|Title||IAB/IESG Recommendations on IPv6 Address Allocations to Sites
Network Working Group IAB
Request for Comments: 3177 IESG
Category: Informational September 2001
IAB/IESG Recommendations on IPv6 Address Allocations to Sites
Status of this Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright (C) The Internet Society (2001). All Rights Reserved.
This document provides recommendations to the addressing registries
(APNIC, ARIN and RIPE-NCC) on policies for assigning IPv6 address
blocks to end sites. In particular, it recommends the assignment of
/48 in the general case, /64 when it is known that one and only one
subnet is needed and /128 when it is absolutely known that one and
only one device is connecting.
The original recommendations were made in an IAB/IESG statement
mailed to the registries on September 1, 2000. This document refines
the original recommendation and documents it for the historical
There have been many discussions between IETF and RIR experts on the
topic of IPv6 address allocation policy. This memo addresses the
issue of the boundary in between the public and the private topology
in the Internet, that is, how much address space should an ISP
allocate to homes, small and large enterprises, mobile networks and
This document does not address the issue of the other boundaries in
the public topology, that is, between the RIRs and the LIRs.
This document was developed by the IPv6 Directorate, IAB and IESG,
and is a recommendation from the IAB and IESG to the RIRs.
The technical principles that apply to address allocation seek to
balance healthy conservation practices and wisdom with a certain ease
of access. On one hand, when managing a potentially limited
resource, one must conserve wisely to prevent exhaustion within an
expected lifetime. On the other hand, the IPv6 address space is in
no sense as limited a resource as the IPv4 address space, and
unwarranted conservatism acts as a disincentive in a marketplace
already dampened by other factors. So from a market development
perspective, we would like to see it be very easy for a user or an
ISP to obtain as many IPv6 addresses as they really need without a
prospect of immediate renumbering or of scaling inefficiencies.
The IETF makes no comment on business issues or relationships.
However, in general, we observe that technical delegation policy can
have strong business impacts. A strong requirement of the address
delegation plan is that it not be predicated on or unduly bias
business relationships or models.
The IPv6 address, as currently defined, consists of 64 bits of
"network number" and 64 bits of "host number". The technical reasons
for this are several. The requirements for IPv6 agreed to in 1993
included a plan to be able to address approximately 2^40 networks and
2^50 hosts; the 64/64 split effectively accomplishes this.
Procedures used in host address assignment, such as the router
advertisement of a network's prefix to hosts [RFC2462], which in turn
place a locally unique number in the host portion, depend on this
split. Subnet numbers must be assumed to come from the network part.
This is not to preclude routing protocols such as IS-IS level 1
(intra-area) routing, which routes individual host addresses, but
says that it may not be depended upon in the world outside that zone.
The 64-bit host field can also be used with EUI-64 for a flat,
uniquely allocated space, and therefore it may not be globally
treated as a subnetting resource. Those concerned with privacy
issues linked to the presence of a globally unique identifier may
note that 64 bits makes a large enough field to maintain excellent
random-number-draw properties for self-configured End System
Designators. That alternative construction of this 64-bit host part
of an IPv6 address is documented in [RFC3041].
While the IETF has also gone to a great deal of effort to minimize
the impacts of network renumbering, renumbering of IPv6 networks is
neither invisible nor completely painless. Therefore, renumbering
should be considered a tolerable event, but to be avoided if
In [RFC2374] and [RFC2450], the IETF's IPNG working group has
recommended that the address block given to a single edge network
which may be recursively subnetted be a 48-bit prefix. This gives
each such network 2^16 subnet numbers to use in routing, and a very
large number of unique host numbers within each network. This is
deemed to be large enough for most enterprises, and to leave plenty
of room for delegation of address blocks to aggregating entities.
It is not obvious, however, that all edge networks are likely to be
recursively subnetted; a single PC in a home or a telephone in a
mobile cellular network, for example, may or may not interface to a
subnetted local network. When a network number is delegated to a
place that will not require subnetting, therefore, it might be
acceptable for an ISP to give a single 64-bit prefix - perhaps shared
among the dial-in connections to the same ISP router. However this
decision may be taken in the knowledge that there is objectively no
shortage of /48s, and the expectation that personal, home networks
will become the norm. Indeed, it is widely expected that all IPv6
subscribers, whether domestic (homes), mobile (vehicles or
individuals), or enterprises of any size, will eventually possess
multiple always-on hosts, at least one subnet with the potential for
additional subnetting, and therefore some internal routing
capability. In other words the subscriber allocation unit is not
always a host; it is always potentially a site. The question this
memo is addressing is how much address space should be delegated to
3. Address Delegation Recommendations
The IESG and the IAB recommend the allocations for the boundary
between the public and the private topology to follow those general
- /48 in the general case, except for very large subscribers.
- /64 when it is known that one and only one subnet is needed by
- /128 when it is absolutely known that one and only one device
In particular, we recommend:
- Home network subscribers, connecting through on-demand or
always-on connections should receive a /48.
- Small and large enterprises should receive a /48.
- Very large subscribers could receive a /47 or slightly shorter
prefix, or multiple /48's.
- Mobile networks, such as vehicles or mobile phones with an
additional network interface (such as bluetooth or 802.11b)
should receive a static /64 prefix to allow the connection of
multiple devices through one subnet.
- A single PC, with no additional need to subnet, dialing-up from
a hotel room may receive its /128 IPv6 address for a PPP style
connection as part of a /64 prefix.
Note that there seems to be little benefit in not giving a /48 if
future growth is anticipated. In the following, we give the
arguments for a uniform use of /48 and then demonstrate that it is
entirely compatible with responsible stewardship of the total IPv6
The arguments for the fixed boundary are:
- That only by having a provider-independent boundary can we
guarantee that a change of ISP will not require a costly
internal restructuring or consolidation of subnets.
- That during straightforward site renumbering from one prefix to
another the whole process, including parallel running of the
two prefixes, would be greatly complicated if the prefixes had
different lengths (depending of course on the size and
complexity of the site).
- There are various possible approaches to multihoming for IPv6
sites, including the techniques already used for IPv4
multihoming. The main open issue is finding solutions that
scale massively without unduly damaging route aggregation
and/or optimal route selection. Much more work remains to be
done in this area, but it seems likely that several approaches
will be deployed in practice, each with their own advantages
and disadvantages. Some (but not all) will work better with a
fixed prefix boundary. (Multihoming is discussed in more
- To allow easy growth of the subscribers' networks without need
to go back to ISPs for more space (except for that relatively
small number of subscribers for which a /48 is not enough).
- To remove the burden from the ISPs and registries of judging
sites' needs for address space, unless the site requests more
space than a /48. This carries several advantages:
- It may become less critical for ISPs to be able to maintain
detailed knowledge of their customers' network architecture
and growth plans,
- ISPs and registries may reduce the effort spent on assessing
rates of address consumption, with address space ample for
long-term growth plans,
- Registry operations may be made more efficient or more
focused, by reducing the urgency of tracking and assessment.
- Address space will no longer be a precious resource for
customers, removing the major incentive for subscribers to
install v6/v6 NATs, which would defeat the IPv6 restoration
of address transparency.
- To allow the site to maintain a single reverse-DNS zone
covering all prefixes.
- If and only if a site can use the same subnetting structure
under each of its prefixes, then it can use the same zone file
for the address-to-name mapping of all of them. And, using the
conventions of [RFC2874], it can roll the reverse mapping data
into the "forward" (name-keyed) zone.
Specific advantages of the fixed boundary being at /48 include
- To leave open the technical option of retro-fitting the GSE
(Global, Site and End-System Designator, a.k.a., "8+8")
proposal for separating locators and identifiers, which assumes
a fixed boundary between global and site addressing at /48.
Although the GSE technique was deferred a couple of years ago,
it still has strong proponents. Also, the IRTF Namespace
Research Group is actively looking into topics closely related
to GSE. It is still possible that GSE or a derivative of GSE
will be used with IPv6 in the future.
- Since the site-local prefix is fec0::/48, global site prefixes
of /48 will allow sites to easily maintain a trivial (identity)
mapping between the global topology and the site-local topology
in the SLA field.
- Similarly, if the 6to4 proposal is widely deployed, migration
from a 6to4 prefix, which is /48 by construction, to a native
IPv6 prefix will be simplified if the native prefix is /48.
4. Conservation of Address Space
The question naturally arises whether giving a /48 to every
subscriber represents a profligate waste of address space. Objective
analysis shows that this is not the case. A /48 prefix under the 001
Global Unicast Address prefix contains 45 variable bits. That is,
the number of available prefixes is 2 to the power 45 or about 35
- [RFC1715] defines an "H ratio" based on experience in address
space assignment in various networks. The H ratio varies
between 0 and 0.3, with larger values denoting denser, more
efficient assignment. Experience shows that problems start to
occur when the H ratio becomes greater than 0.25. At an H
ratio of 0.25, a 45 bit address space would have 178 billion
(178 thousand million) identifiers.
H = log10(178*10^9) / 45 = 0.25
This means that we feel comfortable about the prospect of
allocating 178 billions /48 prefixes under that scheme before
problems start to appear. To understand how big that number
is, one has to compare 178 billion to 10 billion, which is the
projected population on earth in year 2050 (see
http://www.census.gov/ipc/www/world.html). These numbers give
no grounds for concern provided that the ISPs, under the
guidance of the RIRs, allocate /48's prudently, and that the
IETF refrains from new recommendations that further reduce the
remaining 45 variable bits, unless a compelling requirement
- We are highly confident in the validity of this analysis, based
on experience with IPv4 and several other address spaces, and
on extremely ambitious scaling goals for the Internet amounting
to an 80 bit address space *per person*. Even so, being
acutely aware of the history of under-estimating demand, the
IETF has reserved more than 85% of the address space (i.e., the
bulk of the space not under the 001 Global Unicast Address
prefix). Therefore, if the analysis does one day turn out to
be wrong, our successors will still have the option of imposing
much more restrictive allocation policies on the remaining 85%.
However, we must stress that vendors should not encode any of
the boundaries discussed here either in software nor hardware.
Under that assumption, should we ever have to use the remaining
85% of the address space, such a migration may not be devoid of
pain, but it should be far less disruptive than deployment of a
new version of IP.
To summarize, we argue that although careful stewardship of IPv6
address space is essential, this is completely compatible with the
convenience and simplicity of a uniform prefix size for IPv6 sites of
any size. The numbers are such that there seems to be no objective
risk of running out of space, giving an unfair amount of space to
early customers, or of getting back into the over-constrained IPv4
situation where address conservation and route aggregation damage
5. Multihoming Issues
In the realm of multi-homed networks, the techniques used in IPv4 can
all be applied, but they have known scaling problems. Specifically,
if the same prefix is advertised by multiple ISPs, the routing
information will grow as a function of the number of multihomed
sites. To go beyond this for IPv6, we only have initial proposals on
the table at this time, and active work is under way in the IETF IPNG
and Multi6 working groups. Until current or new proposals become
more fully developed, existing techniques known to work in IPv4 will
continue to be used in IPv6.
Key characteristics of an ideal multi-homing proposal include (at
minimum) that it provides routing connectivity to any multi-homed
network globally, conserves address space, produces high quality
routes via any of the network's providers, enables a multi-homed
network to connect to multiple ISPs, does not unintentionally bias
routing to use any proper subset of those networks, does not damage
route aggregation, and scales to very large numbers of multi-homed
One class of solutions being considered amounts to permanent parallel
running of two (or more) prefixes per site. In the absence of a
fixed prefix boundary, such a site might be required to have multiple
different internal subnet numbering strategies, (one for each prefix
length) or, if it only wanted one, be forced to use the most
restrictive one as defined by the longest prefix it received from any
of its ISPs. In this approach, a multi-homed network would have an
address block from each of its upstream providers. Each host would
either have exactly one address picked from the set of upstream
providers, or one address per host from each of the upstream
providers. The first case is essentially a variant on [RFC2260],
with known scaling limits.
In the second case (multiple addresses per host), if two multi-homed
networks communicate, having respectively M and N upstream providers,
then the one initiating the connection will select one address pair
from the N*M potential address pairs to connect between, and in so
doing will select the providers, and therefore the applicable route,
for the life of the connection. Given that each path will have a
different available bit rate, loss rate, and delay, if neither host
is in possession of any routing or metric information, the initiating
host has only a 1/(M*N) probability of selecting the optimal address
pair. Work on better-than-random address selection is in progress in
the IETF, but is incomplete.
The existing IPv4 Internet shows us that a network prefix which is
independent of, and globally advertised to, all upstream providers
permits the routing system to select a reasonably good path within
the applicable policy. Present-day routing policies are not QoS
policies but reachability policies, which means that they will not
necessarily select the optimal delay, bit rate, or loss rate, but the
route will be the best within the metrics that are in use. One may
therefore conclude that this would work correctly for IPv6 networks
as well, apart from scaling issues.
6. Security Considerations
This document does not have any security implications.
This document originated from the IETF IPv6 directorate, with much
input from the IAB and IESG. The original text forming the basis of
this document was contributed by Fred Baker and Brian Carpenter.
Allison Mankin and Thomas Narten merged the original contributions
into a single document, and Alain Durand edited the document through
its final stages.
[RFC1715] Huitema, C., "The H Ratio for Address Assignment
Efficiency", RFC 1715, November 1994.
[RFC2026] Bradner, S., "The Internet Standards Process -- Revision
3", BCP 9, RFC 2026, October 1996.
[RFC2260] Bates, T. and Y. Rekhter, "Scalable Support for Multi-
homed Multi-provider Connectivity", RFC 2260, January
[RFC2374] Hinden, R., O'Dell, M. and S. Deering, "An IPv6
Aggregatable Global Unicast Address Format", RFC 2374,
[RFC2450] Hinden, R., "Proposed TLA and NLA Assignment Rule", RFC
2450, December 1998.
[RFC2462] Thompson, S. and T. Narten, "IPv6 Stateless Address
Autoconfiguration", RFC 2462, December 1998.
[RFC2874] Crawford, M. and C. Huitema, "DNS Extensions to Support
IPv6 Address Aggregation and Renumbering", RFC 2874, July
[RFC3041] Narten, T. and R. Draves, "Privacy Extensions for
Stateless Address Autoconfiguration in IPv6", RFC 3041,
[MobIPv6] Johnson, D. and C. Perkins, "Mobility Support in IPv6",
Work in Progress.
9. Authors Address
Internet Architecture Board
Internet Engineering Steering Group
10. Full Copyright Statement
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