are not syntactically distinguishable from
unicast addresses.

2.4 Unicast Addresses

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

There are several forms of unicast address assignment in IPv6, including
the global provider based unicast address, the neutral-interconnect
unicast address, the NSAP address, the IPX hierarchical address, the
site-local-use address, the link-local-use address, and the IPv4-capable
host address.  Additional addresses types can be defined in the future.

IPv6 nodes may have considerable or little knowledge of the internal
structure of the IPv6 address, depending on the role the node plays (for
instance, host versus router).  At a minimum, a node may consider that
unicast addresses (including its own) have no internal structure:

 |                           128 bits                              |
 |                          node address                           |

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

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

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

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positions the router holds in the routing hierarchy.

2.4.1 Unicast Address Examples

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

 |              n bits            |  m bits   |     48 bits        |
 |        subscriber prefix       | subnet ID |   interface ID     |

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

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

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

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

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

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2.4.2 Provider-Based Global Unicast Addresses

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

                                                      | 125-m-n- |
  | 3 |  n bits   |  m bits   |   o bits    | p bits  | o-p bits |
  |010|registry ID|provider ID|subscriber ID|subnet ID| intf. ID |

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

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

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

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

The subnet ID identifies a specific physical link.  There can be
multiple subnets on the same physical link.  A specific subnet can not
span multiple physical links.  The term "subnet prefix" refers to the
high-order part of the address up to and including the subnet ID.  The
group of nodes identified by the subnet ID must be attached to the same

The interface ID identifies a single interface among the group of
interfaces identified by the subnet prefix.

2.4.3 NSAP Addresses

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

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 |   7   |1|  4  |  12   |  32 bits   | 16 bits|      48 bits      |
 |0000001|G| AFC |  IDI  |   Prefix   |  Area  |        ID         |

The complete definition, motivation, and usage can be found in [NSAP].

2.4.4 Local-use IPv6 Unicast Addresses

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

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

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

Site-Local addresses have the following format:

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

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

2.4.5 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.

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One example of its use is in the Source Address field of any IPv6
datagrams sent by an initializing host before it has learned its own

The unspecified address must not be used as the destination address of
IPv6 datagrams or in IPv6 Routing Headers.

2.4.6 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 a IPv6 datagram to itself.  It may never
be assigned to any interface.

The loopback address must not be used as the source address in IPv6
datagrams that are sent outside of a single node.

2.4.7 IPv6 Addresses with Embedded IPv4 Addresses

The IPv6 transition mechanisms [TMV6] 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

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

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2.5 Region Addresses

Region Addresses provide information abstraction for the purpose of
network layer routing.  A subgraph of an internet forms a "Region" with
an identifier called a "Region-ID" if all the following conditions are

  1. Associated with each Region is a globally unique identifier called
     a Region-ID.  Syntactically, a Region-ID is a 128-bit IPv6 unicast

  2. There is an address administration (at least one, but maybe
     several) that is responsible for unicast address allocation for all
     the nodes within the subgraph.  A Region-ID is assigned by one such
     administration out of the unicast address block used by the
     administration for unicast address allocation within the subgraph.

  3. All the routers in the Region that connect the subgraph (or its
     connected components) with the rest of the internet are called
     "Border Routers". These Region Border Routers (or just Border
     Routers) are preconfigured with the Region-ID.  The configuration
     procedures are outside the scope of the definition.

  4. At least some of a Region's Border Routers advertise into the
     Region's routing system a  prefix that matches (in the sense of the
     "longest match" algorithm) the Region's Region-ID.  In addition, a
     Region's Border Router may advertise into the routing system a
     prefix equal to the Region-ID only (i.e., a host route).

  5. If Region-IDs are used to specify transit policies (i.e., specify
     that a packet should pass through at least one node in a Region
     identified by a particular Region Identifier), in order to enforce
     strict source routes, the Region Border Router must know the
     Region-IDs of all other routers that share a common link with the

Region addresses MUST NOT be used as a source address in any IPv6

The Border Routers of the Region consider the Region-ID to be one of
their addresses, and will accept packets with a destination address
equal to the Region-ID.  All the nodes within each connected component
that forms a Region, including Region Border Routers, are said to be
"members of the region".

Region-IDs can be used to represent Routing Domains, Routing Domain
Confederations, OSPF areas, Subnets, as well as generic anycast
addresses.  This is described as follows:

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  1. A Routing Domain is a Region.  The domain is identified by a
     Routing Domain Identifier (RDI), which is simply the Region-
     Identifier for the domain.  Any Border Router of a domain is a
     Region Border Router.

  2. A Routing Domain Confederation is a Region.  The confederation is
     identified by a Routing Domain Confederation Identifier (RDCI),
     which is the Region Identifier for the confederation.  A Border
     Router of a domain within a confederation that peers with a Border
     Router in a domain that is not in the confederation is a Region
     Border Router. In the case of confederations, note that all Border
     Routers of domains nested within a confederation must be configured
     with the Region-IDs of the confederation(s) in which they are

  3. An OSPF area is a Region.  The Region Identifier of such a Region
     may be any unicast address that matches address prefixes within the
     area.  Note that OSPF area Border Routers do not need to be
     configured with the Region-ID of their containing domain.

  4. An IPv6 subnet is a Region. The Region includes all the nodes
     attached to the subnet.  Any router attached to the subnet is a
     Region Border Router.  The Region-ID is an address out of the

  5. A set of nodes that wish to share an anycast address can be
     represented by a Region-ID where each element in the anycast group
     is configured as a Border Router for that Region.  Note that
     unconstrained usage of anycast addresses can lead to scaling

2.6 Multicast Addresses

A 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.

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     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

          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  community-local scope
          C  (unassigned)
          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 43 (hex), then:

     FF01:0:0:0:0:0:0:43 means all NTP servers on the same node as the

     FF02:0:0:0:0:0:0:43 means all NTP servers on the same link as the

     FF05:0:0:0:0:0:0:43 means all NTP servers at the same site as the

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     FF0E:0:0:0:0:0:0:43 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 FF15:0:0:0:0:0:0:43 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
datagrams or appear in any routing header.

2.6.1 Pre-Defined Multicast Addresses

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

  Reserved Multicast Addresses:   FF00:0:0:0:0:0:0:0

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

The above multicast addresses identify the group of all IPv6 nodes,
within scope 1 (node-local) or 2 (link-local).

  All Hosts Addresses:     FF01:0:0:0:0:0:0:2

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The above multicast addresses identify the group of all IPv6 hosts,
within scope 1 (node-local) or 2 (link-local).

  All Routers Addresses:   FF01:0:0:0:0:0:0:3

The above multicast addresses identify the group of all IPv6 routers,
within scope 1 (node-local) or 2 (link-local).

2.7 A Node's Required Addresses

A host is required to recognize the following addresses as identifying

   o Assigned Unicast Addresses
   o Loopback Address
   o All Nodes Multicast Address
   o All Hosts Multicast Address
   o All other Multicast Addresses to which the host belongs.

A router is required to recognize the following addresses as identifying

   o Assigned Unicast Addresses
   o Region Addresses of all configured regions for which the router is
     a boundary router
   o Loopback Address
   o All Nodes Multicast Address
   o All Router Multicast Address
   o All other Multicast Addresses 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 Pre-Defined Multicast Addresses
   o IPv4 Compatible Prefixes

Implementations should assume all other addresses are unicast unless
specifically configured (e.g., region addresses).


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IPv6 routing algorithms are identical to those used with the CIDR
version of IP, except that the address used is 128 bits rather than 32
(for instance [OSPF], [RIP_]).

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   [ASSN] Yakov Rekhter, "IPv6 Provider Unicast Address Assignment",
          Internet Draft.

   [AUTO] S. Thomson, "Automatic Host Address Assignment in IPv6",
          Internet Draft.

   [CIDR] V. Fuller, T. Li, K. Varadhan, J. Yu, "Supernetting: an
          Address Assignment and Aggregation Strategy", RFC 1338.

   [DISC] W. Simpson, "IPv6 Neighbor Discovery -- ICMP Message Formats",
          Internet Draft.

   [ICMP] S. Deering, A. Conta, "ICMP and IGMP for the Internet Protocol
          Version 6 (IPv6)" Internet-Draft.

   [IPV6]  R. Hinden, Editor, "Internet Protocol, Version 6 (IPv6)
          Specification", Internet Draft.

   [MULT] S. Deering, "Host Extensions for IP multicasting", RFC 1112.

   [NSAP] B. Carpenter, J. Bound, "Recommendations for OSI NSAP usage in
          IPv6", Internet Draft.

   [OSPF] "OSPF for IPv6", Internet Draft, In preparation.

   [RIP_] "RIPv2 for IPv6, Internet Draft, In preparation.

   [TMV6] R. Gilligan, E. Nordmark, "Transition Mechanisms for IPv6
          Hosts and Routers," Internet-Draft.


     Robert M. Hinden
     Ipsilon Network, Inc.
     2465 Latham Street
     Suite 100
     Mt. View, CA 94040

     Phone: (415) 528-4604
     FAX:   (415) 854-4653
     Email: hinden@ipsilon.com

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Changes from Previous Version

   This version of the "IPv6 Addressing Architecture" includes the
   following changes made since the previous version:

        o Added "Addressing Model" section.

        o Changed "Node ID" to "Interface ID" to reflect the current
          Addressing Model.

        o Cluster Address replaced by Region Address.

        o Geographic Addresses changed to be Neutral-Interconnect

        o Changed multicast scope names from inter-*** to ***-local

        o Added mention that address subfields can be assigned a zero or
          ones value.

        o Reduced the amount address space assigned to local use and
          divided the local use address space into link-local and site-
          local unicast.

        o Swapped prefixes for IPv4-compatible and IPv4 mapped

        o Changed definition of loopback address.

        o Added wording about wired in knowledge of address prefixes.

        o Minor clarification's, corrections, and typos fixed.

        o New typos likely added.

draft-ietf-ipngwg-ipv6-addr-arch-00.txt                        [Page 18]