Network Working Group G. Van de Velde
Request for Comments: 5375 C. Popoviciu
Category: Informational Cisco Systems
University of Southampton
O. BonnessC. Hahn
T-Systems Enterprise Services GmbH
December 2008 IPv6 Unicast Address Assignment Considerations
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) 2008 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (http://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document.
One fundamental aspect of any IP communications infrastructure is its
addressing plan. With its new address architecture and allocation
policies, the introduction of IPv6 into a network means that network
designers and operators need to reconsider their existing approaches
to network addressing. Lack of guidelines on handling this aspect of
network design could slow down the deployment and integration of
IPv6. This document aims to provide the information and
recommendations relevant to planning the addressing aspects of IPv6
deployments. The document also provides IPv6 addressing case studies
for both an enterprise and an ISP network.
The Internet Protocol Version 6 (IPv6) Addressing Architecture
[RFC4291] defines three main types of addresses: unicast, anycast,
and multicast. This document focuses on unicast addresses, for which
there are currently two principal allocated types: Globally Unique
Addresses ('globals') [RFC3587] and Unique Local IPv6 Addresses
(ULAs) [RFC4193]. In addition, until recently there has been the
'experimental' 6bone address space [RFC3701], though its use has been
deprecated since June 2006 [RFC3701].
The document covers aspects that should be considered during IPv6
deployment for the design and planning of an addressing scheme for an
IPv6 network. The network's IPv6 addressing plan may be for an IPv6-
only network, or for a dual-stack infrastructure where some or all
devices have addresses in both protocols. These considerations will
help an IPv6 network designer to efficiently and prudently assign the
IPv6 address space that has been allocated to their organization.
The address assignment considerations are analyzed separately for the
two major components of the IPv6 unicast addresses -- namely,
'Network-Level Addressing' (the allocation of subnets) and the
'interface-id' (the identification of the interface within a subnet).
Thus, the document includes a discussion of aspects of address
assignment to nodes and interfaces in an IPv6 network. Finally, the
document provides two examples of deployed addressing plans in a
service provider (ISP) and an enterprise network.
Parts of this document highlight the differences that an experienced
IPv4 network designer should consider when planning an IPv6
deployment, for example:
o IPv6 devices will more likely be multi-addressed in comparison
with their IPv4 counterparts.
o The practically unlimited size of an IPv6 subnet (2^64 bits)
reduces the requirement to size subnets to device counts for the
purposes of (IPv4) address conservation.
o The vastly increased subnet size has implications on the threat of
address-based host scanning and other scanning techniques, as
discussed in [RFC5157].
We do not discuss here how a site or ISP should proceed with
acquiring its globally routable IPv6 address prefix. In each case,
the prefix received is either provider assigned (PA) or provider
We do not discuss PI policy here. The observations and
recommendations of this text are largely independent of the PA or PI
nature of the address block being used. At this time, we assume that
when an IPv6 network changes provider, typically it will need to
undergo a renumbering process, as described in [RFC4192]. A separate
document [THINKABOUT] makes recommendations to ease the IPv6
This document does not discuss implementation aspects related to the
transition from the now obsoleted site-local addresses to ULAs. Some
implementations know about site-local addresses even though they are
deprecated, and do not know about ULAs even though they represent
current specification. As a result, transitioning between these
types of addresses may cause difficulties.
2. Network-Level Addressing Design Considerations
This section discusses the kind of IPv6 addresses used at the network
level for the IPv6 infrastructure. The kind of addresses that can be
considered are Globally Unique Addresses and ULAs. We also comment
here on the deprecated 6bone address space.
2.1. Globally Unique Addresses
The most commonly used unicast addresses will be Globally Unique
Addresses ('globals'). No significant considerations are necessary
if the organization has an address space assignment and a single
prefix is deployed through a single upstream provider.
However, a multihomed site may deploy addresses from two or more
service-provider-assigned IPv6 address ranges. Here, the network
administrator must have awareness on where and how these ranges are
used on the multihomed infrastructure environment. The nature of the
usage of multiple prefixes may depend on the reason for multihoming
(e.g., resilience failover, load balancing, policy-based routing, or
multihoming during an IPv6 renumbering event). IPv6 introduces
improved support for multi-addressed hosts through the IPv6 default
address selection methods described in RFC 3484 [RFC3484]. A
multihomed host may thus have two or more addresses, one per prefix
(provider), and select source and destination addresses to use as
described in that RFC. However, multihoming also has some
operational and administrative burdens besides choosing multiple
addresses per interface [RFC4218] [RFC4219].
2.2. Unique Local IPv6 Addresses
ULAs have replaced the originally conceived site-local addresses in
the IPv6 addressing architecture, for reasons described in [RFC3879].
ULAs improve on site-locals by offering a high probability of the
global uniqueness of the prefix used, which can be beneficial when
there is (deliberate or accidental) leakage or when networks are
merged. ULAs are akin to the private address space [RFC1918]
assigned for IPv4 networks, except that in IPv6 networks we may
expect to see ULAs used alongside global addresses, with ULAs used
internally and globals used externally. Thus, use of ULAs does not
imply use of NAT for IPv6.
The ULA address range allows network administrators to deploy IPv6
addresses on their network without asking for a globally unique
registered IPv6 address range. A ULA prefix is 48 bits, i.e., a /48,
the same as the currently recommended allocation for a site from the
globally routable IPv6 address space [RFC3177].
A site that wishes to use ULAs can have (a) multiple /48 prefixes
(e.g., a /44) (b) one /48, or (c) a less-than-/48 prefix (e.g., a /56
or /64). In all of the above cases, the ULAs can be randomly chosen
according to the principles specified in [RFC4193]. However, in case
(a) the use of randomly chosen ULAs will provide suboptimal
ULAs provide the means to deploy a fixed addressing scheme that is
not affected by a change in service provider and the corresponding PA
global addresses. Internal operation of the network is thus
unaffected during renumbering events. Nevertheless, this type of
address must be used with caution.
A site using ULAs may or may not also deploy global addresses. In an
isolated network, ULAs may be deployed on their own. In a connected
network that also deploys global addresses, both may be deployed,
such that hosts become multi-addressed (one global and one ULA), and
the IPv6 default address selection algorithm will pick the
appropriate source and destination addresses to use, e.g., ULAs will
be selected where both the source and destination hosts have ULAs.
Because a ULA and a global site prefix are both /48 length, an
administrator can choose to use the same subnetting (and host
addressing) plan for both prefixes.
As an example of the problems ULAs may cause, when using IPv6
multicast within the network, the IPv6 default address selection
algorithm prefers the ULA as the source address for the IPv6
multicast streams. This is NOT a valid option when sending an IPv6
multicast stream to the IPv6 Internet for two reasons. For one,
these addresses are not globally routable, so Reverse Path Forwarding
checks for such traffic will fail outside the internal network. The
other reason is that the traffic will likely not cross the network
boundary due to multicast domain control and perimeter security
In principle, ULAs allow easier network mergers than RFC 1918
addresses do for IPv4 because ULA prefixes have a high probability of
uniqueness, if the prefix is chosen as described in the RFC.
2.3. 6bone Address Space
The 6bone address space was used before the Regional Internet
Registries (RIRs) started to distribute 'production' IPv6 prefixes.
The 6bone prefixes have a common first 16 bits in the IPv6 Prefix of
3FFE::/16. This address range has been deprecated as of 6 June 2006
[RFC3701] and must not be used on any new IPv6 network deployments.
Sites using 6bone address space should renumber to production address
space using procedures as defined in [RFC4192].
2.4. Network-Level Design Considerations
IPv6 provides network administrators with a significantly larger
address space, enabling them to be very creative in how they can
define logical and practical addressing plans. The subnetting of
assigned prefixes can be done based on various logical schemes that
involve factors such as:
o Using existing systems
* translate the existing subnet numbers into IPv6 subnet IDs
* translate the VLAN IDs into IPv6 subnet IDs
* allocate according to your need
* Geographical Boundaries - by assigning a common prefix to all
subnets within a geographical area.
* Organizational Boundaries - by assigning a common prefix to an
entire organization or group within a corporate infrastructure.
* Service Type - by reserving certain prefixes for predefined
services such as: VoIP, content distribution, wireless
services, Internet access, security areas, etc. This type of
addressing may create dependencies on IP addresses that can
make renumbering harder if the nodes or interfaces supporting
those services on the network are sparse within the topology.
Such logical addressing plans have the potential to simplify network
operations and service offerings, and to simplify network management
and troubleshooting. A very large network would not need to consider
using private address space for its infrastructure devices, thereby
simplifying network management.
The network designer must however keep in mind several factors when
developing these new addressing schemes for networks with and without
o Prefix aggregation - The larger IPv6 addresses can lead to larger
routing tables unless network designers are actively pursuing
aggregation. While prefix aggregation will be enforced by the
service provider, it is beneficial for the individual
organizations to observe the same principles in their network
o Network growth - The allocation mechanism for flexible growth of a
network prefix, documented in RFC 3531 [RFC3531] can be used to
allow the network infrastructure to grow and be numbered in a way
that is likely to preserve aggregation (the plan leaves 'holes'
o ULA usage in large networks - Networks that have a large number of
'sites' that each deploy a ULA prefix that will by default be a
'random' /48 under fc00::/7 will have no aggregation of those
prefixes. Thus, the end result may be cumbersome because the
network will have large amounts of non-aggregated ULA prefixes.
However, there is no rule to disallow large networks from using a
single ULA prefix for all 'sites', as a ULA still provides 16 bits
for subnetting to be used internally.
o Compact numbering of small sites - It is possible that as registry
policies evolve, a small site may experience an increase in prefix
length when renumbering, e.g., from /48 to /56. For this reason,
the best practice is to number subnets compactly rather than
sparsely, and to use low-order bits as much as possible when
numbering subnets. In other words, even if a /48 is allocated,
act as though only a /56 is available. Clearly, this advice does
not apply to large sites and enterprises that have an intrinsic
need for a /48 prefix.
o Consider assigning more than one /64 to a site - A small site may
want to enable routing amongst interfaces connected to a gateway
device. For example, a residential gateway that receives a /48
and is situated in a home with multiple LANs of different media
types (sensor network, wired, Wi-Fi, etc.), or has a need for
traffic segmentation (home, work, kids, etc.), could benefit
greatly from multiple subnets and routing in IPv6. Ideally,
residential networks would be given an address range of a /48 or
/56 [RIPE_Nov07] such that multiple /64 subnets could be used
within the residence.
2.4.1. Sizing the Network Allocation
We do not discuss here how a network designer sizes their application
for address space. By default, a site will receive a /48 prefix
[RFC3177]; however, different RIR service regions policies may
suggest alternative default assignments or let the ISPs decide on
what they believe is more appropriate for their specific case (see
Section 6.5.4, "Assignments from LIRs/ISPs", of [ARIN]). The default
provider allocation via the RIRs is currently a /32 [RIPE_Nov07].
These allocations are indicators for a first allocation for a
network. Different sizes may be obtained based on the anticipated
address usage [RIPE_Nov07]. At the time of writing, there are
examples of allocations as large as /19 having been made from RIRs to
2.4.2. Address Space Conservation
Despite the large IPv6 address space, which enables easier
subnetting, it still is important to ensure an efficient use of this
resource. Some addressing schemes, while facilitating aggregation
and management, could lead to significant numbers of addresses being
unused. Address conservation requirements are less stringent in
IPv6, but they should still be observed.
The proposed Host-Density (HD) value [RFC3194] for IPv6 is 0.94
compared to the current value of 0.96 for IPv4. Note that with IPv6,
HD is calculated for sites (e.g., on a basis of /56), instead of for
addresses as with IPv4.
3. Subnet Prefix Considerations
An important part of an IPv4 addressing plan is deciding the length
of each subnet prefix. Unlike in IPv4, the IPv6 addressing
architecture [RFC4291] specifies that all subnets using Globally
Unique Addresses and ULAs always have the same prefix length of 64
bits. (This also applies to the deprecated 6bone and site-local
The only exception to this rule are special addresses starting with
the binary value 000, such as IPv4-compatible IPv6 addresses. These
exceptions are largely beyond the scope of this document.
Using a subnet prefix length other than a /64 will break many
features of IPv6, including Neighbor Discovery (ND), Secure Neighbor
Discovery (SEND) [RFC3971], privacy extensions [RFC4941], parts of
Mobile IPv6 [RFC4866], Protocol Independent Multicast - Sparse Mode
(PIM-SM) with Embedded-RP [RFC3956], and Site Multihoming by IPv6
Intermediation (SHIM6) [SHIM6], among others. A number of other
features currently in development, or being proposed, also rely on
/64 subnet prefixes.
Nevertheless, many IPv6 implementations do not prevent the
administrator from configuring a subnet prefix length shorter or
longer than 64 bits. Using subnet prefixes shorter than /64 would
rarely be useful; see Appendix B.1 for discussion.
However, some network administrators have used prefixes longer than
/64 for links connecting routers, usually just two routers on a
point-to-point link. On links where all the addresses are assigned
by manual configuration, and all nodes on the link are routers (not
end hosts) that are known by the network, administrators do not need
any of the IPv6 features that rely on /64 subnet prefixes, this can
work. Using subnet prefixes longer than /64 is not recommended for
general use, and using them for links containing end hosts would be
an especially bad idea, as it is difficult to predict what IPv6
features the hosts will use in the future.
Appendix B.2 describes some practical considerations that need to be
taken into account when using prefixes longer than /64 in limited
cases. In particular, a number of IPv6 features use interface
identifiers that have a special form (such as a certain fixed value
in some bit positions). When using prefixes longer than /64, it is
prudent to avoid certain subnet prefix values so that nodes who
assume that the prefix is /64 will not incorrectly identify the
addresses in that subnet as having a special form. Appendix B.2
describes the subnet prefix values that are currently believed to be
potentially problematic; however, the list is not exhaustive and can
be expected to grow in the future.
Using /64 subnets is strongly recommended, also for links connecting
only routers. A deployment compliant with the current IPv6
specifications cannot use other prefix lengths. However, the V6OPS
WG believes that despite the drawbacks (and a potentially expensive
network redesign, if IPv6 features relying on /64 subnets are needed
in the future), some networks administrators will use prefixes longer
3.1. Considerations for /64 Prefixes
Based on RFC 3177 [RFC3177], 64 bits is the prescribed subnet prefix
length to allocate to interfaces and nodes.
When using a /64 subnet length, the address assignment for these
addresses can be made either by manual configuration, by a Dynamic
Host Configuration Protocol [RFC3315], by stateless autoconfiguration
[RFC4862], or by a combination thereof [RFC3736].
Note that RFC 3177 strongly prescribes 64-bit subnets for general
usage, and that stateless autoconfiguration on most link layers
(including Ethernet) is only defined for 64-bit subnets. While in
theory it might be possible that some future autoconfiguration
mechanisms would allow longer than 64-bit prefix lengths to be used,
the use of such prefixes is not recommended at this time.
4. Allocation of the IID of an IPv6 Address
In order to have a complete IPv6 address, an interface must be
associated with a prefix and an Interface Identifier (IID). Section
3 of this document analyzed the prefix selection considerations.
This section discusses the elements that should be considered when
assigning the IID portion of the IPv6 address.
There are various ways to allocate an IPv6 address to a device or
interface. The option with the least amount of caveats for the
network administrator is that of EUI-64 [RFC4862] based addresses.
For the manual or dynamic options, the overlap with well-known IPv6
addresses should be avoided.
4.1. Automatic EUI-64 Format Option
When using this method, the network administrator has to allocate a
valid 64-bit subnet prefix. Once that allocation has been made, the
EUI-64 [RFC4862] allocation procedure can assign the remaining 64 IID
bits in a stateless manner. All the considerations for selecting a
valid IID have been incorporated into the EUI-64 methodology.
4.2. Using Privacy Extensions
The main purpose of IIDs generated based on RFC 4941 [RFC4941] is to
provide privacy to the entity using an IPv6 address. While there are
no particular constraints in the usage of IPv6 addresses with IIDs as
defined in [RFC4941], there are some implications to be aware of when
using privacy addresses as documented in Section 4 of RFC 4941
4.3. Manual/Dynamic Assignment Option
This section discusses those IID allocations that are not implemented
through stateless address configuration (Section 4.1). They are
applicable regardless of the prefix length used on the link. It is
out of scope for this section to discuss the various assignment
methods (e.g., manual configuration, DHCPv6, etc).
In this situation, the actual allocation is done by human
intervention, and consideration needs to be given to the complete
IPv6 address so that it does not result in overlaps with any of the
well-known IPv6 addresses:
o Subnet Router Anycast Address (Appendix B.2.5.1)
o Reserved Subnet Anycast Address (Appendix B.2.5.2)
o Addresses used by Embedded-RP (Appendix B.2.6)
o Intra-Site Automatic Tunnel Addressing Protocol (ISATAP) Addresses
When using an address assigned by human intervention, it is
recommended to choose IPv6 addresses that are not obvious to guess
and/or to avoid any IPv6 addresses that embed IPv4 addresses used in
the current infrastructure. Following these two recommendations will
make it more difficult for malicious third parties to guess targets
for attack, and thus reduce security threats to a certain extent.
5. Security Considerations
This document doesn't add any new security considerations that aren't
already outlined in the security considerations of the references.
It must be noted that using subnet prefixes other than /64 breaks
security mechanisms such as Cryptographically Generated Addresses
(CGAs) and Hash-Based Addresses (HBAs), and thus makes it impossible
to use protocols that depend on them.
Constructive feedback and contributions have been received during
IESG review cycle and from Marla Azinger, Stig Venaas, Pekka Savola,
John Spence, Patrick Grossetete, Carlos Garcia Braschi, Brian
Carpenter, Mark Smith, Janos Mohacsi, Jim Bound, Fred Templin, Ginny
Listman, Salman Assadullah, Krishnan Thirukonda, and the IESG.
7. Informative References
[RFC1918] Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot,
G., and E. Lear, "Address Allocation for Private
Internets", BCP 5, RFC 1918, February 1996.
[RFC2526] Johnson, D. and S. Deering, "Reserved IPv6 Subnet
Anycast Addresses", RFC 2526, March 1999.
[RFC3021] Retana, A., White, R., Fuller, V., and D. McPherson,
"Using 31-Bit Prefixes on IPv4 Point-to-Point Links",
RFC 3021, December 2000.
[RFC3053] Durand, A., Fasano, P., Guardini, I., and D. Lento,
"IPv6 Tunnel Broker", RFC 3053, January 2001.
[RFC3056] Carpenter, B. and K. Moore, "Connection of IPv6
Domains via IPv4 Clouds", RFC 3056, February 2001.
[RFC3177] IAB and IESG, "IAB/IESG Recommendations on IPv6
Address Allocations to Sites", RFC 3177,
[RFC3180] Meyer, D. and P. Lothberg, "GLOP Addressing in
233/8", BCP 53, RFC 3180, September 2001.
[RFC3194] Durand, A. and C. Huitema, "The H-Density Ratio for
Address Assignment Efficiency An Update on the H
ratio", RFC 3194, November 2001.
[RFC3315] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins,
C., and M. Carney, "Dynamic Host Configuration
Protocol for IPv6 (DHCPv6)", RFC 3315, July 2003.
[RFC3484] Draves, R., "Default Address Selection for Internet
Protocol version 6 (IPv6)", RFC 3484, February 2003.
[RFC3531] Blanchet, M., "A Flexible Method for Managing the
Assignment of Bits of an IPv6 Address Block",
RFC 3531, April 2003.
[RFC3587] Hinden, R., Deering, S., and E. Nordmark, "IPv6
Global Unicast Address Format", RFC 3587,
[RFC3627] Savola, P., "Use of /127 Prefix Length Between
Routers Considered Harmful", RFC 3627,
[RFC3633] Troan, O. and R. Droms, "IPv6 Prefix Options for
Dynamic Host Configuration Protocol (DHCP) version
6", RFC 3633, December 2003.
[RFC3701] Fink, R. and R. Hinden, "6bone (IPv6 Testing Address
Allocation) Phaseout", RFC 3701, March 2004.
[RFC3736] Droms, R., "Stateless Dynamic Host Configuration
Protocol (DHCP) Service for IPv6", RFC 3736,
[RFC3879] Huitema, C. and B. Carpenter, "Deprecating Site Local
Addresses", RFC 3879, September 2004.
[RFC3956] Savola, P. and B. Haberman, "Embedding the Rendezvous
Point (RP) Address in an IPv6 Multicast Address",
RFC 3956, November 2004.
[RFC3971] Arkko, J., Kempf, J., Zill, B., and P. Nikander,
"SEcure Neighbor Discovery (SEND)", RFC 3971,
[RFC4192] Baker, F., Lear, E., and R. Droms, "Procedures for
Renumbering an IPv6 Network without a Flag Day",
RFC 4192, September 2005.
[RFC4193] Hinden, R. and B. Haberman, "Unique Local IPv6
Unicast Addresses", RFC 4193, October 2005.
[RFC4218] Nordmark, E. and T. Li, "Threats Relating to IPv6
Multihoming Solutions", RFC 4218, October 2005.
[RFC4219] Lear, E., "Things Multihoming in IPv6 (MULTI6)
Developers Should Think About", RFC 4219,
[RFC4271] Rekhter, Y., Li, T., and S. Hares, "A Border Gateway
Protocol 4 (BGP-4)", RFC 4271, January 2006.
[RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, February 2006.
[RFC4477] Chown, T., Venaas, S., and C. Strauf, "Dynamic Host
Configuration Protocol (DHCP): IPv4 and IPv6 Dual-
Stack Issues", RFC 4477, May 2006.
Appendix A. Case Studies
This appendix contains two case studies for IPv6 addressing schemas
that have been based on the statements and considerations of this
document. These case studies illustrate how this document has been
used in two specific network scenarios. The case studies may serve
as basic considerations for an administrator who designs the IPv6
addressing schema for an enterprise or ISP network, but are not
intended to serve as a general design proposal for every kind of IPv6
network. All subnet sizes used in this appendix are for practical
visualization and do not dictate RIR policy.
A.1. Enterprise Considerations
In this section, one considers a case study of a campus network that
is deploying IPv6 in parallel with existing IPv4 protocols in a dual-
stack environment. The specific example is the University of
Southampton (UK), focusing on a large department within that network.
The deployment currently spans around 1,000 hosts and over 1,500
A.1.1. Obtaining General IPv6 Network Prefixes
In the case of a campus network, the site will typically take its
connectivity from its National Research and Education Network (NREN).
Southampton connects to JANET, the UK academic network, via its local
regional network LeNSE (Learning Network South East). JANET
currently has a /32 allocation from RIPE NCC. The current
recommended practice is for sites to receive a /48 allocation; on
this basis, Southampton has received such a prefix for its own use.
The regional network also uses its own allocation from the NREN
No ULA addressing is used on site. The campus is not multihomed
(JANET is the sole provider), nor does it expect to change service
provider, and thus does not plan to use ULAs for the (perceived)
benefit of easing network renumbering. Indeed, the campus has
renumbered following the aforementioned renumbering procedure
[RFC4192] on two occasions, and this has proven adequate (with
provisos documented in [THINKABOUT]). The campus does not see any
need to deploy ULAs for in-band or out-of-band network management;
there are enough IPv6 prefixes available in the site allocation for
the infrastructure. In some cases, use of private IP address space
in IPv4 creates problems, so University of Southampton believes that
the availability of ample global IPv6 address space for
infrastructure may be a benefit for many sites.
No 6bone addressing is used on site any more. Since the 6bone
phaseout of June 2006 [RFC3701], most transit ISPs have begun
filtering attempted use of such prefixes.
Southampton does participate in global and organizational scope IPv6
multicast networks. Multicast address allocations are not discussed
here as they are not in scope for the document. It is noted that
IPv6 has advantages for multicast group address allocation. In IPv4,
a site needs to use techniques like GLOP [RFC3180] to pick a globally
unique multicast group to use. This is problematic if the site does
not use the Border Gateway Protocol (BGP) [RFC4271] and does not have
an Autonomous System Number (ASN). In IPv6,0 unicast-prefix-based
IPv6 multicast addresses empower a site to pick a globally unique
group address based on its own unicast site or link prefix.
Embedded-RP is also in use, is seen as a potential advantage for IPv6
and multicast, and has been tested successfully across providers
between sites (including paths to/from the US and UK).
A.1.2. Forming an Address (Subnet) Allocation Plan
The campus has a /16 prefix for IPv4 use; in principle, 256 subnets
of 256 addresses. In reality, the subnetting is muddier, because of
concerns of IPv4 address conservation; subnets are sized to the hosts
within them, e.g., a /26 IPv4 prefix is used if a subnet has 35 hosts
in it. While this is efficient, it increases management burden when
physical deployments change, and IPv4 subnets require resizing (up or
down), even when DHCP is in use.
The /48 IPv6 prefix is considerably larger than the IPv4 allocation
already in place at the site. It is loosely equivalent to a 'Class
A' IPv4 prefix in that it has 2^16 (over 65,000) subnets, but has an
effectively unlimited subnet address size (2^64) compared to 256 in
the IPv4 equivalent. The increased subnet size means that /64 IPv6
prefixes can be used on all subnets, without any requirement to
resize them at a later date. The increased subnet volume allows
subnets to be allocated more generously to schools and departments in
the campus. While address conservation is still important, it is no
longer an impediment to network management. Rather, address (subnet)
allocation is more about embracing the available address space and
planning for future expansion.
In a dual-stack network, it was chosen to deploy the IP subnets
congruently for IPv4 and IPv6. This is because the systems are still
in the same administrative domains and the same geography. It is not
expected to have IPv6-only subnets in production use for a while yet,
outside the test beds and some early Mobile IPv6 trials. With
congruent addressing, the firewall policies are also aligned for IPv4
and IPv6 traffic at the site border.
The subnet allocation plan required a division of the address space
per school or department. Here, a /56 was allocated to the school
level of the university; there are around 30 schools currently. A
/56 of IPv6 address space equates to 256 /64 subnet allocations.
Further /56 allocations were made for central IT infrastructure, the
network infrastructure, and the server side systems.
A.1.3. Other Considerations
The network uses a Demilitarized Zone (DMZ) topology for some level
of protection of 'public' systems. Again, this topology is congruent
with the IPv4 network.
There are no specific transition methods deployed internally to the
campus; everything is using the conventional dual-stack approach.
There is no use of ISATAP [RFC5214] for example.
For the Mobile IPv6 early trials, there is one allocated prefix for
Home Agent (HA) use. However, there has been no detailed
consideration yet regarding how Mobile IPv6 usage may grow, and
whether more subnets (or even every subnet) will require HA support.
The university operates a tunnel broker [RFC3053] service on behalf
of the United Kingdom Education and Research Network Association
(UKERNA) for JANET sites. This uses separate address space from
JANET, not the university site allocation.
A.1.4. Node Configuration Considerations
Currently, stateless autoconfiguration is used on most subnets for
IPv6 hosts. There is no DHCPv6 service deployed yet, beyond tests of
early code releases. It is planned to deploy DHCPv6 for address
assignment when robust client and server code is available (at the
time of writing, the potential for this looks good, e.g., via the
Internet Systems Consortium (ISC) implementation). University of
Southampton is also investigating a common integrated DHCP/DNS
management platform, even if the servers themselves are not co-
located, including integrated DHCPv4 and DHCPv6 server configuration,
as discussed in [RFC4477]. Currently, clients with statelessly
autoconfigured addresses are added to the DNS manually, though
dynamic DNS is an option. The network administrators would prefer
the use of DHCP because they believe it gives them more management
Regarding the implications of the larger IPv6 subnet address space on
scanning attacks [RFC5157], it is noted that all the hosts are dual-
stack, and thus are potentially exposed over both protocols anyway.
All addresses are published in DNS, and the site does not operate a
Currently, there is internal usage of RFC 4941 privacy addresses
[RFC4941] (certain platforms ship with it on by default), but network
administrators may desire to disable this (perhaps via DHCP) to ease
management complexity. However, it is desired to determine the
feasibility of this on all systems, e.g., for guests on wireless LAN
or other user-maintained systems. Network management and monitoring
should be simpler without RFC 4941 in operation, in terms of
identifying which physical hosts are using which addresses. Note
that RFC 4941 is only an issue for outbound connections, and that
there is potential to assign privacy addresses via DHCPv6.
Manually configured server addresses are used to avoid address
changes based upon change of network adaptor. With IPv6 you can pick
::53 for a DNS server, or you can pick 'random' addresses for
obfuscation, though that's not an issue for publicly advertised
addresses (dns, mx, web, etc.).
A.2. Service Provider Considerations
In this section an IPv6 addressing schema is sketched that could
serve as an example for an Internet Service Provider.
Appendix A.2.1 starts with some thoughts regarding objective
requirements of such an addressing schema and derives a few general
rules of thumb that have to be kept in mind when designing an ISP
IPv6 addressing plan.
Appendix A.2.2 illustrates the findings of Appendix A.2.1 with an
exemplary IPv6 addressing schema for an MPLS-based ISP offering
Internet services as well as network access services to several
millions of customers.
A.2.1. Investigation of Objective Requirements for an IPv6 Addressing
Schema of a Service Provider
The first step of the IPv6 addressing plan design for a service
provider should identify all technical, operational, political, and
business requirements that have to be satisfied by the services
supported by this addressing schema.
According to the different technical constraints and business models
as well as the different weights of these requirements (from the
point of view of the corresponding service provider), it is very
likely that different addressing schemas will be developed and
deployed by different ISPs. Nevertheless, the addressing schema of
Appendix A.2.2 is one possible example.
For this document, it is assumed that our exemplary ISP has to
fulfill several roles for its customers such as:
o Local Internet Registry
o Network Access Provider
o Internet Service Provider
A.2.1.1. Recommendations for an IPv6 Addressing Schema from the LIR
Perspective of the Service Provider
In its role as Local Internet Registry (LIR), the service provider
has to care about the policy constraints of the RIRs and the
standards of the IETF regarding IPv6 addressing. In this context,
the following basic recommendations have to be considered and should
be satisfied by the IPv6 address allocation plan of a service
o As recommended in RFC 3177 [RFC3177] and in several RIR policies,
"Common" customers sites (normally private customers) should
receive a /48 prefix from the aggregate of the service provider.
(Note: The addressing plan must be flexible enough and take into
account the possible change of the minimum allocation size for end
users currently under definition by the RIRs.)
o "Big customers" (like big enterprises, governmental agencies,
etc.) may receive shorter prefixes according to their needs, when
their needs can be documented and justified to the RIR.
o The IPv6 address allocation schema has to be able to meet the HD-
ratio that is proposed for IPv6. This requirement corresponds to
the demand for an efficient usage of the IPv6 address aggregate by
the service provider. (Note: The currently valid IPv6 HD-ratio of
0.94 means an effective usage rate of about 22% of a /20 prefix of
the service provider, on the basis of /56 assignments.)
o All assignments to customers have to be documented and stored into
a database that can also be queried by the RIR.
o The LIR has to make available the means for supporting the reverse
DNS mapping of the customer prefixes.
o IPv6 Address Allocation and Assignment Policies can be found at
RIRs and are similar in many aspects. See [RIPE_Nov07],
[RIPE_Jul07], [APNIC_IPv6], [LACNIC_IPv6], [AFRINIC_IPv6], and
Section 6 of [ARIN].
A.2.1.2. IPv6 Addressing Schema Recommendations from the ISP
Perspective of the Service Provider
From the ISP perspective, the following basic requirements can be
o The IPv6 address allocation schema must be able to realize a
maximal aggregation of all IPv6 address delegations to customers
into the address aggregate of the service provider. Only this
provider aggregate will be routed and injected into the global
routing table (DFZ, "Default-Free Zone"). This strong aggregation
keeps the routing tables of the DFZ small and eases filtering and
access control very much.
o The IPv6 addressing schema of the SP should contain optimal
flexibility since the infrastructure of the SP will change over
time with new customers, transport technologies, and business
cases. The requirement of optimal flexibility is contrary to the
recommendation of strong IPv6 address aggregation and efficient
address usage, but each SP has to decide which of these
requirements to prioritize.
o While keeping the multilevel network hierarchy of an ISP in mind,
note that due to addressing efficiency reasons, not all hierarchy
levels can and should be mapped into the IPv6 addressing schema of
an ISP. Sometimes it is much better to implement a more "flat"
addressing for the ISP network than to lose big chunks of the IPv6
address aggregate in addressing each level of network hierarchy.
(Note: In special cases, it is even recommended for really "small"
ISPs to design and implement a totally flat IPv6 addressing schema
without any level of hierarchy.)
o A decoupling of provider network addressing and customer
addressing is recommended. (Note: A strong aggregation (e.g., on
POP, Aggregation Router (AG), or Label Edge Router (LER) level)
limits the numbers of customer routes that are visible within the
ISP network, but also brings down the efficiency of the IPv6
addressing schema. That's why each ISP has to decide how many
internal aggregation levels it wants to deploy.)
A.2.1.3. IPv6 Addressing Schema Recommendations from the Network Access
Provider Perspective of the Service Provider
As already done for the LIR and the ISP roles of the SP it is also
necessary to identify requirements that come from its Network Access
Provider role. Some of the basic requirements are:
o The IPv6 addressing schema of the SP, it must be chosen in a way
that it can handle new requirements that are triggered from
customer side. For instance, this can be the customer's growing
needs for IPv6 addresses as well as customer-driven modifications
within the access network topology (e.g., when the customer moves
from one point of network attachment (POP) to another). (See
Appendix A.2.3.4, "Changing the Point of Network Attachment".)
o For each IPv6 address assignment to customers, a "buffer zone"
should be reserved that allows the customer to grow in its
addressing range without renumbering or assignment of additional
o The IPv6 addressing schema of the SP must deal with multiple
attachments of a single customer to the SP network infrastructure
(i.e., multihomed network access with the same SP).
These few requirements are only part of the requirements a service
provider has to investigate and keep in mind during the definition
phase of its addressing architecture. Each SP will most likely add
more constraints to this list.
A.2.1.4. A Few Rules of Thumb for Designing an ISP IPv6 Addressing
As a result of the above enumeration of requirements regarding an ISP
IPv6 addressing plan, the following design "rules of thumb" have been
o No "One size fits all". Each ISP must develop its own IPv6
address allocation schema depending on its concrete business
needs. It is not practical to design one addressing plan that
fits for all kinds of ISPs (small / big, routed / MPLS-based,
access / transit, LIR / No LIR, etc.).
o The levels of IPv6 address aggregation within the ISP addressing
schema should strongly correspond to the implemented network
structure, and their number should be minimized because of
efficiency reasons. It is assumed that the SP's own
infrastructure will be addressed in a fairly flat way, whereas
part of the customer addressing architecture should contain
several levels of aggregation.
o Keep the number of IPv6 customer routes inside your network as
small as possible. A totally flat customer IPv6 addressing
architecture without any intermediate aggregation level will lead
to lots of customer routes inside the SP network. A fair trade-
off between address aggregation levels (and hence the size of the
internal routing table of the SP) and address conservation of the
addressing architecture has to be found.
o The ISP IPv6 addressing schema should provide maximal flexibility.
This has to be realized for supporting different sizes of customer
IPv6 address aggregates ("big" customers vs. "small" customers) as
well as to allow future growth rates (e.g., of customer
aggregates) and possible topological or infrastructural changes.
o A limited number of aggregation levels and sizes of customer
aggregates will ease the management of the addressing schema.
This has to be weighed against the previous "rule of thumb" --
A.2.2. Exemplary IPv6 Address Allocation Plan for a Service Provider
In this example, the service provider is assumed to operate an MPLS-
based backbone and to implement IPv6 Provider Edge Routers (6PE)
[RFC4798] to provide IPv6 backbone transport between the different
locations (POPs) of a fully dual-stacked network access and
In addition, it is assumed that the service provider:
o has received a /20 from its RIR
o operates its own LIR
o has to address its own IPv6 infrastructure
o delegates prefixes from this aggregate to its customers
This addressing schema should illustrate how the /20 IPv6 prefix of
the SP can be used to address the SP's own infrastructure and to
delegate IPv6 prefixes to its customers, following the above-
mentioned requirements and rules of thumb as far as possible.
The figure below summarizes the device types in an SP network and the
typical network design of a MPLS-based service provider. The network
hierarchy of the SP has to be taken into account for the design of an
IPv6 addressing schema; it defines the basic shape of the addressing
schema and the various levels of aggregation.
| LSRs of the MPLS Backbone of the SP |
| | | | |
| | | | |
+-----+ +-----+ +--------+ +--------+ +--------+
| LER | | LER | | LER-BB | | LER-BB | | LER-BB |
+-----+ +-----+ +--------+ +--------+ +--------+
| | | | | | / | | |
| | | | | | / | | |
| | | | +------+ +------+ +------+ | |
| | | | |BB-RAR| |BB-RAR| | AG | | |
| | | | +------+ +------+ +------+ | |
| | | | | | | | | | | |
| | | | | | | | | | | |
| | | | | | | | +-----+ +-----+ +-----+ +-----+
| | | | | | | | | RAR | | RAR | | RAR | | RAR |
| | | | | | | | +-----+ +-----+ +-----+ +-----+
| | | | | | | | | | | | | | | |
| | | | | | | | | | | | | | | |
| Customer networks |
LSR Label Switch Router
LER Label Edge Router
LER-BB Broadband Label Edge Router
RAR Remote Access Router
BB-RAR Broadband Remote Access Router
AG Aggregation Router
Exemplary Service Provider Network
The following should be taken into consideration when making the
basic design decisions for the exemplary service provider IPv6
addressing plan regarding customer prefixes.
o The prefixes assigned to all customers behind the same LER (or
LER-BB) are aggregated under one LER prefix. This ensures that
the number of labels that have to be used for 6PE is limited and
hence provides strong MPLS label conservation.
o The /20 prefix of the SP is separated into 3 different pools that
are used to allocate IPv6 prefixes to the customers of the SP:
1. A pool (e.g., /24) for satisfying the addressing needs of
really "big" customers (as defined in Appendix A.184.108.40.206) that
need IPv6 prefixes larger than /48 (e.g., /32). These
customers are assumed to be connected to several POPs of the
access network, so that this customer prefix will be visible
in each of these POPs.
2. A pool (e.g., /24) for the LERs with direct customer
connections (e.g., dedicated line access) and without an
additional aggregation area between the customer and the LER.
(These LERs are mostly connected to a limited number of
customers because of the limited number of interfaces/ports.)
3. A larger pool (e.g., 14*/24) for LERs (or LER-BBs) that serve
a high number of customers that are normally connected via
some kind of aggregation network (e.g., DSL customers behind a
BB-RAR or dial-in customers behind a RAR).
o The IPv6 address delegation within each pool (the end customer
delegation or the aggregates that are dedicated to the LER itself)
should be chosen with an additional buffer zone of 100-300% for
future growth. That is, 1 or 2 additional prefix bits should be
reserved according to the expected future growth rate of the
corresponding customer or the corresponding network device
A.2.2.1. Defining an IPv6 Address Allocation Plan for Customers of the
A.220.127.116.11. "Big" Customers
The SP's "big" customers receive their prefix from the /24 IPv6
address aggregate that has been reserved for their "big" customers.
A customer is considered a "big" customer if it has a very complex
network infrastructure and/or huge IPv6 address needs (e.g., because
of very large customer numbers) and/or several uplinks to different
POPs of the SP network.
The assigned IPv6 address prefixes can have a prefix length in the
range 32-48 and for each assignment a 100 or 300% future growing zone
is marked as "reserved" for this customer. For instance, this means
that with a delegation of a /34 to a customer the corresponding /32
prefix (which contains this /34) is reserved for the customer's
The prefixes for the "big" customers can be chosen from the
corresponding "big customer" pool by either using an equidistant
algorithm or using mechanisms similar to the Sparse Allocation
Algorithm (SAA) [RIPE_Nov07].
A.18.104.22.168. "Common" Customers
All customers that are not "big" customers are considered as "common"
customers. They represent the majority of customers, hence they
receive a /48 out of the IPv6 customer address pool of the LER where
they are directly connected or aggregated.
Again a 100-300% future growing IPv6 address range is reserved for
each customer, so that a "common" customer receives a /48 allocation
but has a /47 or /46 reserved.
(Note: If it is obvious that the likelihood of needing a /47 or /46
in the future is very small for a "common" customer, then no growing
buffer should be reserved for it, and only a /48 will be assigned
without any growing buffer.)
In the network access scenarios where the customer is directly
connected to the LER, the customer prefix is directly taken out of
the customer IPv6 address aggregate (e.g., /38) of the corresponding
For other cases (e.g., the customer is attached to a RAR that is
itself aggregated to an AG or to a LER-BB), at least 2 different
approaches are possible.
1) Mapping of Aggregation Network Hierarchy into Customer IPv6
Addressing Schema. The aggregation network hierarchy could be
mapped into the design of the customer prefix pools of each
network level in order to achieve a maximal aggregation at the
LER level as well as at the intermediate levels. (Example:
Customer - /48, RAR - /38, AG - /32, LER-BB - /30). At each
network level, an adequate growing zone should be reserved.
(Note: Of course, this approach requires some "fine tuning" of
the addressing schema based on a very good knowledge of the
Service Provider network topology including actual growing ranges
When the IPv6 customer address pool of a LER (or another device
of the aggregation network -- AG or RAR) is exhausted, the
related LER (or AG or RAR) prefix is shortened by 1 or 2 bits
(e.g., from /38 to /37 or /36) so that the originally reserved
growing zone can be used for further IPv6 address allocations to
customers. In the case where this growing zone is exhausted as
well, a new prefix range from the corresponding pool of the next-
higher hierarchy level can be requested.
2) "Flat" Customer IPv6 Addressing Schema. The other option is to
allocate all the customer prefixes directly out of the customer
IPv6 address pool of the LER where the customers are attached and
aggregated and to ignore the intermediate aggregation network
infrastructure. Of course, this approach leads to a higher
amount of customer routes at the LER and aggregation network
level, but it takes a great amount of complexity out of the
addressing schema. Nevertheless, the aggregation of the customer
prefixes to one prefix at the LER level is realized as required
Note: The handling of changes (e.g., technically triggered changes)
within the ISP access network is discussed briefly in
If the actual observed growing rates show that the reserved growing
zones are not needed, then they can be freed and used for assignments
for prefix pools to other devices at the same level of the network
A.2.2.2. Defining an IPv6 Address Allocation Plan for the Service
Provider Network Infrastructure
For the IPv6 addressing of the SP's own network infrastructure, a /32
(or /40) from the "big" customers address pool can be chosen.
This SP infrastructure prefix is used to code the network
infrastructure of the SP by assigning a /48 to every POP/location and
using (for instance) a /56 for coding the corresponding router within
this POP. Each SP internal link behind a router interface could be
coded using a /64 prefix. (Note: While it is suggested to choose a
/48 for addressing the POP/location of the SP network, it is left to
each SP to decide what prefix length to assign to the routers and
links within the POP.)
The IIDs of the router interfaces may be generated by using EUI-64 or
through plain manual configuration, e.g., for coding additional
network or operational information into the IID.
Again, it is assumed that 100-300% growing zones are needed for each
level of network hierarchy, and additional prefix bits may be
assigned to POPs and/or routers if needed.
Loopback interfaces of routers may be chosen from the first /64 of
the /56 router prefix (in the example above).
(Note: The /32 (or /40) prefix that has been chosen for addressing
the SP's own IPv6 network infrastructure leaves enough space to code
additional functionalities like security levels or private and test
infrastructure, although such approaches haven't been considered in
more detail for the above-described SP until now.)
Point-to-point links to customers (e.g., PPP links, dedicated lines,
etc.) may be addressed using /126 prefixes out of the first /64 of
the access routers that could be reserved for this reason.
A.2.3. Additional Remarks
There are no compelling reasons for service providers to use ULAs.
See Section 2.2.
ULAs could be used inside the SP network in order to have an
additional "site-local scoped" IPv6 address for the SP's own
infrastructure, for instance, for network management reasons and in
order to have an addressing schema that can't be reached from outside
the SP network.
When ULAs are used, it is possible to map the proposed internal IPv6
addressing of the SP's own network infrastructure (as described in
Appendix A.2.2.2) directly to the ULA addressing schema by
substituting the /48 POP prefix with a /48 ULA site prefix.
IPv6 multicast-related addressing issues are out of the scope of this
A.2.3.3. POP Multihoming
POP multihoming (or better, LER multihoming) of customers with the
same SP can be realized within the proposed IPv6 addressing schema of
the SP by assigning multiple LER-dependent prefixes to this customer
(i.e., considering each customer location as a single customer) or by
choosing a customer prefix out of the pool of "big" customers. The
second solution has the disadvantage that in every LER where the
customer is attached, this prefix will appear inside the IGP routing
table, thus requiring an explicit MPLS label.
Note: The negative effects (described above) of POP/LER multihoming
on the addressing architecture in the SP access network are not
resolved by implementing the Site Multihoming by IPv6 Intermediation
(SHIM6) approach. SHIM6 only targets a mechanism for dealing with
multiple prefixes in end systems. The SP is expected to have
unaggregated customer prefixes in its internal routing tables.
A.2.3.4. Changing the Point of Network Attachment
In the possible case that a customer has to change its point of
network attachment to another POP/LER within the ISP access network,
two different approaches can be applied, assuming that the customer
uses PA addresses out of the SP aggregate:
1) The customer has to renumber its network with an adequate
customer prefix out of the aggregate of the corresponding LER/RAR
of its new network attachment. To minimize the administrative
burden for the customer, the prefix should be of the same size as
the former. This conserves the IPv6 address aggregation within
the SP network (and the MPLS label space) but adds additional
burden to the customer. Hence, this approach will most likely
only be chosen in the case of "small customers" with temporary
addressing needs and/or prefix delegation with address
2) The customer does not need to renumber its network and keeps its
This approach leads to additional more-specific routing entries
within the IGP routing table of the LER and will hence consume
additional MPLS labels, but it is totally transparent to the
customer. Because this results in additional administrative
effort and will stress the router resources (label space, memory)
of the ISP, this solution will only be offered to the most
valuable customers of an ISP (e.g., "big customers" or
Nevertheless, the ISP again has to find a fair trade-off between
customer renumbering and sub-optimal address aggregation (i.e.,
the generation of additional more-specific routing entries within
the IGP and the waste of MPLS label space).
A.2.3.5. Restructuring of SP (Access) Network and Renumbering
A technically triggered restructuring of the SP (access) network (for
instance, because of split of equipment or installation of new
equipment) should not lead to a customer network renumbering. This
challenge should be handled in advance by an intelligent network
design and IPv6 address planning.
In the worst case, the customer network renumbering could be avoided
through the implementation of more-specific customer routes. (Note:
Since this kind of network restructuring will mostly happen within
the access network (at the level) below the LER, the LER aggregation
level will not be harmed and the more-specific routes will not
consume additional MPLS label space.)
A.2.3.6. Extensions Needed for the Later IPv6 Migration Phases
The proposed IPv6 addressing schema for an SP needs some slight
enhancements / modifications for the later phases of IPv6
integration, for instance, when the whole MPLS backbone
infrastructure (LDP, IGP, etc.) is realized over IPv6 transport, and
an IPv6 addressing of the LSRs is needed. Other changes may be
necessary as well but should not be explained at this point.
Appendix B. Considerations for Subnet Prefixes Different than /64
B.1. Considerations for Subnet Prefixes Shorter than /64
An allocation of a prefix shorter then 64 bits to a node or interface
is considered bad practice. One exception to this statement is when
using 6to4 technology where a /16 prefix is utilized for the pseudo-
interface [RFC3056]. The shortest subnet prefix that could
theoretically be assigned to an interface or node is limited by the
size of the network prefix allocated to the organization.
A possible reason for choosing the subnet prefix for an interface
shorter than /64 is that it would allow more nodes to be attached to
that interface compared to a prescribed length of 64 bits. The
prescribed /64 does include 2 functional bits, the 'g' bit and the
inverted 'u' (universal/local) bit and these can not be chosen at
will. However, a larger address space then a /64 is unnecessary for
most networks, considering that 2^62 provides plenty of node
The subnet prefix assignments can be made by manual configuration, by
a stateful Host Configuration Protocol [RFC3315], by a stateful
prefix delegation mechanism [RFC3633], or implied by stateless
autoconfiguration from prefix Router Advertisements (RAs).
B.2. Considerations for Subnet Prefixes Longer than /64
The following subsections describe subnet prefix values that should
be avoided in deployments because nodes who assume that the subnet
prefix is /64 could treat them incorrectly.
B.2.1. /126 Addresses
126-bit subnet prefixes are typically used for point-to-point links
similar to a the IPv4 address-conservative /30 allocation for point-
to-point links. The usage of this subnet address length does not
lead to any considerations beyond those discussed earlier in this
section, particularly those related to the 'u' and 'g' bits (see
B.2.2. /127 Addresses
The usage of the /127 addresses, the equivalent of IPv4's RFC 3021
[RFC3021], is not valid and should be strongly discouraged as
documented in RFC 3627 [RFC3627].
B.2.3. /128 Addresses
The 128-bit address prefix may be used in those situations where we
know that one, and only one, address is sufficient. Example usage
would be the off-link loopback address of a network device.
When choosing a 128 bit prefix, it is recommended to take the 'u' and
'g' bits into consideration and to make sure that there is no overlap
with any of the following well-known addresses:
o Subnet Router Anycast Address
o Reserved Subnet Anycast Address
o Addresses used by Embedded-RP
o ISATAP Addresses
B.2.4. EUI-64 'u' and 'g' Bits
When using subnet prefix lengths other than /64, the interface
identifier cannot be in Modified EUI-64 format as required by
[RFC4291]. However, nodes not aware that a prefix length other than
/64 is used might still think it's an EUI-64; therefore, it's prudent
to take into account the following points when setting the bits.
Address space conservation is the main motivation for using a subnet
prefix length longer than 64 bits; however, this kind of address
conservation is of little benefit compared with the additional
considerations one must make when creating and maintaining an IPv6
The address assignment can be made either by manual configuration or
by a stateful Host Configuration Protocol [RFC3315].
When assigning a subnet prefix of more then 70 bits, according to RFC
4291 [RFC4291], 'u' and 'g' bits (the 71st and 72nd bit,
respectively) need to be taken into consideration and should be set
The 71st bit of a IPv6 address is the inverted 'u' (universal/local)
bit and is used to determine whether the address is universally or
locally administered. If 1, the IEEE, through the designation of a
unique company ID, has administered the address. If 0, the address
is locally administered. The network administrator has overridden
the manufactured address and specified a different address.
The 'g' (the individual/group) bit is the 72nd bit and is used to
determine whether the address is an individual address (unicast) or a
group address (multicast). If '0', the address is a unicast address.
If '1', the address is a multicast address.
In current IPv6 protocol stacks, the relevance of the 'u' and 'g'
bits is marginal and typically will not give an error when configured
wrongly; however, future implementations may turn out differently if
they process the 'u' and 'g' bits in IEEE-like behavior.
When using subnet lengths longer then 64 bits, it is important to
avoid selecting addresses that may have a predefined use and could
confuse IPv6 protocol stacks. The alternate usage may not be a
simple unicast address in all cases. The following points should be
considered when selecting a subnet length longer then 64 bits.
B.2.5. Anycast Addresses
B.2.5.1. Subnet Router Anycast Address
RFC 4291 [RFC4291] provides a definition for the required Subnet
Router Anycast Address as follows:
| n bits | 128-n bits |
| subnet prefix | 00000000000000 |
It is recommended to avoid allocating this IPv6 address to a device
that expects to have a normal unicast address.
B.2.5.2. Reserved IPv6 Subnet Anycast Addresses
RFC 2526 [RFC2526] stated that within each subnet, the highest 128
interface identifier values are reserved for assignment as subnet
The construction of a reserved subnet anycast address depends on the
type of IPv6 addresses used within the subnet, as indicated by the
format prefix in the addresses.
The first type of Subnet Anycast addresses have been defined as
follows for the Modified EUI-64 format:
| 64 bits | 57 bits | 7 bits |
| subnet prefix | 1111110111...111 | anycast ID |
The anycast address structure implies that it is important to avoid
creating a subnet prefix where the bits 65 to 121 are defined as
"1111110111...111" (57 bits in total) in order to prevent confusion.
For other IPv6 address types (that is, with format prefixes other
than those listed above), the interface identifier is not in 64-bit
extended unique identifier (EUI-64) format and may not be 64 bits in
length. The reserved subnet anycast addresses for such address types
are constructed as follows:
| n bits | 121-n bits | 7 bits |
| subnet prefix | 1111111...111111 | anycast ID |
| interface identifier field |
It is recommended to avoid allocating this IPv6 address to a device
that expects to have a normal unicast address.
B.2.6. Addresses Used by Embedded-RP (RFC 3956)
Embedded-RP [RFC3956] reflects the concept of integrating the
Rendezvous Point (RP) IPv6 address into the IPv6 multicast group
address. Due to this embedding and the fact that the length of the
IPv6 address AND the IPv6 multicast address are 128 bits, it is not
possible to have the complete IPv6 address of the multicast RP
embedded as such.
This results in a restriction of 15 possible RP-addresses per prefix
that can be used with embedded-RP. The space assigned for the
embedded-RP is based on the 4 low-order bits, while the remainder of
the Rendezvous Interface ID (RIID) is set to all '0'. The format of
the IPv6 multicast group address used by embedded-RP is as follows:
(IPv6-prefix (64 bits))(60 bits all '0')(RIID)
where: (RIID) = 4 bits.
This format implies that when selecting subnet prefixes longer than
64, and when the bits beyond the 64th bit are non-zero, the subnet
cannot use embedded-RP.
In addition, it is discouraged to assign a matching embedded-RP IPv6
address to a device that is not a real Multicast Rendezvous Point,
even though it would not generate major problems.
B.2.7. ISATAP Addresses
ISATAP [RFC5214] is an experimental automatic tunneling protocol used
to provide IPv6 connectivity over an IPv4 campus or enterprise
environment. In order to leverage the underlying IPv4
infrastructure, the IPv6 addresses are constructed in a special
An IPv6 ISATAP address has the IPv4 address embedded, based on a
predefined structure policy that identifies them as an ISATAP
address. The format is as follows:
[IPv6 Prefix (64 bits)][0000:5EFE][IPv4 address]
When using a subnet prefix length longer then 64 bits, it is good
engineering practice to ensure that the portion of the IPv6 prefix
from bit 65 to the end of the host-ID does not match with the well-
known ISATAP [0000:5EFE] address when assigning an IPv6 address to a
Note that the definition of ISATAP does not support multicast.