Internet Engineering Task Force (IETF)
Request for Comments: 7421
Category: Informational
ISSN: 2070-1721
B. Carpenter, Ed.
Univ. of Auckland
T. Chown
Univ. of Southampton
F. Gont
SI6 Networks / UTN-FRH
S. Jiang
Huawei Technologies Co., Ltd
A. Petrescu
A. Yourtchenko
January 2015

Analysis of the 64-bit Boundary in IPv6 Addressing


The IPv6 unicast addressing format includes a separation between the prefix used to route packets to a subnet and the interface identifier used to specify a given interface connected to that subnet. Currently, the interface identifier is defined as 64 bits long for almost every case, leaving 64 bits for the subnet prefix. This document describes the advantages of this fixed boundary and analyzes the issues that would be involved in treating it as a variable boundary.

Status of This Memo

This document is not an Internet Standards Track specification; it is published for informational purposes.

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). Not all documents approved by the IESG are a candidate for any level of Internet Standard; see 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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Copyright © 2015 IETF Trust and the persons identified as the document authors. All rights reserved.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Advantages of a Fixed Identifier Length . . . . . . . . . . .   4
   3.  Arguments for Shorter Identifier Lengths  . . . . . . . . . .   5
     3.1.  Insufficient Address Space Delegated  . . . . . . . . . .   5
     3.2.  Hierarchical Addressing . . . . . . . . . . . . . . . . .   6
     3.3.  Audit Requirement . . . . . . . . . . . . . . . . . . . .   7
     3.4.  Concerns over ND Cache Exhaustion . . . . . . . . . . . .   7
   4.  Effects of Varying the Interface Identifier Length  . . . . .   8
     4.1.  Interaction with IPv6 Specifications  . . . . . . . . . .   8
     4.2.  Possible Failure Modes  . . . . . . . . . . . . . . . . .  10
     4.3.  Experimental Observations . . . . . . . . . . . . . . . .  12
       4.3.1.  Survey of the processing of Neighbor Discovery
               Options with Prefixes Other than /64  . . . . . . . .  12
       4.3.2.  Other Observations  . . . . . . . . . . . . . . . . .  14
     4.4.  Implementation and Deployment Issues  . . . . . . . . . .  14
     4.5.  Privacy Issues  . . . . . . . . . . . . . . . . . . . . .  16
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  16
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  17
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .  17
     6.2.  Informative References  . . . . . . . . . . . . . . . . .  21
   Acknowledgements .  . . . . . . . . . . . . . . . . . . . . . . .  23
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  24

1. Introduction

Rather than simply overcoming the IPv4 address shortage by doubling the address size to 64 bits, IPv6 addresses were originally chosen to be 128 bits long to provide flexibility and new possibilities. In particular, the notion of a well-defined interface identifier was added to the IP addressing model. The IPv6 addressing architecture [RFC4291] specifies that a unicast address is divided into n bits of subnet prefix followed by (128-n) bits of interface identifier (IID). The bits in the IID may have significance only in the process of deriving the IID; once it is derived, the entire identifier should be treated as an opaque value [RFC7136]. Also, since IPv6 routing is entirely based on variable length prefixes (also known as variable length subnet masks), there is no basic architectural assumption that n has any particular fixed value. All IPv6 routing protocols support prefixes of any length up to /128.

The IID is of basic importance in the IPv6 stateless address autoconfiguration (SLAAC) process [RFC4862]. However, it is important to understand that its length is a parameter in the SLAAC process, and it is determined in a separate link-type specific document (see the definition of "interface identifier" in Section 2 of RFC 4862). The SLAAC protocol does not define its length or assume any particular length. Similarly, DHCPv6 [RFC3315] does not include a prefix length in its address assignment.

The notion of a /64 boundary in the address was introduced after the initial design of IPv6, following a period when it was expected to be at /80. There were two motivations for setting it at /64. One was the original "8+8" proposal [ODELL] that eventually led to the Identifier-Locator Network Protocol (ILNP) [RFC6741], which required a fixed point for the split between local and wide-area parts of the address. The other was the expectation that 64-bit Extended Unique Identifier (EUI-64) Media Access Control (MAC) addresses would become widespread in place of 48-bit addresses, coupled with the plan at that time that auto-configured addresses would normally be based on interface identifiers derived from MAC addresses.

As a result, RFC 4291 describes a method of forming interface identifiers from IEEE EUI-64 hardware addresses [IEEE802], and this specifies that such interface identifiers are 64 bits long. Various other methods of forming interface identifiers also specify a length of 64 bits. The addressing architecture, as modified by [RFC7136], states that:

For all unicast addresses, except those that start with the binary value 000, Interface IDs are required to be 64 bits long. If derived from an IEEE MAC-layer address, they must be constructed in Modified EUI-64 format.

The de facto length of almost all IPv6 interface identifiers is therefore 64 bits. The only documented exception is in [RFC6164], which standardizes 127-bit prefixes for point-to-point links between routers, among other things, to avoid a loop condition known as the ping-pong problem.

With that exception, and despite the comments above about the routing architecture and the design of SLAAC, using an IID shorter than 64 bits and a subnet prefix longer than 64 bits is outside the current IPv6 specifications, so results may vary.

The question is often asked why the subnet prefix boundary is set rigidly at /64. The first purpose of this document is to explain the advantages of the fixed IID length. Its second purpose is to analyze, in some detail, the effects of hypothetically varying the IID length. The fixed-length limits the practical length of a routing prefix to 64 bits, whereas architecturally, and from the point of view of routing protocols, it could be any value up to /128, as in the case of host routes. Whatever the length of the IID, the longest match is done on the concatenation of prefix and IID. Here, we mainly discuss the question of a shorter IID, which would allow a longer subnet prefix. The document makes no proposal for a change to the IID length.

The following three sections describe, in turn, the advantages of the fixed-length IID, some arguments for shorter lengths, and the expected effects of varying the length.

2. Advantages of a Fixed Identifier Length

As mentioned in Section 1, the existence of an IID of a given length is a necessary part of IPv6 stateless address autoconfiguration (SLAAC) [RFC4862]. This length is normally the same for all nodes on a given link that is running SLAAC. Even though this length is a parameter for SLAAC, determined separately for the link-layer media type of each interface, a globally fixed IID length for all link- layer media is the simplest solution and is consistent with the principles of Internet host configuration described in [RFC5505].

An interface identifier of significant length, clearly separated from the subnet prefix, makes it possible to limit the traceability of a host computer by varying the identifier. This is discussed further in Section 4.5.

An interface identifier of significant length guarantees that there are always enough addresses in any subnet to add one or more real or virtual interfaces. There might be other limits, but IP addressing will never get in the way.

The addressing architecture [RFC4291] [RFC7136] sets the IID length at 64 bits for all unicast addresses and therefore for all media supporting SLAAC. An immediate effect of fixing the IID length at 64 bits is, of course, that it fixes the subnet prefix length also at 64 bits, regardless of the aggregate prefix assigned to the site concerned, which in accordance with [RFC6177] should be /56 or shorter. This situation has various specific advantages:

  • Everything is the same. Compared to IPv4, there is no more calculating leaf subnet sizes, no more juggling between subnets, and fewer consequent errors. Network design is therefore simpler and much more straightforward. This is of importance for all types of networks -- enterprise, campus, small office, or home networks -- and for all types of operator, from professional to consumer.
  • Adding a subnet is easy -- just take another /64 from the pool. No estimates, calculations, consideration, or judgement is needed.
  • Router configurations are homogeneous and easier to understand.
  • Documentation is easier to write and easier to read; training is easier.

The remainder of this document describes arguments that have been made against the current fixed IID length and analyzes the effects of a possible change. However, the consensus of the IETF is that the benefits of keeping the length fixed at 64 bits and the practical difficulties of changing it outweigh the arguments for change.

3. Arguments for Shorter Identifier Lengths

In this section, we describe arguments for scenarios where shorter IIDs, implying prefixes longer than /64, have been used or proposed.

3.1. Insufficient Address Space Delegated

A site may not be delegated a sufficiently generous prefix from which to allocate a /64 prefix to all of its internal subnets. In this case, the site may either determine that it does not have enough address space to number all its network elements and thus, at the very best, be only partially operational, or it may choose to use internal prefixes longer than /64 to allow multiple subnets and the hosts within them to be configured with addresses.

In this case, the site might choose, for example, to use a /80 per subnet in combination with hosts using either manually configured addressing or DHCPv6 [RFC3315].

Scenarios that have been suggested where an insufficient prefix might be delegated include home or small office networks, vehicles, building services, and transportation services (e.g., road signs). It should be noted that the homenet architecture text [RFC7368] states that Customer Premises Equipment (CPE) should consider the lack of sufficient address space to be an error condition, rather than using prefixes longer than /64 internally.

Another scenario occasionally suggested is one where the Internet address registries actually begin to run out of IPv6 prefix space, such that operators can no longer assign reasonable prefixes to users in accordance with [RFC6177]. It is sometimes suggested that assigning a prefix such as /48 or /56 to every user site (including the smallest) as recommended by [RFC6177] is wasteful. In fact, the currently released unicast address space, 2000::/3, contains 35 trillion /48 prefixes ((2**45 = 35,184,372,088,832), of which only a small fraction have been allocated. Allowing for a conservative estimate of allocation efficiency, i.e., an HD-ratio of 0.94 [RFC4692], approximately 5 trillion /48 prefixes can be allocated. Even with a relaxed HD-ratio of 0.89, approximately one trillion /48 prefixes can be allocated. Furthermore, with only 2000::/3 currently committed for unicast addressing, we still have approximately 85% of the address space in reserve. Thus, there is no objective risk of prefix depletion by assigning /48 or /56 prefixes even to the smallest sites.

3.2. Hierarchical Addressing

Some operators have argued that more prefix bits are needed to allow an aggregated hierarchical addressing scheme within a campus or corporate network. However, if a campus or enterprise gets a /48 prefix (or shorter), then that already provides 16 bits for hierarchical allocation. In any case, flat IGP routing is widely and successfully used within rather large networks, with hundreds of routers and thousands of end systems. Therefore, there is no objective need for additional prefix bits to support hierarchy and aggregation within enterprises.

3.3. Audit Requirement

Some network operators wish to know and audit nodes that are active on a network, especially those that are allowed to communicate off- link or off-site. They may also wish to limit the total number of active addresses and sessions that can be sourced from a particular host, LAN, or site, in order to prevent potential resource-depletion attacks or other problems spreading beyond a certain scope of control. It has been argued that this type of control would be easier if only long network prefixes with relatively small numbers of possible hosts per network were used, reducing the discovery problem. However, such sites most typically operate using DHCPv6, which means that all legitimate hosts are automatically known to the DHCPv6 servers, which is sufficient for audit purposes. Such hosts could, if desired, be limited to a small range of IID values without changing the /64 subnet length. Any hosts inadvertently obtaining addresses via SLAAC can be audited through Neighbor Discovery (ND) logs.

3.4. Concerns over ND Cache Exhaustion

A site may be concerned that it is open to ND cache exhaustion attacks [RFC3756], whereby an attacker sends a large number of messages in rapid succession to a series of (most likely inactive) host addresses within a specific subnet. Such an attack attempts to fill a router's ND cache with ND requests pending completion, which results in denying correct operation to active devices on the network.

One potential way to mitigate this attack would be to consider using a /120 prefix, thus limiting the number of addresses in the subnet to be similar to an IPv4 /24 prefix, which should not cause any concerns for ND cache exhaustion. Note that the prefix does need to be quite long for this scenario to be valid. The number of theoretically possible ND cache slots on the segment needs to be of the same order of magnitude as the actual number of hosts. Thus, small increases from the /64 prefix length do not have a noticeable impact; even 2^32 potential entries, a factor of two billion decrease compared to 2^64, is still more than enough to exhaust the memory on current routers. Given that most link-layer mappings cause SLAAC to assume a 64-bit network boundary, in such an approach hosts would likely need to use DHCPv6 or be manually configured with addresses.

It should be noted that several other mitigations of the ND cache attack are described in [RFC6583], and that limiting the size of the cache and the number of incomplete entries allowed would also defeat the attack. For the specific case of a point-to-point link between routers, this attack is indeed mitigated by a /127 prefix [RFC6164].

4. Effects of Varying the Interface Identifier Length

This section of the document analyzes the impact and effects of varying the length of an IPv6 unicast IID by reducing it to less than 64 bits.

4.1. Interaction with IPv6 Specifications

The precise 64-bit length of the IID is widely mentioned in numerous RFCs describing various aspects of IPv6. It is not straightforward to distinguish cases where this has normative impact or affects interoperability. This section aims to identify specifications that contain an explicit reference to the 64-bit length. Regardless of implementation issues, the RFCs themselves would all need to be updated if the 64-bit rule was changed, even if the updates were small, which would involve considerable time and effort.

First and foremost, the RFCs describing the architectural aspects of IPv6 addressing explicitly state, refer, and repeat this apparently immutable value: Addressing Architecture [RFC4291], IPv6 Address Assignment to End Sites [RFC6177], Reserved IIDs [RFC5453], and ILNP Node Identifiers [RFC6741]. Customer edge routers impose /64 for their interfaces [RFC7084]. The IPv6 Subnet Model [RFC5942] points out that the assumption of a /64 prefix length is a potential implementation error.

   Numerous IPv6-over-foo documents make mandatory statements with
   respect to the 64-bit length of the IID to be used during the
   Stateless Autoconfiguration.  These documents include [RFC2464]
   (Ethernet), [RFC2467] (Fiber Distributed Data Interface (FDDI)),
   [RFC2470] (Token Ring), [RFC2492] (ATM), [RFC2497] (ARCnet),
   [RFC2590] (Frame Relay), [RFC3146] (IEEE 1394), [RFC4338] (Fibre
   Channel), [RFC4944] (IEEE 802.15.4), [RFC5072] (PPP), [RFC5121]
   [RFC5692] (IEEE 802.16), [RFC2529] (6over4), [RFC5214] (Intra-Site
   Automatic Tunnel Addressing Protocol (ISATAP)), [AERO-TRANS]
   (Asymmetric Extended Route Optimization (AERO)), [BLUETOOTH-LE]
   (BLUETOOTH Low Energy), [IPv6-TRANS] (IPv6 over MS/TP), and
   [IPv6-G9959] (IPv6 packets over ITU-T G.9959).

To a lesser extent, the address configuration RFCs themselves may in some ways assume the 64-bit length of an IID (e.g, RFC 4862 for the link-local addresses, DHCPv6 for the potentially assigned EUI- 64-based IP addresses, and Optimistic Duplicate Address Detection [RFC4429] that computes 64-bit-based collision probabilities).

The Multicast Listener Discovery Version 1 (MLDv1) [RFC2710] and MLDv2 [RFC3810] protocols mandate that all queries be sent with a link-local source address, with the exception of MLD messages sent using the unspecified address when the link-local address is tentative [RFC3590]. At the time of publication of RFC 2710, the IPv6 addressing architecture specified link-local addresses with 64-bit interface identifiers. MLDv2 explicitly specifies the use of the fe80::/64 link-local prefix and bases the querier election algorithm on the link-local subnet prefix of length /64.

The "IPv6 Flow Label Specification" [RFC6437] gives an example of a 20-bit hash function generation, which relies on splitting an IPv6 address in two equally sized, 64-bit-length parts.

The basic transition mechanisms [RFC4213] refer to IIDs of length 64 for link-local addresses; other transition mechanisms such as Teredo [RFC4380] assume the use of IIDs of length 64. Similar assumptions are found in 6to4 [RFC3056] and 6rd [RFC5969]. Translation-based transition mechanisms such as NAT64 and NPTv6 have some dependency on prefix length, discussed below.

The proposed method [RFC7278] of extending an assigned /64 prefix from a smartphone's cellular interface to its WiFi link relies on prefix length, and implicitly on the length of the IID, to be valued at 64.

The Cryptographically Generated Addresses (CGA) and Hash-Based Addresses (HBA) specifications rely on the 64-bit identifier length (see below), as do the Privacy extensions [RFC4941] and some examples in "Internet Key Exchange Version 2 (IKEv2)" [RFC7296].

464XLAT [RFC6877] explicitly mentions acquiring /64 prefixes. However, it also discusses the possibility of using the interface address on the device as the end point for the traffic, thus potentially removing this dependency.

[RFC2526] reserves a number of subnet anycast addresses by reserving some anycast IIDs. An anycast IID so reserved cannot be less than 7 bits long. This means that a subnet prefix length longer than /121 is not possible, and a subnet of exactly /121 would be useless since all its identifiers are reserved. It also means that half of a /120 is reserved for anycast. This could of course be fixed in the way described for /127 in [RFC6164], i.e., avoiding the use of anycast within a /120 subnet. Note that support for "on-link anycast" is a standard IPv6 neighbor discovery capability [RFC4861] [RFC7094]; therefore, applications and their developers would expect it to be available.

The Mobile IP home network models [RFC4887] rely heavily on the /64 subnet length and assume a 64-bit IID.

While preparing this document, it was noted that many other IPv6 specifications refer to mandatory alignment on 64-bit boundaries, 64-bit data structures, 64-bit counters in MIBs, 64-bit sequence numbers and cookies in security, etc. Finally, the number "64" may be considered "magic" in some RFCs, e.g., 64k limits in DNS and Base64 encodings in MIME. None of this has any influence on the length of the IID but might confuse a careless reader.

4.2. Possible Failure Modes

This section discusses several specific aspects of IPv6 where we can expect operational failures with subnet prefixes other than /64.

  • Router implementations: Router implementors might interpret IETF specifications such as [RFC6164] and [RFC7136] as indicating that prefixes between /65 and /126 (inclusive) for unicast packets on- the-wire are invalid and that operational practices that utilize prefix lengths in this range may fail on some devices, as discussed in Section 4.3.2.
  • Multicast: [RFC3306] defines a method for generating IPv6 multicast group addresses based on unicast prefixes. This method assumes a longest prefix of 64 bits. If a longer prefix is used, there is no way to generate a specific multicast group address using this method. In such cases, the administrator would need to use an "artificial" prefix from within their allocation (a /64 or shorter) from which to generate the group address. This prefix would not correspond to a real subnet.

Similarly, [RFC3956], which specifies the Embedded Rendezvous Point (RP)) allowing IPv6 multicast rendezvous point addresses to be embedded in the multicast group address, would also fail, as the scheme assumes a maximum prefix length of 64 bits.

  • CGA: The Cryptographically Generated Address format [RFC3972] is heavily based on a /64 interface identifier. [RFC3972] has defined a detailed algorithm showing how to generate a 64-bit interface identifier from a public key and a 64-bit subnet prefix. Changing the /64 boundary would certainly invalidate the current CGA definition. However, the CGA might benefit in a redefined version if more bits are used for interface identifiers (which means shorter prefix length). For now, 59 bits are used for cryptographic purposes. The more bits are available, the stronger CGA could be. Conversely, longer prefixes would weaken CGA.
   o  NAT64: Both stateless NAT64 [RFC6052] and stateful NAT64 [RFC6146]
      are flexible for the prefix length.  [RFC6052] has defined
      multiple address formats for NAT64.  In Section 2 of

"IPv4-Embedded IPv6 Address Prefix and Format" [RFC6052], the network-specific prefix could be one of /32, /40, /48, /56, /64, and /96. The remaining part of the IPv6 address is constructed by a 32-bit IPv4 address, an 8-bit u byte and a variable length suffix (there is no u byte and suffix in the case of the 96-bit Well-Known Prefix). NAT64 is therefore OK with a subnet boundary out to /96 but not longer.

   o  NPTv6: IPv6-to-IPv6 Network Prefix Translation [RFC6296] is also
      bound to /64 boundary.  NPTv6 maps a /64 prefix to another /64
      prefix.  When the NPTv6 Translator is configured with a /48 or
      shorter prefix, the 64-bit interface identifier is kept unmodified
      during translation.  However, the /64 boundary might be changed as
      long as the "inside" and "outside" prefixes have the same length.
   o  ILNP: Identifier-Locator Network Protocol (ILNP) [RFC6741] is
      designed around the /64 boundary, since it relies on locally
      unique 64-bit node identifiers (in the interface identifier
      field).  While a redesign to use longer prefixes is not
      inconceivable, this would need major changes to the existing
      specification for the IPv6 version of ILNP.
  • Shim6: The Multihoming Shim Protocol for IPv6 (Shim6) [RFC5533] in its insecure form treats IPv6 addresses as opaque 128-bit objects. However, to secure the protocol against spoofing, it is essential to either use CGAs (see above) or HBAs [RFC5535]. Like CGAs, HBAs are generated using a procedure that assumes a 64-bit identifier. Therefore, in effect, secure shim6 is affected by the /64 boundary exactly like CGAs.
  • Duplicate address risk: If SLAAC was modified to work with shorter IIDs, the statistical risk of hosts choosing the same pseudo- random identifier [RFC7217] would increase correspondingly. The practical impact of this would range from slight to dramatic, depending on how much the IID length was reduced. In particular, a /120 prefix would imply an 8-bit IID and address collisions would be highly probable.
  • The link-local prefix: While RFC 4862 is careful not to define any specific length of link-local prefix within fe80::/10, the addressing architecture [RFC4291] does define the link-local IID length to be 64 bits. If different hosts on a link used IIDs of different lengths to form a link-local address, there is potential for confusion and unpredictable results. Typically today the choice of 64 bits for the link-local IID length is hard-coded per interface, in accordance with the relevant IPv6-over-foo specification, and systems behave as if the link-local prefix was actually fe80::/64. There might be no way to change this except conceivably by manual configuration, which will be impossible if the host concerned has no local user interface.

It goes without saying that if prefixes longer than /64 are to be used, all hosts must be capable of generating IIDs shorter than 64 bits, in order to follow the auto-configuration procedure correctly [RFC4862].

4.3. Experimental Observations

4.3.1. Survey of the processing of Neighbor Discovery Options with

Prefixes Other than /64

This section provides a survey of the processing of Neighbor Discovery options that include prefixes that are different than /64.

The behavior of nodes was assessed with respect to the following options:

   o  PIO-A: Prefix Information Option (PIO) [RFC4861] with the A bit
   o  PIO-L: Prefix Information Option (PIO) [RFC4861] with the L bit
   o  PIO-AL: Prefix Information Option (PIO) [RFC4861] with both the A
      and L bits set.
  • RIO: Route Information Option (RIO) [RFC4191].

In the tables below, the following notation is used:


This option is not supported (i.e., it is ignored no matter the prefix length used).


The corresponding prefix is considered "on-link".


The corresponding route is added to the IPv6 routing table.


The default configuration is NOT-SUP, but there is an option to enable ROUTE.


The option is ignored as an error.

        |  Operating System  | PIO-A  | PIO-L | PIO-AL |   RIO   |
        |    FreeBSD 9.0     | IGNORE | LOCAL | LOCAL  | NOT-SUP |
        |   Linux 3.0.0-15   | IGNORE | LOCAL | LOCAL  | NOT-DEF |
        |   Linux-current    | IGNORE | LOCAL | LOCAL  | NOT-DEF |
        |     NetBSD 5.1     | IGNORE | LOCAL | LOCAL  | NOT-SUP |
        |  OpenBSD-current   | IGNORE | LOCAL | LOCAL  | NOT-SUP |
        |     Win XP SP2     | IGNORE | LOCAL | LOCAL  |  ROUTE  |
        | Win 7 Home Premium | IGNORE | LOCAL | LOCAL  |  ROUTE  |

Table 1: Processing of ND options with prefixes longer than /64

        |  Operating System  | PIO-A  | PIO-L | PIO-AL |   RIO   |
        |    FreeBSD 9.0     | IGNORE | LOCAL | LOCAL  | NOT-SUP |
        |   Linux 3.0.0-15   | IGNORE | LOCAL | LOCAL  | NOT-DEF |
        |   Linux-current    | IGNORE | LOCAL | LOCAL  | NOT-DEF |
        |     NetBSD 5.1     | IGNORE | LOCAL | LOCAL  | NOT-SUP |
        |  OpenBSD-current   | IGNORE | LOCAL | LOCAL  | NOT-SUP |
        |     Win XP SP2     | IGNORE | LOCAL | LOCAL  |  ROUTE  |
        | Win 7 Home Premium | IGNORE | LOCAL | LOCAL  |  ROUTE  |

Table 2: Processing of ND options with prefixes shorter than /64

The results obtained can be summarized as follows:

  • The "A" bit in the Prefix Information Options is honored only if the prefix length is 64. This is consistent with [RFC4862], at least for the case where the IID length is defined to be 64 bits in the corresponding link-type-specific document, which is the case for all currently published such documents. [RFC4862] defines the case where the sum of the advertised prefix length and the IID length does not equal 128 as an error condition.
  • The "L" bit in the Prefix Information Options is honored for any arbitrary prefix length (whether shorter or longer than /64).
  • Nodes that support the Route Information Option allow such routes to be specified with prefixes of any arbitrary length (whether shorter or longer than /64)

4.3.2. Other Observations

Participants in the V6OPS working group have indicated that some forwarding devices have been shown to work correctly with long prefixes such as /80 or /96. Indeed, it is to be expected that forwarding based on the longest prefix match will work for any prefix length, and no reports of this completely failing have been noted. Also, DHCPv6 is in widespread use without any dependency on the /64 boundary. Reportedly, there are deployments of /120 subnets configured using DHCPv6.

There have been definite reports that some routers have a performance drop-off or even resource exhaustion for prefixes longer than /64 due to design issues. In particular, some routing chip designs allocate much less space for longer prefixes than for prefixes up to /64 for the sake of savings in memory, power, and lookup latency. Some devices need special-case code to handle point-to-point links according to [RFC6164].

It has been reported that at least one type of switch has a content- addressable memory limited to 144 bits, which is indeed a typical value for commodity components [TCAM]. This means that packet filters or access control lists cannot be defined based on 128-bit addresses and two 16-bit port numbers; the longest prefix that could be used in such a filter is a /112.

4.4. Implementation and Deployment Issues

From an early stage, implementations and deployments of IPv6 assumed the /64 subnet length, even though routing was based on prefixes of any length. As shown above, this became anchored in many specifications (Section 4.1) and in important aspects of implementations commonly used in local area networks (Section 4.3). In fact, a programmer might be lulled into assuming a comfortable rule of thumb that subnet prefixes are always /64 and an IID is always of length 64. Apart from the limited evidence in Section 4.3.1, we cannot tell without code inspections or tests whether existing stacks are able to handle a flexible IID length or whether they would require modification to do so. A conforming implementation of an IPv6-over-foo that specifies a 64 bit IID for foo links will of course only support 64. But in a well designed stack, the IP layer itself will treat that 64 as a parameter, so changing the IID length in the IPv6-over-foo code should be all that is necessary.

The main practical consequence of the existing specifications is that deployments in which longer subnet prefixes are used cannot make use of SLAAC-configured addresses and require either manually configured addresses or DHCPv6. To reverse this argument, if it was considered desirable to allow auto-configured addresses with subnet prefixes longer than /64, all of the specifications identified above as depending on /64 would have to be modified with due regard to interoperability with unmodified stacks. In fact, [RFC7217] allows for this possibility. Then, modified stacks would have to be developed and deployed. It might be the case that some stacks contain dependencies on the /64 boundary that are not directly implied by the specifications, and any such hidden dependencies would also need to be found and removed.

At least one DHCPv6 client unconditionally installs a /64 prefix as on-link when it configures an interface with an address, although some specific operating system vendors seem to change this default behavior by tweaking a client-side script. This is in clear violation of the IPv6 subnet model [RFC5942]. The motivation for this choice is that if there is no router on the link, the hosts would fail to communicate with each other using the configured addresses because the "on-link assumption" was removed in [RFC4861]. This is not really about the magic number of 64, but an implementation may sometimes pick an arbitrary value of prefix length due to the removal of the on-link assumption, and the value chosen will most likely be 64.

Typical IP Address Management (IPAM) tools treat /64 as the default subnet length but allow users to specify longer subnet prefixes if desired. Clearly, all IPAM tools and network management systems would need to be checked in detail.

Finally, IPv6 is already deployed at many sites, with a large number of staff trained on the basis of the existing standards, supported by documentation and tools based on those standards. Numerous existing middlebox devices are also based on those standards. These people, documents, tools, and devices represent a very large investment that would be seriously impacted by a change in the /64 boundary.

4.5. Privacy Issues

The length of the interface identifier has implications for privacy [ADDRESS-PRIVACY]. In any case in which the value of the identifier is intended to be hard to guess, whether or not it is cryptographically generated, it is apparent that more bits are better. For example, if there are only 20 bits to be guessed, then at most just over a million guesses are needed, which is well within the capacity of a low-cost attack mechanism. It is hard to state in general how many bits are enough to protect privacy, since this depends on the resources available to the attacker, but it seems clear that a privacy solution needs to resist an attack requiring billions rather than millions of guesses. Trillions would be better, suggesting that at least 40 bits should be available. Thus, we can argue that subnet prefixes longer than say /80 might raise privacy concerns by making the IID guessable.

A prefix long enough to limit the number of addresses comparably to an IPv4 subnet, such as /120, would create exactly the same situation for privacy as IPv4 except for the absence of NAT. In particular, a host would be forced to pick a new IID when roaming to a new network to avoid collisions. As mentioned earlier, it is likely that SLAAC will not be used on such a subnet.

5. Security Considerations

In addition to the privacy issues mentioned in Section 4.5 and the issues mentioned with CGAs and HBAs in Section 4.2, the length of the subnet prefix affects the matter of defense against scanning attacks [HOST-SCANNING]. Assuming the attacker has discovered or guessed the prefix length, a longer prefix reduces the space that the attacker needs to scan, e.g., to only 256 addresses if the prefix is /120. On the other hand, if the attacker has not discovered the prefix length and assumes it to be /64, routers can trivially discard attack packets that do not fall within an actual subnet.

However, assume that an attacker finds one valid address "A" and assumes that it is within a long prefix such as a /120. The attacker then starts a scanning attack by scanning "outwards" from A, by trying A+1, A-1, A+2, A-2, etc. This attacker will easily find all hosts in any subnet with a long prefix, because they will have addresses close to A. We therefore conclude that any prefix containing densely packed valid addresses is vulnerable to a scanning attack, without the attacker needing to guess the prefix length. Therefore, to preserve IPv6's advantage over IPv4 in resisting scanning attacks, it is important that subnet prefixes are short enough to allow sparse allocation of identifiers within each subnet. The considerations are similar to those for privacy, and we can again argue that prefixes longer than say /80 might significantly increase vulnerability. Ironically, this argument is exactly converse to the argument for longer prefixes to resist an ND cache attack, as described in Section 3.4.

Denial-of-service attacks related to Neighbor Discovery are discussed in Section 3.4 and in [RFC6583]. One of the mitigations suggested by that document is "sizing subnets to reflect the number of addresses actually in use", but the fact that this greatly simplifies scanning attacks is not noted. For further discussion of scanning attacks, see [HOST-SCANNING].

Note that, although not known at the time of writing, there might be other resource exhaustion attacks available, similar in nature to the ND cache attack. We cannot exclude that such attacks might be exacerbated by sparsely populated subnets such as a /64. It should also be noted that this analysis assumes a conventional deployment model with a significant number of end-systems located in a single LAN broadcast domain. Other deployment models might lead to different conclusions.

6. References

6.1. Normative References

   [RFC2464]  Crawford, M., "Transmission of IPv6 Packets over Ethernet
              Networks", RFC 2464, December 1998,
   [RFC2467]  Crawford, M., "Transmission of IPv6 Packets over FDDI
              Networks", RFC 2467, December 1998,
   [RFC2470]  Crawford, M., Narten, T., and S. Thomas, "Transmission of
              IPv6 Packets over Token Ring Networks", RFC 2470, December
              1998, <>.
   [RFC2492]  Armitage, G., Schulter, P., and M. Jork, "IPv6 over ATM
              Networks", RFC 2492, January 1999,
   [RFC2497]  Souvatzis, I., "Transmission of IPv6 Packets over ARCnet
              Networks", RFC 2497, January 1999,
   [RFC2526]  Johnson, D. and S. Deering, "Reserved IPv6 Subnet Anycast
              Addresses", RFC 2526, March 1999,
   [RFC2529]  Carpenter, B. and C. Jung, "Transmission of IPv6 over IPv4
              Domains without Explicit Tunnels", RFC 2529, March 1999,
   [RFC2590]  Conta, A., Malis, A., and M. Mueller, "Transmission of
              IPv6 Packets over Frame Relay Networks Specification", RFC
              2590, May 1999, <>.
   [RFC2710]  Deering, S., Fenner, W., and B. Haberman, "Multicast
              Listener Discovery (MLD) for IPv6", RFC 2710, October
              1999, <>.
   [RFC3056]  Carpenter, B. and K. Moore, "Connection of IPv6 Domains
              via IPv4 Clouds", RFC 3056, February 2001,
   [RFC3146]  Fujisawa, K. and A. Onoe, "Transmission of IPv6 Packets
              over IEEE 1394 Networks", RFC 3146, October 2001,
   [RFC3306]  Haberman, B. and D. Thaler, "Unicast-Prefix-based IPv6
              Multicast Addresses", RFC 3306, August 2002,
   [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,
   [RFC3590]  Haberman, B., "Source Address Selection for the Multicast
              Listener Discovery (MLD) Protocol", RFC 3590, September
              2003, <>.
   [RFC3810]  Vida, R. and L. Costa, "Multicast Listener Discovery
              Version 2 (MLDv2) for IPv6", RFC 3810, June 2004,
   [RFC3956]  Savola, P. and B. Haberman, "Embedding the Rendezvous
              Point (RP) Address in an IPv6 Multicast Address", RFC
              3956, November 2004,
   [RFC3972]  Aura, T., "Cryptographically Generated Addresses (CGA)",
              RFC 3972, March 2005,
   [RFC4191]  Draves, R. and D. Thaler, "Default Router Preferences and
              More-Specific Routes", RFC 4191, November 2005,
   [RFC4213]  Nordmark, E. and R. Gilligan, "Basic Transition Mechanisms
              for IPv6 Hosts and Routers", RFC 4213, October 2005,
   [RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", RFC 4291, February 2006,
   [RFC4338]  DeSanti, C., Carlson, C., and R. Nixon, "Transmission of
              IPv6, IPv4, and Address Resolution Protocol (ARP) Packets
              over Fibre Channel", RFC 4338, January 2006,
   [RFC4380]  Huitema, C., "Teredo: Tunneling IPv6 over UDP through
              Network Address Translations (NATs)", RFC 4380, February
              2006, <>.
   [RFC4429]  Moore, N., "Optimistic Duplicate Address Detection (DAD)
              for IPv6", RFC 4429, April 2006,
   [RFC4861]  Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
              "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
              September 2007, <>.
   [RFC4862]  Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
              Address Autoconfiguration", RFC 4862, September 2007,
   [RFC4941]  Narten, T., Draves, R., and S. Krishnan, "Privacy
              Extensions for Stateless Address Autoconfiguration in
              IPv6", RFC 4941, September 2007,
   [RFC4944]  Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler,
              "Transmission of IPv6 Packets over IEEE 802.15.4
              Networks", RFC 4944, September 2007,
   [RFC5072]  Varada, S., Haskins, D., and E. Allen, "IP Version 6 over
              PPP", RFC 5072, September 2007,
   [RFC5121]  Patil, B., Xia, F., Sarikaya, B., Choi, JH., and S.
              Madanapalli, "Transmission of IPv6 via the IPv6
              Convergence Sublayer over IEEE 802.16 Networks", RFC 5121,
              February 2008, <>.
   [RFC5214]  Templin, F., Gleeson, T., and D. Thaler, "Intra-Site
              Automatic Tunnel Addressing Protocol (ISATAP)", RFC 5214,
              March 2008, <>.
   [RFC5453]  Krishnan, S., "Reserved IPv6 Interface Identifiers", RFC
              5453, February 2009,
   [RFC5533]  Nordmark, E. and M. Bagnulo, "Shim6: Level 3 Multihoming
              Shim Protocol for IPv6", RFC 5533, June 2009,
   [RFC5535]  Bagnulo, M., "Hash-Based Addresses (HBA)", RFC 5535, June
              2009, <>.
   [RFC5692]  Jeon, H., Jeong, S., and M. Riegel, "Transmission of IP
              over Ethernet over IEEE 802.16 Networks", RFC 5692,
              October 2009, <>.
   [RFC5942]  Singh, H., Beebee, W., and E. Nordmark, "IPv6 Subnet
              Model: The Relationship between Links and Subnet
              Prefixes", RFC 5942, July 2010,
   [RFC5969]  Townsley, W. and O. Troan, "IPv6 Rapid Deployment on IPv4
              Infrastructures (6rd) -- Protocol Specification", RFC
              5969, August 2010,
   [RFC6052]  Bao, C., Huitema, C., Bagnulo, M., Boucadair, M., and X.
              Li, "IPv6 Addressing of IPv4/IPv6 Translators", RFC 6052,
              October 2010, <>.
   [RFC6146]  Bagnulo, M., Matthews, P., and I. van Beijnum, "Stateful
              NAT64: Network Address and Protocol Translation from IPv6
              Clients to IPv4 Servers", RFC 6146, April 2011,
   [RFC6164]  Kohno, M., Nitzan, B., Bush, R., Matsuzaki, Y., Colitti,
              L., and T. Narten, "Using 127-Bit IPv6 Prefixes on Inter-
              Router Links", RFC 6164, April 2011,
   [RFC6177]  Narten, T., Huston, G., and L. Roberts, "IPv6 Address
              Assignment to End Sites", BCP 157, RFC 6177, March 2011,
   [RFC6296]  Wasserman, M. and F. Baker, "IPv6-to-IPv6 Network Prefix
              Translation", RFC 6296, June 2011,
   [RFC6437]  Amante, S., Carpenter, B., Jiang, S., and J. Rajahalme,
              "IPv6 Flow Label Specification", RFC 6437, November 2011,
   [RFC7084]  Singh, H., Beebee, W., Donley, C., and B. Stark, "Basic
              Requirements for IPv6 Customer Edge Routers", RFC 7084,
              November 2013, <>.
   [RFC7136]  Carpenter, B. and S. Jiang, "Significance of IPv6
              Interface Identifiers", RFC 7136, February 2014,
   [RFC7296]  Kaufman, C., Hoffman, P., Nir, Y., Eronen, P., and T.
              Kivinen, "Internet Key Exchange Protocol Version 2
              (IKEv2)", STD 79, RFC 7296, October 2014,

6.2. Informative References


              Cooper, A., Gont, F., and D. Thaler, "Privacy
              Considerations for IPv6 Address Generation Mechanisms",
              Work in Progress, draft-ietf-6man-ipv6-address-generation-
              privacy-02, October 2014.


              Templin, F., "Transmission of IP Packets over AERO Links",
              Work in Progress, draft-templin-aerolink-46, October 2014.


              Nieminen, J., Savolainen, T., Isomaki, M., Patil, B.,
              Shelby, Z., and C. Gomez, "Transmission of IPv6 Packets
              over BLUETOOTH Low Energy", Work in Progress, draft-ietf-
              6lowpan-btle-12, February 2013.


              Gont, F. and T. Chown, "Network Reconnaissance in IPv6
              Networks", Work in Progress, draft-ietf-opsec-ipv6-host-
              scanning-04, June 2014.
   [IEEE802]  IEEE, "IEEE Standard for Local and Metropolitan Area
              Networks: Overview and Architecture", IEEE Std 802-2001
              (R2007), 2007.


              Brandt, A. and J. Buron, "Transmission of IPv6 packets
              over ITU-T G.9959 Networks", Work in Progress, draft-ietf-
              6lo-lowpanz-08, October 2014.


              Lynn, K., Ed., Martocci, J., Neilson, C., and S.
              Donaldson, "Transmission of IPv6 over MS/TP Networks",
              Work in Progress, draft-ietf-6lo-6lobac-00, July 2014.
   [ODELL]    O'Dell, M., "8+8 - An Alternate Addressing Architecture
              for IPv6", Work in Progress, draft-odell-8+8-00, October
   [RFC2629]  Rose, M., "Writing I-Ds and RFCs using XML", RFC 2629,
              June 1999, <>.
   [RFC3756]  Nikander, P., Kempf, J., and E. Nordmark, "IPv6 Neighbor
              Discovery (ND) Trust Models and Threats", RFC 3756, May
              2004, <>.
   [RFC4692]  Huston, G., "Considerations on the IPv6 Host Density
              Metric", RFC 4692, October 2006,
   [RFC4887]  Thubert, P., Wakikawa, R., and V. Devarapalli, "Network
              Mobility Home Network Models", RFC 4887, July 2007,
   [RFC5505]  Aboba, B., Thaler, D., Andersson, L., and S. Cheshire,
              "Principles of Internet Host Configuration", RFC 5505, May
              2009, <>.
   [RFC6583]  Gashinsky, I., Jaeggli, J., and W. Kumari, "Operational
              Neighbor Discovery Problems", RFC 6583, March 2012,
   [RFC6741]  Atkinson,, RJ., "Identifier-Locator Network Protocol
              (ILNP) Engineering Considerations", RFC 6741, November
              2012, <>.
   [RFC6877]  Mawatari, M., Kawashima, M., and C. Byrne, "464XLAT:
              Combination of Stateful and Stateless Translation", RFC
              6877, April 2013,
   [RFC7094]  McPherson, D., Oran, D., Thaler, D., and E. Osterweil,
              "Architectural Considerations of IP Anycast", RFC 7094,
              January 2014, <>.
   [RFC7217]  Gont, F., "A Method for Generating Semantically Opaque
              Interface Identifiers with IPv6 Stateless Address
              Autoconfiguration (SLAAC)", RFC 7217, April 2014,
   [RFC7278]  Byrne, C., Drown, D., and A. Vizdal, "Extending an IPv6
              /64 Prefix from a Third Generation Partnership Project
              (3GPP) Mobile Interface to a LAN Link", RFC 7278, June
              2014, <>.
   [RFC7368]  Chown, T., Ed., Arkko, J., Brandt, A., Troan, O., and J.
              Weil, "IPv6 Home Networking Architecture Principles", RFC,
              7368, October 2014.
   [TCAM]     Meiners, C., Liu, A., and E. Torng, "Algorithmic
              Approaches to Redesigning TCAM-Based Systems", ACM
              SIGMETRICS'08 467-468, 2008.


This document was inspired by a vigorous discussion on the V6OPS working group mailing list with at least 20 participants. Later, valuable comments were received from Ran Atkinson, Fred Baker, Steven Blake, Lorenzo Colitti, David Farmer, Bill Fenner, Ray Hunter, Paraskevi Iliadou, Jen Linkova, Philip Matthews, Matthew Petach, Scott Schmit, Tatuya Jinmei, Fred Templin, Ole Troan, Stig Venaas, and numerous other participants in the 6MAN working group. An extremely detailed review by Mark Smith was especially helpful.

This document was originally produced using the xml2rfc tool [RFC2629].

Authors' Addresses

   Brian Carpenter (editor)
   Department of Computer Science
   University of Auckland
   PB 92019
   Auckland  1142
   New Zealand
   Tim Chown
   University of Southampton
   Southampton, Hampshire  SO17 1BJ
   United Kingdom
   Fernando Gont
   SI6 Networks / UTN-FRH
   Evaristo Carriego 2644
   Haedo, Provincia de Buenos Aires  1706
   Sheng Jiang
   Huawei Technologies Co., Ltd
   Q14, Huawei Campus
   No.156 Beiqing Road
   Hai-Dian District, Beijing  100095
   P.R. China
   Alexandru Petrescu
   CEA Saclay
   Gif-sur-Yvette, Ile-de-France  91190
   Andrew Yourtchenko
   7a de Kleetlaan
   Diegem  1830