Internet Engineering Task Force (IETF) T. Narten
Request for Comments: 6177 IBM
BCP: 157 G. Huston
Obsoletes: 3177 APNIC
Category: Best Current Practice L. Roberts
ISSN: 2070-1721 Stanford University
March 2011 IPv6 Address Assignment to End Sites
RFC 3177 argued that in IPv6, end sites should be assigned /48 blocks
in most cases. The Regional Internet Registries (RIRs) adopted that
recommendation in 2002, but began reconsidering the policy in 2005.
This document obsoletes the RFC 3177 recommendations on the
assignment of IPv6 address space to end sites. The exact choice of
how much address space to assign end sites is an issue for the
operational community. The IETF's role in this case is limited to
providing guidance on IPv6 architectural and operational
considerations. This document reviews the architectural and
operational considerations of end site assignments as well as the
motivations behind the original recommendations in RFC 3177.
Moreover, this document clarifies that a one-size-fits-all
recommendation of /48 is not nuanced enough for the broad range of
end sites and is no longer recommended as a single default.
This document obsoletes RFC 3177.
Status of This Memo
This memo documents an Internet Best Current Practice.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
BCPs is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
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Table of Contents
1. Introduction ....................................................32. On /48 Assignments to End Sites .................................43. Other RFC 3177 Considerations ...................................64. Impact on IPv6 Standards ........................................64.1. RFC 3056: Connection of IPv6 Domains via IPv4 Clouds .......64.2. IPv6 Multicast Addressing ..................................75. Summary .........................................................76. Security Considerations .........................................87. Acknowledgments .................................................88. Informative References ..........................................8
There are a number of considerations that factor into address
assignment policies. For example, to provide for the long-term
health and scalability of the public routing infrastructure, it is
important that addresses aggregate well [ROUTE-SCALING]. Likewise,
giving out an excessive amount of address space could result in
premature depletion of the address space. This document focuses on
the (more narrow) question of what is an appropriate IPv6 address
assignment size for end sites. That is, when end sites request IPv6
address space from ISPs, what is an appropriate assignment size.
RFC 3177 [RFC3177] called for a default end site IPv6 assignment size
of /48. Subsequently, the Regional Internet Registries (RIRs)
developed and adopted IPv6 address assignment and allocation policies
consistent with the recommendations of RFC 3177 [RIR-IPV6]. In 2005,
the RIRs began discussing IPv6 address assignment policy again.
Since then, APNIC [APNIC-ENDSITE], ARIN [ARIN-ENDSITE], and RIPE
[RIPE-ENDSITE] have revised the end site assignment policy to
encourage the assignment of smaller (i.e., /56) blocks to end sites.
This document obsoletes RFC 3177, updating its recommendations in the
1) It is no longer recommended that /128s be given out. While
there may be some cases where assigning only a single address
may be justified, a site, by definition, implies multiple
subnets and multiple devices.
2) RFC 3177 specifically recommended using prefix lengths of /48,
/64, and /128. Specifying a small number of fixed boundaries
has raised concerns that implementations and operational
practices might become "hard-coded" to recognize only those
fixed boundaries (i.e., a return to "classful addressing").
The actual intention has always been that there be no hard-
coded boundaries within addresses, and that Classless Inter-
Domain Routing (CIDR) continues to apply to all bits of the
3) This document moves away from the previous recommendation that
a single default assignment size (e.g., a /48) makes sense for
all end sites in the general case. End sites come in different
shapes and sizes, and a one-size-fits-all approach is not
necessary or appropriate.
This document does, however, reaffirm an important assumption behind
A key principle for address management is that end sites always be
able to obtain a reasonable amount of address space for their
actual and planned usage, and over time ranges specified in years
rather than just months. In practice, that means at least one
/64, and in most cases significantly more. One particular
situation that must be avoided is having an end site feel
compelled to use IPv6-to-IPv6 Network Address Translation or other
burdensome address conservation techniques because it could not
get sufficient address space.
This document does not make a formal recommendation on what the exact
assignment size should be. The exact choice of how much address
space to assign end sites is an issue for the operational community.
The IETF's role in this case is limited to providing guidance on IPv6
architectural and operational considerations. This document provides
input into those discussions. The focus of this document is to
examine the architectural issues and some of the operational
considerations relating to the size of the end site assignment.
2. On /48 Assignments to End Sites
Looking back at some of the original motivations behind the /48
recommendation [RFC3177], there were three main concerns. The first
motivation was to ensure that end sites could easily obtain
sufficient address space without having to "jump through hoops" to do
so. For example, if someone felt they needed more space, just the
act of asking would at some level be sufficient justification. As a
comparison point, in IPv4, typical home users are given a single
public IP address (though even this is not always assured), but
getting any more than one address is often difficult or even
impossible -- unless one is willing to pay a (significantly)
increased fee for what is often considered to be a "higher grade" of
service. (It should be noted that increased ISP charges to obtain a
small number of additional addresses cannot usually be justified by
the real per-address cost levied by RIRs, but additional addresses
are frequently only available to end users as part of a different
type or "higher grade" of service, for which an additional charge is
levied. The point here is that the additional cost is not due to the
RIR fee structures, but to business choices ISPs make.) An important
goal in IPv6 is to significantly change the default and minimal end
site assignment, from "a single address" to "multiple networks" and
to ensure that end sites can easily obtain address space.
A second motivation behind the original /48 recommendation was to
simplify the management of an end site's addressing plan in the
presence of renumbering (e.g., when switching ISPs). In IPv6, a site
may simultaneously use multiple prefixes, including one or more
public prefixes from ISPs as well as Unique Local Addresses
[ULA-ADDRESSES]. In the presence of multiple prefixes, it is
significantly less complex to manage a numbering plan if the same
subnet numbering plan can be used for all prefixes. That is, for a
link that has (say) three different prefixes assigned to it, the
subnet portion of those prefixes would be identical for all assigned
addresses. In contrast, renumbering from a larger set of "subnet
bits" into a smaller set is often painful, as it can require making
changes to the network itself (e.g., collapsing subnets). Hence,
renumbering a site into a prefix that has (at least) the same number
of subnet bits is more straightforward, because only the top-level
bits of the address need to change. A key goal of the
recommendations in RFC 3177 is to ensure that upon renumbering, one
does not have to deal with renumbering into a smaller subnet size.
It should be noted that similar arguments apply to the management of
zone files in the DNS. In particular, managing the reverse
(ip6.arpa) tree is simplified when all links are numbered using the
same subnet ids.
A third motivation behind the /48 recommendation was to better
support network growth common at many sites. In IPv4, it is usually
difficult (or impossible) to obtain public address space for more
than a few months worth of projected growth. Thus, even slow growth
over several years can lead to the need to renumber into a larger
address block. With IPv6's vast address space, end sites can easily
be given more address space (compared with IPv4) to support expected
growth over multi-year time periods.
While the /48 recommendation does simplify address space management
for end sites, it has also been widely criticized as being wasteful.
For example, a large business (which may have thousands of employees)
would, by default, receive the same amount of address space as a home
user, who today typically has a single (or small number of) LAN and a
small number of devices (dozens or less). While it seems likely that
the size of a typical home network will grow over the next few
decades, it is hard to argue that home sites will make use of 65K
subnets within the foreseeable future. At the same time, it might be
tempting to give home sites a single /64, since that is already
significantly more address space compared with today's IPv4 practice.
However, this precludes the expectation that even home sites will
grow to support multiple subnets going forward. Hence, it is
strongly intended that even home sites be given multiple subnets
worth of space, by default. Hence, this document still recommends
giving home sites significantly more than a single /64, but does not
recommend that every home site be given a /48 either.
A change in policy (such as above) would have a significant impact on
address consumption projections and the expected longevity for IPv6.
For example, changing the default assignment from a /48 to /56 (for
the vast majority of end sites, e.g., home sites) would result in a
savings of up to 8 bits, reducing the "total projected address
consumption" by (up to) 8 bits or two orders of magnitude. (The
exact amount of savings depends on the relative number of home users
compared with the number of larger sites.)
The above-mentioned goals of RFC 3177 can easily be met by giving
home users a default assignment of less than /48, such as a /56.
3. Other RFC 3177 Considerations
RFC 3177 suggested that some multihoming approaches (e.g.,
Generalized Structure Element (GSE)) might benefit from having a
fixed /48 boundary. This no longer appears to be a consideration.
RFC 3177 argued that having a "one-size-fits-all" default assignment
size reduced the need for customers to continually or repeatedly
justify the usage of existing address space in order to get "a little
more". Likewise, it also reduces the need for ISPs to evaluate such
requests. Given the large amount of address space in IPv6, there is
plenty of space to grant end sites enough space to be consistent with
reasonable growth projections over multi-year time frames. Thus, it
remains highly desirable to provide end sites with enough space (on
both initial and subsequent assignments) to last several years.
Fortunately, this goal can be achieved in a number of ways and does
not require that all end sites receive the same default size
4. Impact on IPv6 Standards
4.1. RFC 3056: Connection of IPv6 Domains via IPv4 Clouds
RFC 3056 [RFC3056] describes a way of generating IPv6 addresses from
an existing public IPv4 address. That document describes an address
format in which the first 48 bits concatenate a well-known prefix
with a globally unique public IPv4 address. The "SLA ID" field is
assumed to be 16 bits, consistent with a 16-bit "subnet id" field.
To facilitate transitioning from the address numbering scheme in RFC
3056 to one based on a prefix obtained from an ISP, an end site would
be advised to number out of the right most bits first, using the
leftmost bits only if the size of the site made that necessary.
Similar considerations apply to other documents that allow for a
subnet id of 16 bits, including [ULA-ADDRESSES].
4.2. IPv6 Multicast Addressing
Some IPv6 multicast address assignment schemes embed a unicast IPv6
prefix into the multicast address itself [RFC3306]. Such documents
do not assume a particular size for the subnet id, per se, but do
assume that the IPv6 prefix is a /64. Thus, the relative size of the
subnet id has no direct impact on multicast address schemes.
The exact choice of how much address space to assign end sites is an
issue for the operational community. The recommendation in RFC 3177
[RFC3177] to assign /48s as a default is not a requirement of the
IPv6 architecture; anything of length /64 or shorter works from a
standards perspective. However, there are important operational
considerations as well, some of which are important if users are to
share in the key benefit of IPv6: expanding the usable address space
of the Internet. The IETF recommends that any policy on IPv6 address
assignment policy to end sites take into consideration the following:
- it should be easy for an end site to obtain address space to
number multiple subnets (i.e., a block larger than a single /64)
and to support reasonable growth projections over long time
periods (e.g., a decade or more).
- the default assignment size should take into consideration the
likelihood that an end site will have need for multiple subnets
in the future and avoid the IPv4 practice of having frequent and
continual justification for obtaining small amounts of
- Although a /64 can (in theory) address an almost unlimited
number of devices, sites should be given sufficient address
space to be able to lay out subnets as appropriate, and not be
forced to use address conservation techniques such as using
bridging. Whether or not bridging is an appropriate choice is
an end site matter.
- assigning a longer prefix to an end site, compared with the
existing prefixes the end site already has assigned to it, is
likely to increase operational costs and complexity for the end
site, with insufficient benefit to anyone.
[ULA-ADDRESSES] Hinden, R. and B. Haberman, "Unique Local IPv6
Unicast Addresses", RFC 4193, October 2005.
3039 Cornwallis Ave.
PO Box 12195
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Rosalea G Roberts
Stanford University, Networking Systems
P.O. Box 19131
Stanford, CA 94309-9131