rfc2373.txt
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IPv6 unicast addresses are aggregatable with contiguous bit-wise
masks similar to IPv4 addresses under Class-less Interdomain Routing
[CIDR].
There are several forms of unicast address assignment in IPv6,
including the global aggregatable global unicast address, the NSAP
address, the IPX hierarchical address, the site-local address, the
link-local address, and the IPv4-capable host address. Additional
address types can be defined in the future.
IPv6 nodes may have considerable or little knowledge of the internal
structure of the IPv6 address, depending on the role the node plays
(for instance, host versus router). At a minimum, a node may
consider that unicast addresses (including its own) have no internal
structure:
| 128 bits |
+-----------------------------------------------------------------+
| node address |
+-----------------------------------------------------------------+
A slightly sophisticated host (but still rather simple) may
additionally be aware of subnet prefix(es) for the link(s) it is
attached to, where different addresses may have different values for
n:
Hinden & Deering Standards Track [Page 7]
RFC 2373 IPv6 Addressing Architecture July 1998
| n bits | 128-n bits |
+------------------------------------------------+----------------+
| subnet prefix | interface ID |
+------------------------------------------------+----------------+
Still more sophisticated hosts may be aware of other hierarchical
boundaries in the unicast address. Though a very simple router may
have no knowledge of the internal structure of IPv6 unicast
addresses, routers will more generally have knowledge of one or more
of the hierarchical boundaries for the operation of routing
protocols. The known boundaries will differ from router to router,
depending on what positions the router holds in the routing
hierarchy.
2.5.1 Interface Identifiers
Interface identifiers in IPv6 unicast addresses are used to identify
interfaces on a link. They are required to be unique on that link.
They may also be unique over a broader scope. In many cases an
interface's identifier will be the same as that interface's link-
layer address. The same interface identifier may be used on multiple
interfaces on a single node.
Note that the use of the same interface identifier on multiple
interfaces of a single node does not affect the interface
identifier's global uniqueness or each IPv6 addresses global
uniqueness created using that interface identifier.
In a number of the format prefixes (see section 2.4) Interface IDs
are required to be 64 bits long and to be constructed in IEEE EUI-64
format [EUI64]. EUI-64 based Interface identifiers may have global
scope when a global token is available (e.g., IEEE 48bit MAC) or may
have local scope where a global token is not available (e.g., serial
links, tunnel end-points, etc.). It is required that the "u" bit
(universal/local bit in IEEE EUI-64 terminology) be inverted when
forming the interface identifier from the EUI-64. The "u" bit is set
to one (1) to indicate global scope, and it is set to zero (0) to
indicate local scope. The first three octets in binary of an EUI-64
identifier are as follows:
0 0 0 1 1 2
|0 7 8 5 6 3|
+----+----+----+----+----+----+
|cccc|ccug|cccc|cccc|cccc|cccc|
+----+----+----+----+----+----+
Hinden & Deering Standards Track [Page 8]
RFC 2373 IPv6 Addressing Architecture July 1998
written in Internet standard bit-order , where "u" is the
universal/local bit, "g" is the individual/group bit, and "c" are the
bits of the company_id. Appendix A: "Creating EUI-64 based Interface
Identifiers" provides examples on the creation of different EUI-64
based interface identifiers.
The motivation for inverting the "u" bit when forming the interface
identifier is to make it easy for system administrators to hand
configure local scope identifiers when hardware tokens are not
available. This is expected to be case for serial links, tunnel end-
points, etc. The alternative would have been for these to be of the
form 0200:0:0:1, 0200:0:0:2, etc., instead of the much simpler ::1,
::2, etc.
The use of the universal/local bit in the IEEE EUI-64 identifier is
to allow development of future technology that can take advantage of
interface identifiers with global scope.
The details of forming interface identifiers are defined in the
appropriate "IPv6 over <link>" specification such as "IPv6 over
Ethernet" [ETHER], "IPv6 over FDDI" [FDDI], etc.
2.5.2 The Unspecified Address
The address 0:0:0:0:0:0:0:0 is called the unspecified address. It
must never be assigned to any node. It indicates the absence of an
address. One example of its use is in the Source Address field of
any IPv6 packets sent by an initializing host before it has learned
its own address.
The unspecified address must not be used as the destination address
of IPv6 packets or in IPv6 Routing Headers.
2.5.3 The Loopback Address
The unicast address 0:0:0:0:0:0:0:1 is called the loopback address.
It may be used by a node to send an IPv6 packet to itself. It may
never be assigned to any physical interface. It may be thought of as
being associated with a virtual interface (e.g., the loopback
interface).
The loopback address must not be used as the source address in IPv6
packets that are sent outside of a single node. An IPv6 packet with
a destination address of loopback must never be sent outside of a
single node and must never be forwarded by an IPv6 router.
Hinden & Deering Standards Track [Page 9]
RFC 2373 IPv6 Addressing Architecture July 1998
2.5.4 IPv6 Addresses with Embedded IPv4 Addresses
The IPv6 transition mechanisms [TRAN] include a technique for hosts
and routers to dynamically tunnel IPv6 packets over IPv4 routing
infrastructure. IPv6 nodes that utilize this technique are assigned
special IPv6 unicast addresses that carry an IPv4 address in the low-
order 32-bits. This type of address is termed an "IPv4-compatible
IPv6 address" and has the format:
| 80 bits | 16 | 32 bits |
+--------------------------------------+--------------------------+
|0000..............................0000|0000| IPv4 address |
+--------------------------------------+----+---------------------+
A second type of IPv6 address which holds an embedded IPv4 address is
also defined. This address is used to represent the addresses of
IPv4-only nodes (those that *do not* support IPv6) as IPv6 addresses.
This type of address is termed an "IPv4-mapped IPv6 address" and has
the format:
| 80 bits | 16 | 32 bits |
+--------------------------------------+--------------------------+
|0000..............................0000|FFFF| IPv4 address |
+--------------------------------------+----+---------------------+
2.5.5 NSAP Addresses
This mapping of NSAP address into IPv6 addresses is defined in
[NSAP]. This document recommends that network implementors who have
planned or deployed an OSI NSAP addressing plan, and who wish to
deploy or transition to IPv6, should redesign a native IPv6
addressing plan to meet their needs. However, it also defines a set
of mechanisms for the support of OSI NSAP addressing in an IPv6
network. These mechanisms are the ones that must be used if such
support is required. This document also defines a mapping of IPv6
addresses within the OSI address format, should this be required.
2.5.6 IPX Addresses
This mapping of IPX address into IPv6 addresses is as follows:
| 7 | 121 bits |
+-------+---------------------------------------------------------+
|0000010| to be defined |
+-------+---------------------------------------------------------+
The draft definition, motivation, and usage are under study.
Hinden & Deering Standards Track [Page 10]
RFC 2373 IPv6 Addressing Architecture July 1998
2.5.7 Aggregatable Global Unicast Addresses
The global aggregatable global unicast address is defined in [AGGR].
This address format is designed to support both the current provider
based aggregation and a new type of aggregation called exchanges.
The combination will allow efficient routing aggregation for both
sites which connect directly to providers and who connect to
exchanges. Sites will have the choice to connect to either type of
aggregation point.
The IPv6 aggregatable global unicast address format is as follows:
| 3| 13 | 8 | 24 | 16 | 64 bits |
+--+-----+---+--------+--------+--------------------------------+
|FP| TLA |RES| NLA | SLA | Interface ID |
| | ID | | ID | ID | |
+--+-----+---+--------+--------+--------------------------------+
Where
001 Format Prefix (3 bit) for Aggregatable Global
Unicast Addresses
TLA ID Top-Level Aggregation Identifier
RES Reserved for future use
NLA ID Next-Level Aggregation Identifier
SLA ID Site-Level Aggregation Identifier
INTERFACE ID Interface Identifier
The contents, field sizes, and assignment rules are defined in
[AGGR].
2.5.8 Local-Use IPv6 Unicast Addresses
There are two types of local-use unicast addresses defined. These
are Link-Local and Site-Local. The Link-Local is for use on a single
link and the Site-Local is for use in a single site. Link-Local
addresses have the following format:
| 10 |
| bits | 54 bits | 64 bits |
+----------+-------------------------+----------------------------+
|1111111010| 0 | interface ID |
+----------+-------------------------+----------------------------+
Link-Local addresses are designed to be used for addressing on a
single link for purposes such as auto-address configuration, neighbor
discovery, or when no routers are present.
Hinden & Deering Standards Track [Page 11]
RFC 2373 IPv6 Addressing Architecture July 1998
Routers must not forward any packets with link-local source or
destination addresses to other links.
Site-Local addresses have the following format:
| 10 |
| bits | 38 bits | 16 bits | 64 bits |
+----------+-------------+-----------+----------------------------+
|1111111011| 0 | subnet ID | interface ID |
+----------+-------------+-----------+----------------------------+
Site-Local addresses are designed to be used for addressing inside of
a site without the need for a global prefix.
Routers must not forward any packets with site-local source or
destination addresses outside of the site.
2.6 Anycast Addresses
An IPv6 anycast address is an address that is assigned to more than
one interface (typically belonging to different nodes), with the
property that a packet sent to an anycast address is routed to the
"nearest" interface having that address, according to the routing
protocols' measure of distance.
Anycast addresses are allocated from the unicast address space, using
any of the defined unicast address formats. Thus, anycast addresses
are syntactically indistinguishable from unicast addresses. When a
unicast address is assigned to more than one interface, thus turning
it into an anycast address, the nodes to which the address is
assigned must be explicitly configured to know that it is an anycast
address.
For any assigned anycast address, there is a longest address prefix P
that identifies the topological region in which all interfaces
belonging to that anycast address reside. Within the region
identified by P, each member of the anycast set must be advertised as
a separate entry in the routing system (commonly referred to as a
"host route"); outside the region identified by P, the anycast
address may be aggregated into the routing advertisement for prefix
P.
Note that in, the worst case, the prefix P of an anycast set may be
the null prefix, i.e., the members of the set may have no topological
locality. In that case, the anycast address must be advertised as a
separate routing entry throughout the entire internet, which presents
Hinden & Deering Standards Track [Page 12]
RFC 2373 IPv6 Addressing Architecture July 1998
a severe scaling limit on how many such "global" anycast sets may be
supported. Therefore, it is expected that support for global anycast
sets may be unavailable or very restricted.
One expected use of anycast addresses is to identify the set of
routers belonging to an organization providing internet service.
Such addresses could be used as intermediate addresses in an IPv6
Routing header, to cause a packet to be delivered via a particular
aggregation or sequence of aggregations. Some other possible uses
are to identify the set of routers attached to a particular subnet,
or the set of routers providing entry into a particular routing
domain.
There is little experience with widespread, arbitrary use of internet
anycast addresses, and some known complications and hazards when
using them in their full generality [ANYCST]. Until more experience
has been gained and solutions agreed upon for those problems, the
following restrictions are imposed on IPv6 anycast addresses:
o An anycast address must not be used as the source address of an
IPv6 packet.
o An anycast address must not be assigned to an IPv6 host, that
is, it may be assigned to an IPv6 router only.
2.6.1 Required Anycast Address
The Subnet-Router anycast address is predefined. Its format is as
follows:
| n bits | 128-n bits |
+------------------------------------------------+----------------+
| subnet prefix | 00000000000000 |
+------------------------------------------------+----------------+
The "subnet prefix" in an anycast address is the prefix which
identifies a specific link. This anycast address is syntactically
the same as a unicast address for an interface on the link with the
interface identifier set to zero.
Packets sent to the Subnet-Router anycast address will be delivered
to one router on the subnet. All routers are required to support the
Subnet-Router anycast addresses for the subnets which they have
interfaces.
Hinden & Deering Standards Track [Page 13]
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