INTERNET-DRAFT R. Hinden, Nokia
September 24, 2004 B. Haberman, JHU-APL
Unique Local IPv6 Unicast Addresses
<draft-ietf-ipv6-unique-local-addr-06.txt>
Status of this Memo
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This internet draft expires on March 29, 2005.
Abstract
This document defines an IPv6 unicast address format that is globally
unique and is intended for local communications, usually inside of a
site. They are not expected to be routable on the global Internet.
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Table of Contents
1.0 Introduction....................................................2
2.0 Acknowledgments.................................................3
3.0 Local IPv6 Unicast Addresses....................................3
3.1 Format..........................................................3
3.1.1 Background....................................................4
3.2 Global ID.......................................................4
3.2.1 Locally Assigned Global IDs...................................5
3.2.2 Sample Code for Pseudo-Random Global ID Algorithm.............6
3.2.3 Analysis of the Uniqueness of Global IDs......................6
3.3 Scope Definition................................................7
4.0 Routing.........................................................7
5.0 Renumbering and Site Merging....................................8
6.0 Site Border Router and Firewall Packet Filtering................8
7.0 DNS Issues......................................................9
8.0 Application and Higher Level Protocol Issues....................9
9.0 Use of Local IPv6 Addresses for Local Communications............9
10.0 Use of Local IPv6 Addresses with VPNs.........................10
11.0 Advantages and Disadvantages..................................10
12.0 Security Considerations.......................................11
13.0 IANA Considerations...........................................11
14.0 References....................................................12
14.1 Normative References..........................................12
14.2 Informative References........................................13
15.0 Authors' Addresses............................................13
16.0 Change Log....................................................14
1.0 Introduction
This document defines an IPv6 unicast address format that is globally
unique and is intended for local communications [IPV6]. These
addresses are called Unique Local IPv6 Unicast Addresses and are
abbreviated in this document as Local IPv6 addresses. They are not
expected to be routable on the global Internet. They are routable
inside of a more limited area such as a site. They may also be
routed between a limited set of sites.
Local IPv6 unicast addresses have the following characteristics:
- Globally unique prefix.
- Well known prefix to allow for easy filtering at site
boundaries.
- Allows sites to be combined or privately interconnected without
creating any address conflicts or requiring renumbering of
interfaces using these prefixes.
- Internet Service Provider independent and can be used for
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communications inside of a site without having any permanent or
intermittent Internet connectivity.
- If accidentally leaked outside of a site via routing or DNS,
there is no conflict with any other addresses.
- In practice, applications may treat these addresses like global
scoped addresses.
This document defines the format of Local IPv6 addresses, how to
allocate them, and usage considerations including routing, site
border routers, DNS, application support, VPN usage, and guidelines
for how to use for local communication inside a site.
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 Acknowledgments
The underlying idea of creating Local IPv6 addresses described in
this document been proposed a number of times by a variety of people.
The authors of this draft do not claim exclusive credit. Credit goes
to Brian Carpenter, Christian Huitema, Aidan Williams, Andrew White,
Charlie Perkins, and many others. The authors would also like to
thank Brian Carpenter, Charlie Perkins, Harald Alvestrand, Keith
Moore, Margaret Wasserman, Shannon Behrens, Alan Beard, Hans Kruse,
Geoff Huston, Pekka Savola, Christian Huitema, and Tim Chown for
their comments and suggestions on this document.
3.0 Local IPv6 Unicast Addresses
3.1 Format
The Local IPv6 addresses are created using a pseudo-randomly
allocated global ID. They have the following format:
| 7 bits | 41 bits | 16 bits | 64 bits |
+--------+------------+-----------+-----------------------------+
| prefix | global ID | subnet ID | interface ID |
+--------+------------+-----------+-----------------------------+
Where:
prefix FC00::/7 prefix to identify Local IPv6 unicast
addresses.
global ID 41-bit global identifier used to create a
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globally unique prefix. See section 3.2 for
additional information.
subnet ID 16-bit subnet ID is an identifier of a subnet
within the site.
interface ID 64-bit interface ID as defined in [ADDARCH].
3.1.1 Background
There were a range of choices available when choosing the size of the
prefix and global ID field length. There is a direct tradeoff
between having a global ID field large enough to support foreseeable
future growth and not using too much of the IPv6 address space
needlessly. A reasonable way of evaluating a specific field length
is to compare it to a projected 2050 world population of 9.3 billion
[POPUL] and the number of resulting /48 prefixes per person. A range
of prefix choices is shown in the following table:
Prefix Global ID Number of Prefixes % of IPv6
Length /48 Prefixes per Person Address Space
/11 37 137,438,953,472 15 0.049%
/10 38 274,877,906,944 30 0.098%
/9 39 549,755,813,888 59 0.195%
/8 40 1,099,511,627,776 118 0.391%
/7 41 2,199,023,255,552 236 0.781%
/6 42 4,398,046,511,104 473 1.563%
A very high utilization ratio of these allocations can be assumed
because the global ID field does not require internal structure, and
there is no reason to be able to aggregate the prefixes.
The authors believe that a /7 prefix resulting in a 41 bit global ID
is a good choice. It provides for a large number of assignments
(i.e., 2.2 trillion) and at the same time uses less than .8% of the
total IPv6 address space. It is unlikely that this space will be
exhausted. If more than this were to be needed, then additional IPv6
address space could be allocated for this purpose.
3.2 Global ID
The allocation of global IDs should be pseudo-random [RANDOM]. They
should not be assigned sequentially or with well known numbers. This
is to ensure that there is not any relationship between allocations
and to help clarify that these prefixes are not intended to be routed
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globally. Specifically, these prefixes are designed to not
aggregate.
There are two ways to allocate Global IDs. These are centrally by a
allocation authority and locally by the site. The Global ID is split
into two parts for each type of allocation. The prefixes for each
type are:
FC00::/8 Centrally assigned
FD00::/8 Locally assigned
Each results in a 40-bit space to allocate.
Two assignment methods are included because they have different
properties. The centrally assigned global IDs are uniquely assigned.
The local assignments are self generated and do not need any central
coordination or assignment, but have a lower (but still adequate)
probability of being unique. It is expected that large managed sites
will prefer central assignments and small or disconnected sites will
prefer local assignments. It is recommended that sites planning to
use Local IPv6 addresses for extensive inter-site communication,
initially or as a future possibility, use a centrally assigned prefix
as there is no possibility of assignment conflicts. Sites are free
to choose either approach.
This document only allocates the prefix (FC00::/8) for centrally
assigned local IPv6 addresses. The characteristics and technical
allocation requirements for centrally assigned Local IPv6 addresses
will be defined in a separate document.
3.2.1 Locally Assigned Global IDs
Global IDs can be generated locally by an individual site. This
makes it easy to get a prefix without the need to contact an
assignment authority or internet service provider. There is not as
high a degree of assurance that the prefix will not conflict with
another locally generated prefix, but the likelihood of conflict is
small. Sites that are not comfortable with this degree of
uncertainty should use a centrally assigned global ID.
Locally assigned global IDs MUST be generated with a pseudo-random
algorithm consistent with [RANDOM]. Section 3.2.2 describes a
suggested algorithm. It is important to ensure a reasonable
likelihood uniqueness that all sites generating a Global IDs use a
functionally similar algorithm.
The use of a pseudo-random algorithm to generate global IDs in the
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locally assigned prefix gives an assurance that any network numbered
using such a prefix is highly unlikely to have that address space
clash with any other network that has another locally assigned prefix
allocated to it. This is a particularly useful property when
considering a number of scenarios including networks that merge,
overlapping VPN address space, or hosts mobile between such networks.
3.2.2 Sample Code for Pseudo-Random Global ID Algorithm
The algorithm described below is intended to be used for locally
assigned Global IDs. In each case the resulting global ID will be
used in the appropriate prefix as defined in section 3.2.
1) Obtain the current time of day in 64-bit NTP format [NTP].
2) Obtain an EUI-64 identifier from the system running this
algorithm. If an EUI-64 does not exist, one can be created from
a 48-bit MAC address as specified in [ADDARCH]. If an EUI-64
cannot be obtained or created, a suitably unique identifier,
local to the node, should be used (e.g. system serial number).
3) Concatenate the time of day with the system-specific identifier
creating a key.
4) Compute an MD5 digest on the key as specified in [MD5DIG].
5) Use the least significant 40 bits as the Global ID.
This algorithm will result in a global ID that is reasonably unique
and can be used as a Global ID.
3.2.3 Analysis of the Uniqueness of Global IDs
The selection of a pseudo random global ID is similar to the
selection of an SSRC identifier in RTP/RTCP defined in section 8.1 of
[RTP]. This analysis is adapted from that document.
Since the global ID is chosen randomly, it is possible that two or
more networks that have an inter-network connection using globally-
unique local addresses will chose the same global ID. The
probability of collision can be approximated based on the number of
connections between networks using globally-unique local addresses
and the length of the ID (40 bits). The formula
P = 1 - exp(-N**2 / 2**(L+1))
approximates the probability of collision (where N is the number
connections and L is the length of the global ID).
The following table shows the probability of a collision for a range
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of connections using a 40 bit global ID field.
Connections Probability of Collision
2 1.81*10^-12
10 4.54*10^-11
100 4.54*10^-09
1000 4.54*10^-07
10000 4.54*10^-05
Based on this analysis the uniqueness of locally generated global IDs
is adequate for sites planning a small to moderate amount of inter-
site communication using locally generated global IDs. Sites
planning more extensive inter-site communication should consider
using the centrally assigned global ID.
3.3 Scope Definition
By default, the scope of these addresses is global. That is, they
are not limited by ambiguity like the site-local addresses defined in
[ADDARCH]. Rather, these prefixes are globally unique, and as such,
their applicability is greater than site-local addresses. Their
limitation is in the routability of the prefixes, which is limited to
a site and any explicit routing agreements with other sites to
propagate them. Also, unlike site-locals, a site may have more than
one of these prefixes and use them at the same time.
4.0 Routing
Local IPv6 addresses are designed to be routed inside of a site in
the same manner as other types of unicast addresses. They can be
carried in any IPv6 routing protocol without any change.
It is expected that they would share the same subnet IDs with
provider based global unicast addresses if they were being used
concurrently [GLOBAL].
Any router that is used between sites must be configured to filter
out any incoming or outgoing Local IPv6 unicast routes. The
exception to this is if specific /48 or longer IPv6 local unicast
routes have been configured to allow for inter-site communication.
If BGP is being used at the site border with an ISP, the default BGP
configuration must be set to to keep any Local IPv6 address prefixes
from being advertised outside of the site or for these prefixes to be
learned from another site. The exception to this is if there are
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specific /48 or longer routes created for one or more Local IPv6
prefixes.
5.0 Renumbering and Site Merging
The use of Local IPv6 addresses in a site results in making
communication using these addresses independent of renumbering a
site's provider based global addresses.
When merging multiple sites the addresses created with these prefixes
are unlikely to need to be renumbered because all of the addresses
have a high probability of being unique. Routes for each specific
prefix would have to be configured to allow routing to work correctly
between the formerly separate sites.
6.0 Site Border Router and Firewall Packet Filtering
While no serious harm will be done if packets with these addresses
are sent outside of a site via a default route, it is recommended
that routers be configured by default to keep any packets with Local
IPv6 destination addresses from leaking outside of the site and to
keep any site prefixes from being advertised outside of their site.
Site border routers should install a "reject" route for the Local
IPv6 prefix FC00::/7. This will ensure that packets with Local IPv6
destination addresses will not be forwarded outside of the site via a
default route. Site border routers should respond with the
appropriate ICMPv6 Destination Unreachable message to inform the
source that the packet was not forwarded [ICMPV6]. This feedback is
important to avoid transport protocol timeouts.
Site border routers and firewalls should not forward any packets with
Local IPv6 source or destination addresses outside of the site unless
they have been explicitly configured with routing information about
specific /48 or longer Local IPv6 prefixes. The default behavior of
these devices should be to install a "reject" route for these
prefixes. Site border routers should respond with the appropriate
ICMPv6 Destination Unreachable message to inform the source that the
packet was not forwarded.
Routers that maintain peering arrangements between Autonomous Systems
throughout the Internet should obey the recommendations for site
border routers unless configured otherwise.
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7.0 DNS Issues
AAAA and PTR records for locally assigned local IPv6 addresses are
not recommended to be installed in the global DNS.
8.0 Application and Higher Level Protocol Issues
Application and other higher level protocols can treat Local IPv6
addresses in the same manner as other types of global unicast
addresses. No special handling is required. This type of addresses
may not be reachable, but that is no different from other types of
IPv6 global unicast addresses. Applications need to be able to
handle multiple addresses that may or may not be reachable any point
in time. In most cases this complexity should be hidden in APIs.
From a host's perspective this difference shows up as different
reachability than global unicast and could be handled by default that
way. In some cases it is better for nodes and applications to treat
them differently from global unicast addresses. A starting point
might be to give them preference over global unicast, but fall back
to global unicast if a particular destination is found to be
unreachable. Much of this behavior can be controlled by how they are
allocated to nodes and put into the DNS. However it is useful if a
host can have both types of addresses and use them appropriately.
Note that the address selection mechanisms of [ADDSEL], and in
particular the policy override mechanism replacing default address
selection, are expected to be used on a site where Local IPv6
addresses are configured.
9.0 Use of Local IPv6 Addresses for Local Communications
Local IPv6 addresses, like global scope unicast addresses, are only
assigned to nodes if their use has been enabled (via IPv6 address
autoconfiguration [ADDAUTO], DHCPv6 [DHCP6], or manually). They are
not created automatically the way that IPv6 link-local addresses are
and will not appear or be used unless they are purposely configured.
In order for hosts to autoconfigure Local IPv6 addresses, routers
have to be configured to advertise Local IPv6 /64 prefixes in router
advertisements, or a DHCPv6 server must have been configured to
assign them. In order for a node to learn the Local IPv6 address of
another node, the Local IPv6 address must have been installed in the
DNS or another naming system. For these reasons, it is straight
forward to control their usage in a site.
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To limit the use of Local IPv6 addresses the following guidelines
apply:
- Nodes that are to only be reachable inside of a site: The local
DNS should be configured to only include the Local IPv6
addresses of these nodes. Nodes with only Local IPv6 addresses
must not be installed in the global DNS.
- Nodes that are to be limited to only communicate with other
nodes in the site: These nodes should be set to only
autoconfigure Local IPv6 addresses via [ADDAUTO] or to only
receive Local IPv6 addresses via [DHCP6]. Note: For the case
where both global and Local IPv6 prefixes are being advertised
on a subnet, this will require a switch in the devices to only
autoconfigure Local IPv6 addresses.
- Nodes that are to be reachable from inside of the site and from
outside of the site: The DNS should be configured to include
the global addresses of these nodes. The local DNS may be
configured to also include the Local IPv6 addresses of these
nodes.
- Nodes that can communicate with other nodes inside of the site
and outside of the site: These nodes should autoconfigure global
addresses via [ADDAUTO] or receive global address via [DHCP6].
They may also obtain Local IPv6 addresses via the same
mechanisms.
10.0 Use of Local IPv6 Addresses with VPNs
Local IPv6 addresses can be used for inter-site Virtual Private
Networks (VPN) if appropriate routes are set up. Because the
addresses are unique these VPNs will work reliably and without the
need for translation. They have the additional property that they
will continue to work if the individual sites are renumbered or
merged.
11.0 Advantages and Disadvantages
11.1 Advantages
This approach has the following advantages:
- Provides Local IPv6 prefixes that can be used independently of
any provider based IPv6 unicast address allocations. This is
useful for sites not always connected to the Internet or sites
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that wish to have a distinct prefix that can be used to localize
traffic inside of the site.
- Applications can treat these addresses in an identical manner as
any other type of global IPv6 unicast addresses.
- Sites can be merged without any renumbering of the Local IPv6
addresses.
- Sites can change their provider based IPv6 unicast address
without disrupting any communication using Local IPv6 addresses.
- Well known prefix that allows for easy filtering at site
boundary.
- Can be used for inter-site VPNs.
- If accidently leaked outside of a site via routing or DNS, there
is no conflict with any other addresses.
11.2 Disadvantages
This approach has the following disadvantages:
- Not possible to route Local IPv6 prefixes on the global Internet
with current routing technology. Consequentially, it is
necessary to have the default behavior of site border routers to
filter these addresses.
- There is a very low probability of non-unique locally assigned
global IDs being generated by the algorithm in section 3.2.3.
This risk can be ignored for all practical purposes, but it
leads to a theoretical risk of clashing address prefixes.
12.0 Security Considerations
Local IPv6 addresses do not provide any inherent security to the
nodes that use them. They may be used with filters at site
boundaries to keep Local IPv6 traffic inside of the site, but this is
no more or less secure than filtering any other type of global IPv6
unicast addresses.
Local IPv6 addresses do allow for address-based security mechanisms,
including IPSEC, across end to end VPN connections.
13.0 IANA Considerations
The IANA is instructed to assign the FC00::/7 prefix for Unique Local
IPv6 unicast addresses.
The prefix is divided in half for the following purposes:
FC00::/8 Centrally assigned
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FD00::/8 Locally assigned
The IANA is instructed to reserve the prefix FC00::/8 for Centrally
assigned Unique Local IPv6 unicast addresses.
The FD00::/8 prefix is defined in this specification for Locally
assigned Unique Local IPv6 unicast addresses.
14.0 References
14.1 Normative References
[ADDARCH] Hinden, R., S. Deering, S., "IP Version 6 Addressing
Architecture", RFC 3513, April 2003.
[GLOBAL] Hinden, R., S. Deering, E. Nordmark, "IPv6 Global Unicast
Address Format", RFC 3587, August 2003.
[ICMPV6] Conta, A., S. Deering, "Internet Control Message Protocol
(ICMPv6) for the Internet Protocol Version 6 (IPv6)
Specification", RFC2463, December 1998.
[IPV6] Deering, S., R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998.
[MD5DIG] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
April 1992.
[NTP] Mills, David L., "Network Time Protocol (Version 3)
Specification, Implementation and Analysis", RFC 1305,
March 1992.
[POPUL] Population Reference Bureau, "World Population Data Sheet
of the Population Reference Bureau 2002", August 2002.
[RANDOM] Eastlake, D. 3rd, S. Crocker, J. Schiller, "Randomness
Recommendations for Security", RFC 1750, December 1994.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, BCP14, March 1997.
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14.2 Informative References
[ADDAUTO] Thomson, S., T. Narten, "IPv6 Stateless Address
Autoconfiguration", RFC 2462, December 1998.
[ADDSEL] Draves, R., "Default Address Selection for Internet
Protocol version 6 (IPv6)", RFC 3484, February 2003.
[DHCP6] Droms, R., et. al., "Dynamic Host Configuration Protocol
for IPv6 (DHCPv6)", RFC3315, July 2003.
[RTP] Schulzrinne, H., S. Casner, R. Frederick, V. Jacobson,
"RTP: A Transport Protocol for Real-Time Applications"
RFC3550, July 2003.
15.0 Authors' Addresses
Robert M. Hinden
Nokia
313 Fairchild Drive
Mountain View, CA 94043
USA
phone: +1 650 625-2004
email: bob.hinden@nokia.com
Brian Haberman
Johns Hopkins University
Applied Physics Lab
11100 Johns Hopkins Road
Laurel, MD 20723
USA
phone: +1 443 778 1319
email: brian@innovationslab.net
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16.0 Change Log
Draft <draft-ietf-ipv6-unique-local-addr-06.txt>
o Clarify text to permit prefixes longer than /48 to be
configured.
o Changed text in section 7.0 to recommend that locally assigned
ULA addresses are not installed in the global DNS and removed
text about consequences of if they were installed in the global
DNS.
o Clarify the text in section 5.1 to state that there is a high
probability that there will be no address conflict when
renumbering.
o Several minor editorial changes.
Draft <draft-ietf-ipv6-unique-local-addr-05.txt>
o Removed the definition and technical requirements for centrally
assigned local address. The Centrally assigned local addresses
will be defined in a separate document. This document defines
the specific prefixes to be used for centrally and locally
assigned IPv6 local addresses, but only the locally assigned
local addresses are defined here.
Draft <draft-ietf-ipv6-unique-local-addr-04.txt>
o Clarified text in section 3.2.1 that central assigned prefixes
should be assigned under the authority of a single allocation
organization.
o Added step to suggested pseudo-random algorithm that in the case
of centrally assigned prefixes the computed global IDs should be
verified against the escrow.
o Added new text to section 3.2.2 that explains in more detail the
need for pseudo-random global IDs (i.e., avoid duplicate
allocations).
o Rewrote section 7.0 to describe DNS AAAA and PTR records, and
clarify when they might be installed in the global DNS.
o Various editorial changes.
Draft <draft-ietf-ipv6-unique-local-addr-03.txt>
o Removed requirement of a fee per central allocation and updated
IANA considerations to reflect this. Rewrote text to focus on
the requirement to avoid hoarding of allocations.
o Changed "filtering" and "black hole routes" to "reject" routes.
o Changed uppers case requirements (i.e., MUST, SHOULD, etc.) to
lower case in sections giving operational advice.
o Removed paragraph mentioning "Multicast DNS".
o Various editorial changes.
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Draft <draft-ietf-ipv6-unique-local-addr-02.txt>
o Removed mention of 10 euro charge and changed text in section
3.2.1 and IANA considerations to restate the requirement for low
cost allocations and added specific requirement to prevent
hording.
o Added need to send ICMPv6 destination unreachable messages if
packets are filtered or dropped at site boundaries.
o Changed format section to list prefix sizes and definition, and
changed discussion of prefix sizes to new background section.
o Various editorial changes.
Draft <draft-ietf-ipv6-unique-local-addr-01.txt>
o Removed example of PIR as an example of an allocation authority
and clarified the text that the IANA should delegate the
centrally assigned prefix to an allocation authority.
o Changed sample code for generating pseudo random Global IDs to
not require any human input. Changes from using birthday to
unique token (e.g., EUI-64, serial number, etc.) available on
machine running the algorithm.
o Added a new section analyzing the uniqueness properties of the
pseudo random number algorithm.
o Added text to recommend that centrally assigned local addresses
be used for site planning extensive inter-site communication.
o Clarified that domain border routers should follow site border
router recommendations.
o Clarified that AAAA DNS records should not be installed in the
global DNS.
o Several editorial changes.
Draft <draft-ietf-ipv6-unique-local-addr-00.txt>
o Changed file name to become an IPv6 w.g. group document.
o Clarified language in Routing and Firewall sections.
o Several editorial changes.
Draft <draft-hinden-ipv6-global-local-addr-02.txt>
o Changed title and name of addresses defined in this document to
"Unique Local IPv6 Unicast Addresses" with abbreviation of
"Local IPv6 addresses".
o Several editorial changes.
Draft <draft-hinden-ipv6-global-local-addr-01.txt>
o Added section on scope definition and updated application
requirement section.
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o Clarified that, by default, the scope of these addresses is
global.
o Renumbered sections and general text improvements
o Removed reserved global ID values
o Added pseudo code for local allocation submitted by Brian
Haberman and added him as an author.
o Split Global ID values into centrally assigned and local
assignments and added text to describe local assignments
Draft <draft-hinden-ipv6-global-local-addr-00.txt>
o Initial Draft
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