Network Working Group R. Hinden
Request for Comments: 2374 Nokia
Obsoletes: 2073 M. O'Dell
Category: Standards Track UUNET
July 1998 An IPv6 Aggregatable Global Unicast Address Format
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 (C) The Internet Society (1998). All Rights Reserved.
This document defines an IPv6 aggregatable global unicast address
format for use in the Internet. The address format defined in this
document is consistent with the IPv6 Protocol [IPV6] and the "IPv6
Addressing Architecture" [ARCH]. It is designed to facilitate
scalable Internet routing.
This documented replaces RFC 2073, "An IPv6 Provider-Based Unicast
Address Format". RFC 2073 will become historic. The Aggregatable
Global Unicast Address Format is an improvement over RFC 2073 in a
number of areas. The major changes include removal of the registry
bits because they are not needed for route aggregation, support of
EUI-64 based interface identifiers, support of provider and exchange
based aggregation, separation of public and site topology, and new
aggregation based terminology.
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 Overview of the IPv6 Address
IPv6 addresses are 128-bit identifiers for interfaces and sets of
interfaces. There are three types of addresses: Unicast, Anycast,
and Multicast. This document defines a specific type of Unicast
In this document, fields in addresses are given specific names, for
example "subnet". When this name is used with the term "ID" (for
"identifier") after the name (e.g., "subnet ID"), it refers to the
contents of the named field. When it is used with the term "prefix"
(e.g. "subnet prefix") it refers to all of the addressing bits to
the left of and including this field.
IPv6 unicast addresses are designed assuming that the Internet
routing system makes forwarding decisions based on a "longest prefix
match" algorithm on arbitrary bit boundaries and does not have any
knowledge of the internal structure of IPv6 addresses. The structure
in IPv6 addresses is for assignment and allocation. The only
exception to this is the distinction made between unicast and
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).
This document defines an address format for the 001 (binary) Format
Prefix for Aggregatable Global Unicast addresses. The same address
format could be used for other Format Prefixes, as long as these
Format Prefixes also identify IPv6 unicast addresses. Only the "001"
Format Prefix is defined here.
3.0 IPv6 Aggregatable Global Unicast Address Format
This document defines an address format for the IPv6 aggregatable
global unicast address assignment. The authors believe that this
address format will be widely used for IPv6 nodes connected to the
Internet. This address format is designed to support both the
current provider-based aggregation and a new type of exchange-based
aggregation. The combination will allow efficient routing
aggregation for sites that connect directly to providers and for
sites that connect to exchanges. Sites will have the choice to
connect to either type of aggregation entity.
While this address format is designed to support exchange-based
aggregation (in addition to current provider-based aggregation) it is
not dependent on exchanges for it's overall route aggregation
properties. It will provide efficient route aggregation with only
Aggregatable addresses are organized into a three level hierarchy:
- Public Topology
- Site Topology
- Interface Identifier
Public topology is the collection of providers and exchanges who
provide public Internet transit services. Site topology is local to
a specific site or organization which does not provide public transit
service to nodes outside of the site. Interface identifiers identify
interfaces on links.
--+/ \+--------------+/ \+----------
( P1 ) +----+ ( P3 ) +----+
+\______________/ | |----+\______________/+--| |--
| +--| X1 | +| X2 |
| ______________ / | |-+ ______________ / | |--
+/ \+ +-+--+ \ / \+ +----+
( P2 ) / \ +( P4 )
--+\______________/ / \ \______________/
| / \ | |
| / | | |
| / | | |
_|_ _/_ _|_ _|_ _|_
/ \ / \ / \ / \ / \
( S.A ) ( S.B ) ( P5 ) ( P6 )( S.C )
\___/ \___/ \___/ \___/ \___/
| / \
_|_ _/_ \ ___
/ \ / \ +-/ \
( S.D ) ( S.E ) ( S.F )
\___/ \___/ \___/
As shown in the figure above, the aggregatable address format is
designed to support long-haul providers (shown as P1, P2, P3, and
P4), exchanges (shown as X1 and X2), multiple levels of providers
(shown at P5 and P6), and subscribers (shown as S.x) Exchanges
(unlike current NAPs, FIXes, etc.) will allocate IPv6 addresses.
Organizations who connect to these exchanges will also subscribe
(directly, indirectly via the exchange, etc.) for long-haul service
from one or more long-haul providers. Doing so, they will achieve
addressing independence from long-haul transit providers. They will
be able to change long-haul providers without having to renumber
their organization. They can also be multihomed via the exchange to
more than one long-haul provider without having to have address
prefixes from each long-haul provider. Note that the mechanisms used
for this type of provider selection and portability are not discussed
in the document.
3.1 Aggregatable Global Unicast Address Structure
The 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 | |
<--Public Topology---> Site
FP Format Prefix (001)
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 following sections specify each part of the IPv6 Aggregatable
Global Unicast address format.
3.2 Top-Level Aggregation ID
Top-Level Aggregation Identifiers (TLA ID) are the top level in the
routing hierarchy. Default-free routers must have a routing table
entry for every active TLA ID and will probably have additional
entries providing routing information for the TLA ID in which they
are located. They may have additional entries in order to optimize
routing for their specific topology, but the routing topology at all
levels must be designed to minimize the number of additional entries
fed into the default free routing tables.
This addressing format supports 8,192 (2^13) TLA ID's. Additional
TLA ID's may be added by either growing the TLA field to the right
into the reserved field or by using this format for additional format
The issues relating to TLA ID assignment are beyond the scope of this
document. They will be described in a document under preparation.
The Reserved field is reserved for future use and must be set to
The Reserved field allows for future growth of the TLA and NLA fields
as appropriate. See section 4.0 for a discussion.
3.4 Next-Level Aggregation Identifier
Next-Level Aggregation Identifier's are used by organizations
assigned a TLA ID to create an addressing hierarchy and to identify
sites. The organization can assign the top part of the NLA ID in a
manner to create an addressing hierarchy appropriate to its network.
It can use the remainder of the bits in the field to identify sites
it wishes to serve. This is shown as follows:
| n | 24-n bits | 16 | 64 bits |
|NLA1 | Site ID | SLA ID | Interface ID |
Each organization assigned a TLA ID receives 24 bits of NLA ID space.
This NLA ID space allows each organization to provide service to
approximately as many organizations as the current IPv4 Internet can
support total networks.
Organizations assigned TLA ID's may also support NLA ID's in their
own Site ID space. This allows the organization assigned a TLA ID to
provide service to organizations providing public transit service and
to organizations who do not provide public transit service. These
organizations receiving an NLA ID may also choose to use their Site
ID space to support other NLA ID's. This is shown as follows:
| n | 24-n bits | 16 | 64 bits |
|NLA1 | Site ID | SLA ID | Interface ID |
| m | 24-n-m | 16 | 64 bits |
|NLA2 | Site ID | SLA ID | Interface ID |
| o |24-n-m-o| 16 | 64 bits |
|NLA3 | Site ID| SLA ID | Interface ID |
The design of the bit layout of the NLA ID space for a specific TLA
ID is left to the organization responsible for that TLA ID. Likewise
the design of the bit layout of the next level NLA ID is the
responsibility of the previous level NLA ID. It is recommended that
organizations assigning NLA address space use "slow start" allocation
procedures similar to [RFC2050].
The design of an NLA ID allocation plan is a tradeoff between routing
aggregation efficiency and flexibility. Creating hierarchies allows
for greater amount of aggregation and results in smaller routing
tables. Flat NLA ID assignment provides for easier allocation and
attachment flexibility, but results in larger routing tables.
3.5 Site-Level Aggregation Identifier
The SLA ID field is used by an individual organization to create its
own local addressing hierarchy and to identify subnets. This is
analogous to subnets in IPv4 except that each organization has a much
greater number of subnets. The 16 bit SLA ID field support 65,535
Organizations may choose to either route their SLA ID "flat" (e.g.,
not create any logical relationship between the SLA identifiers that
results in larger routing tables), or to create a two or more level
hierarchy (that results in smaller routing tables) in the SLA ID
field. The latter is shown as follows:
| n | 16-n | 64 bits |
|SLA1 | Subnet | Interface ID |
| m |16-n-m | 64 bits |
|SLA2|Subnet | Interface ID |
The approach chosen for structuring an SLA ID field is the
responsibility of the individual organization.
The number of subnets supported in this address format should be
sufficient for all but the largest of organizations. Organizations
which need additional subnets can arrange with the organization they
are obtaining Internet service from to obtain additional site
identifiers and use this to create additional subnets.
3.6 Interface ID
Interface identifiers 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 interfaces identifier will be
the same or be based on the interface's link-layer address.
Interface IDs used in the aggregatable global unicast address format
are required to be 64 bits long and to be constructed in IEEE EUI-64
format [EUI-64]. These identifiers may have global scope when a
global token (e.g., IEEE 48bit MAC) is available or may have local
scope where a global token is not available (e.g., serial links,
tunnel end-points, etc.). The "u" bit (universal/local bit in IEEE
EUI-64 terminology) in the EUI-64 identifier must be set correctly,
as defined in [ARCH], to indicate global or local scope.
The procedures for creating EUI-64 based Interface Identifiers is
defined in [ARCH]. The details on forming interface identifiers is
defined in the appropriate "IPv6 over <link>" specification such as
"IPv6 over Ethernet" [ETHER], "IPv6 over FDDI" [FDDI], etc.
4.0 Technical Motivation
The design choices for the size of the fields in the aggregatable
address format were based on the need to meet a number of technical
requirements. These are described in the following paragraphs.
The size of the Top-Level Aggregation Identifier is 13 bits. This
allows for 8,192 TLA ID's. This size was chosen to insure that the
default-free routing table in top level routers in the Internet is
kept within the limits, with a reasonable margin, of the current
routing technology. The margin is important because default-free
routers will also carry a significant number of longer (i.e., more-
specific) prefixes for optimizing paths internal to a TLA and between
The important issue is not only the size of the default-free routing
table, but the complexity of the topology that determines the number
of copies of the default-free routes that a router must examine while
computing a forwarding table. Current practice with IPv4 it is
common to see a prefix announced fifteen times via different paths.
The complexity of Internet topology is very likely to increase in the
future. It is important that IPv6 default-free routing support
additional complexity as well as a considerably larger internet.
It should be noted for comparison that at the time of this writing
(spring, 1998) the IPv4 default-free routing table contains
approximately 50,000 prefixes. While this shows that it is possible
to support more routes than 8,192 it is matter of debate if the
number of prefixes supported today in IPv4 is already too high for
current routing technology. There are serious issues of route
stability as well as cases of providers not supporting all top level
prefixes. The technical requirement was to pick a TLA ID size that
was below, with a reasonable margin, what was being done with IPv4.
The choice of 13 bits for the TLA field was an engineering
compromise. Fewer bits would have been too small by not supporting
enough top level organizations. More bits would have exceeded what
can be reasonably accommodated, with a reasonable margin, with
current routing technology in order to deal with the issues described
in the previous paragraphs.
If in the future, routing technology improves to support a larger
number of top level routes in the default-free routing tables there
are two choices on how to increase the number TLA identifiers. The
first is to expand the TLA ID field into the reserved field. This
would increase the number of TLA ID's to approximately 2 million.
The second approach is to allocate another format prefix (FP) for use
with this address format. Either or a combination of these
approaches allows the number of TLA ID's to increase significantly.
The size of the Reserved field is 8 bits. This size was chosen to
allow significant growth of either the TLA ID and/or the NLA ID
The size of the Next-Level Aggregation Identifier field is 24 bits.
This allows for approximately sixteen million NLA ID's if used in a
flat manner. Used hierarchically it allows for a complexity roughly
equivalent to the IPv4 address space (assuming an average network
size of 254 interfaces). If in the future additional room for
complexity is needed in the NLA ID, this may be accommodated by
extending the NLA ID into the Reserved field.
The size of the Site-Level Aggregation Identifier field is 16 bits.
This supports 65,535 individual subnets per site. The design goal
for the size of this field was to be sufficient for all but the
largest of organizations. Organizations which need additional
subnets can arrange with the organization they are obtaining Internet
service from to obtain additional site identifiers and use this to
create additional subnets.
The Site-Level Aggregation Identifier field was given a fixed size in
order to force the length of all prefixes identifying a particular
site to be the same length (i.e., 48 bits). This facilitates
movement of sites in the topology (e.g., changing service providers
and multi-homing to multiple service providers).
The Interface ID Interface Identifier field is 64 bits. This size
was chosen to meet the requirement specified in [ARCH] to support
EUI-64 based Interface Identifiers.
The authors would like to express our thanks to Thomas Narten, Bob
Fink, Matt Crawford, Allison Mankin, Jim Bound, Christian Huitema,
Scott Bradner, Brian Carpenter, John Stewart, and Daniel Karrenberg
for their review and constructive comments.
[ALLOC] IAB and IESG, "IPv6 Address Allocation Management",
RFC 1881, December 1995.
[ARCH] Hinden, R., "IP Version 6 Addressing Architecture",
RFC 2373, July 1998.
[AUTH] Atkinson, R., "IP Authentication Header", RFC 1826, August
[AUTO] Thompson, S., and T. Narten., "IPv6 Stateless Address
Autoconfiguration", RFC 1971, August 1996.
[ETHER] Crawford, M., "Transmission of IPv6 Packets over Ethernet
Networks", Work in Progress.
[EUI64] IEEE, "Guidelines for 64-bit Global Identifier (EUI-64)
[FDDI] Crawford, M., "Transmission of IPv6 Packets over FDDI
Networks", Work in Progress.
[IPV6] Deering, S., and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 1883, December 1995.
[RFC2050] Hubbard, K., Kosters, M., Conrad, D., Karrenberg, D.,
and J. Postel, "Internet Registry IP Allocation
Guidelines", BCP 12, RFC 1466, November 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
7.0 Security Considerations
IPv6 addressing documents do not have any direct impact on Internet
infrastructure security. Authentication of IPv6 packets is defined
8.0 Authors' Addresses
Robert M. Hinden
232 Java Drive
Sunnyvale, CA 94089
Phone: 1 408 990-2004
UUNET Technologies, Inc.
3060 Williams Drive
Fairfax, VA 22030
Phone: 1 703 206-5890
Stephen E. Deering
Cisco Systems, Inc.
170 West Tasman Drive
San Jose, CA 95134-1706
Phone: 1 408 527-8213
9.0 Full Copyright Statement
Copyright (C) The Internet Society (1998). All Rights Reserved.
This document and translations of it may be copied and furnished to
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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
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
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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
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