Internet DRAFT - draft-iesg-ipv6-addressing-recommendations


INTERNET-DRAFT                                                  IAB/IESG
July 20, 2001

       IAB/IESG Recommendations on IPv6 Address Allocations to Sites


                          Status of this Memo

   [Note: This document is a draft position of the IAB and IESG.
   Comments should be directed to and  This
   note will be removed upon publication as an RFC.]

   This document is an Internet-Draft and is in full conformance with
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   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 IESG/IAB statement
   mailed to the registries on September 1, 2000. This document refines
   the original recommendation and documents it for the historical

1. Introduction

   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
   transient customers.

   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.

2. Background

   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 [PRIVACY].

   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 an tolerable event, but to be avoided if
   reasonably feasible.

   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
   such sites.

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 is

   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
   address space.

   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 detail below.)

    - 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 format prefix contains 45 variable bits.  That
   is, the number of available prefixes is 2 to the power 45 or about 35
   trillion (35,184,372,088,832).

   More precisely,

    - [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)

         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  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 emerges.

    - 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 format 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

   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
   each other.

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.

7. Acknowledgments

   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.

8. References

   RFC1715 The H Ratio for Address Assignment Efficiency.  C. Huitema.
           November 1994.

   RFC2026 The Internet Standards Process -- Revision 3.  S. Bradner.
           October 1996.

   RFC2260 Scalable Support for Multi-homed Multi-provider Connectivity.
           T. Bates, Y. Rekhter.  January 1998.

   RFC2374 An IPv6 Aggregatable Global Unicast Address Format.  R.
           Hinden, M.O'Dell, S. Deering.  July 1998.

   RFC2450 Proposed TLA and NLA Assignment Rule.  R. Hinden.  December

   RFC2462 IPv6 Stateless Address Autoconfiguration.  S. Thomson, T.
           Narten.  December 1998.

   RFC2874 DNS Extensions to Support IPv6 Address Aggregation and
           Renumbering.  M. Crawford, C. Huitema.  July 2000.

   PRIVACY Privacy Extensions for Stateless Address Autoconfiguration in
           IPv6.  draft-ietf-ipngwg-addrconf-privacy-04.txt

   MobIPv6 Mobility Support in IPv6.  draft-ietf-mobileip-ipv6-13.txt