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
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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
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].
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.
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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.
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.
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:
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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
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RFC 2373 IPv6 Addressing Architecture July 1998
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
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RFC 2373 IPv6 Addressing Architecture July 1998
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.
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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.
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:
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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.
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|
+----+----+----+----+----+----+
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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.
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.
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.
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RFC 2373 IPv6 Addressing Architecture July 1998
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 |
+--------------------------------------+----+---------------------+
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.
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.
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RFC 2373 IPv6 Addressing Architecture July 1998
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].
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.
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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.
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
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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.
The Subnet-Router anycast address is predefined. Its format is as
follows:
| n bits | 128-n bits |
+------------------------------------------------+----------------+