IPv6-Mostly Networks: Deployment and Operations Considerations
draft-ietf-v6ops-6mops-02

IPv6 operations                                              N. Buraglio
Internet-Draft                                   Energy Sciences Network
Intended status: Informational                                O. Caletka
Expires: 27 February 2026                                       RIPE NCC
                                                              J. Linkova
                                                                  Google
                                                          26 August 2025

     IPv6-Mostly Networks: Deployment and Operations Considerations
                       draft-ietf-v6ops-6mops-02

Abstract

   This document discusses a deployment scenario called "an IPv6-Mostly
   network", when IPv6-only and IPv4-enabled endpoints coexist on the
   same network (network segment, VLAN, SSID etc).

Status of This Memo

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   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on 27 February 2026.

Copyright Notice

   Copyright (c) 2025 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
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   Please review these documents carefully, as they describe your rights
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   provided without warranty as described in the Revised BSD License.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Requirements Language . . . . . . . . . . . . . . . . . . . .   4
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  Solution Overview . . . . . . . . . . . . . . . . . . . . . .   4
     4.1.  IPv6-Only Capable Endpoints . . . . . . . . . . . . . . .   5
     4.2.  IPv6-Only and IPv4-enabled Endpoints Coexistence  . . . .   6
     4.3.  Access to IPv4-only Destinations  . . . . . . . . . . . .   6
       4.3.1.  NAT64 . . . . . . . . . . . . . . . . . . . . . . . .   6
       4.3.2.  464XLAT . . . . . . . . . . . . . . . . . . . . . . .   7
       4.3.3.  Signalling NAT64 Prefix to Hosts  . . . . . . . . . .   7
       4.3.4.  DNS vs DNS64  . . . . . . . . . . . . . . . . . . . .   8
   5.  Solution Analysis . . . . . . . . . . . . . . . . . . . . . .   9
     5.1.  IPv6-Only Compared to Dual-Stack  . . . . . . . . . . . .   9
     5.2.  IPv6-Mostly Compared to a Dedicated IPv6-Only Network . .  10
   6.  Incremental Rollout Considerations  . . . . . . . . . . . . .  11
     6.1.  Per-Device and Per-Subnet Incremental Rollout . . . . . .  11
     6.2.  Rollback Approach . . . . . . . . . . . . . . . . . . . .  12
     6.3.  Opt-In and Opt-Out Modes  . . . . . . . . . . . . . . . .  12
   7.  Operational Considerations  . . . . . . . . . . . . . . . . .  12
     7.1.  Address Assignment Policy . . . . . . . . . . . . . . . .  12
     7.2.  Extension Headers . . . . . . . . . . . . . . . . . . . .  13
     7.3.  Onlink Communication and Service Discovery  . . . . . . .  13
     7.4.  Typical Issues  . . . . . . . . . . . . . . . . . . . . .  14
       7.4.1.  Hosts with Disabled or Disfunctional IPv6 . . . . . .  14
       7.4.2.  Network Extension . . . . . . . . . . . . . . . . . .  14
       7.4.3.  Multiple Addresses per Device . . . . . . . . . . . .  15
       7.4.4.  Host Mobility and Renumbering . . . . . . . . . . . .  16
       7.4.5.  Fragmentation . . . . . . . . . . . . . . . . . . . .  16
       7.4.6.  Representing IPv6 Addresses by CLAT . . . . . . . . .  17
       7.4.7.  IPv4-Dependencies in Network Admission Control  . . .  17
       7.4.8.  Custom DNS Configuration on Endpoints . . . . . . . .  17
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  18
   9.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .  18
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  18
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  18
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  18
     11.2.  Informative References . . . . . . . . . . . . . . . . .  20
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  22
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  22

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1.  Introduction

   While most network operators initially deploy IPv6 alongside their
   existing IPv4 infrastructure, pure IPv6-only networks remain uncommon
   outside of the mobile carrier space.  This dual-stack approach is
   seen as a necessary transition phase, allowing operators to gain
   experience with IPv6 while minimizing disruption.

   However, dual-stack networks do not address the core problem driving
   IPv6 adoption: IPv4 address exhaustion.  They still require the same
   amount of IPv4 resources as IPv4-only networks.  Even worse, this
   dual-stack approach has been demonstrated to be a long-term crutch,
   frequently masking problems that would otherwise be exposed and
   remedied as normal operational workflows.  Many applications still
   rely on IPv4, creating a chicken-and-egg problem: IPv6-only networks
   seem impractical with so many incompatible applications, yet
   applications continue to rely on IPv4 because IPv6-only networks are
   rare.

   The less control a network operator has over devices and
   applications, the more difficult it is to break IPv4 dependencies and
   move to IPv6-only.  This is particularly challenging in enterprise
   networks with legacy IPv4-dependent applications and public Wi-Fi
   networks where operators cannot guarantee device compatibility.

   To enable a gradual transition, operators need to identify which
   devices can function in IPv6-only mode and which cannot.  This still
   leaves the challenge of how to provide IPv4 addresses only to devices
   with IPv4 dependencies.  Creating separate network segments for each
   type of devices introduces complexity and scalability issues – a
   major hurdle to IPv6-only adoption.

   A more desirable approach is to deploy an "IPv6-mostly" network that
   provides IPv4 on demand.  This allows IPv6-capable devices to remain
   IPv6-only while the network is seamlessly supplying IPv4 to those
   that require it.  An IPv6-mostly network allows endpoints to operate
   at the highest level of their network stack evolution, on demand,
   while still allowing for legacy compatibility.

   This document explores the requirements, recommendations, and
   challenges associated with deploying IPv6-mostly networks in
   enterprise and public Wi-Fi environments.  While the principles
   discussed may be applicable to other network types, this document's
   focus remains on these specific use cases.

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2.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

3.  Terminology

   This document reuses most of Terminology section from [RFC8925] .

   CLAT: a customer-side translator (CLAT) as specified in [RFC6877] and
   [I-D.ietf-v6ops-claton].

   Endpoint: A device connected to a network and considered a host From
   the operator's perspective.  However, some endpoint can also extend
   the network to other physical or logical systems, thereby assuming
   routing functions.  Examples include:

   *  Corporate laptop: While primarily a host, it might run virtual
      systems and route traffic to them, extending the network and
      acting as a router.

   *  Mobile phone with tethering enabled: Acts as a host on the Wi-Fi
      network, but also as a router for tethered devices, potentially
      without the operator's knowledge or consent.

   Native IPv4 connectivity: IPv4 connectivity or default gateway
   provided by the network without using any form of translation
   mechanisms (such as 464XLAT, [RFC6877]).

   Network segment: a link (VLAN, a broadcast domain etc) where hosts
   share the same IP subnet.

   PREF64: (or NAT64 prefix): An IPv6 prefix used for IPv6 address
   synthesis and for network addresses and protocols translation from
   IPv6 clients to IPv4 servers, [RFC6146].

4.  Solution Overview

   In a nutshell, an IPv6-mostly network is very much similar to a dual-
   stack one with two additional key elements:

   *  The network provides NAT64 ([RFC6146] ) functionality, enabling
      IPv6-only clients to communicate with IPv4-only destinations.

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      -  The network also provides the information about the PREF64, for
         example via RAs ([RFC8781]), via DNS64 ([RFC6147], [RFC7050]),
         or both.  This is to ensure that clients and the network's
         NAT64 use the same PREF64 to translate between IPv6 and IPv4.
         Section 4.3.3 and Section 4.3.4 discuss those mechanisms in
         more detail.

   *  The DHCPv4 server infrastructure offers DHCPv4 Option 108 as per
      [RFC8925].  This is to ensure that IPv6-only capable devices are
      not consuming IPv4 addresses.  Section 4.2 discusses other
      approaches to provide IPv4 addresses on demand.

   Upon connecting to an IPv6-mostly network segment, an endpoint
   configures its IP stack based on its capabilities:

   *  IPv4-Only Endpoint: Acquires an IPv4 address through DHCPv4.

   *  Dual-Stack Endpoint (Not IPv6-Only Capable): Configures IPv6
      addresses using any supported protocol.  Additionally, it obtains
      an IPv4 address via DHCPv4.

   *  IPv6-only capable endpoint configures its IPv6 addresses and,
      while performing DHCPv4, includes option 108 ([RFC8925] ) into the
      Parameter Request List.  The DHCP server returns the option and,
      as per [RFC8925] , the endpoint forgoes requesting an IPv4
      address, remaining in IPv6-only mode.

   An IPv6-mostly network segment can support a mix of IPv4-only, dual-
   stack, and IPv6-only devices.  IPv6-only endpoints utilize the
   network-provided NAT64 to reach IPv4-only destinations.

   The following sections discussed those solution elements in more
   details.

4.1.  IPv6-Only Capable Endpoints

   The term "IPv6-only capable endpoint" lacks a strict technical
   definition.  It broadly describes a device that can function without
   native IPv4 connectivity/IPv4 addresses, providing the same user
   experience.  Examples include but are not limited to:

   *  An endpoint capable of communicating only with a limited set of
      endpoints, all of which are available over IPv6.

   *  An endpoint capable of utilizing only IPv6 sockets, employing
      NAT64 for accessing IPv4 resources.

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   *  An endpoint implementing CLAT to provide IPv4 connectivity for the
      legacy applications.

4.2.  IPv6-Only and IPv4-enabled Endpoints Coexistence

   One effective way to restrict IPv4 addresses solely to devices that
   require them is to enable support for the IPv6-Only Preferred DHCPv4
   option (Option 108, [RFC8925] ) on the network's DHCP infrastructure.
   Currently, most IPv6-only capable systems also support Option 108 and
   have it enabled by default.  By recognizing this option, the network
   infrastructure can enforce the IPv6-only mode for those devices.

   Certain devices, such as resource-constrained embedded systems, may
   operate in IPv6-only mode without specific adjustments if their
   communication is limited to IPv6-enabled destinations.  Since these
   systems often lack Option 108 support, administrators may need
   alternative methods to prevent IPv4 address assignment.  One approach
   is to block IPv4 traffic at the network attachment level (such as a
   switchport or wireless access point).  This could involve methods
   like:

   *  Static ACL: Applying a static filter with a "deny ip any any"
      rule.

   *  Dynamic ACL via RADIUS: If 802.1x authentication is in use, RADIUS
      can provide an ACL blocking all IPv4 traffic.

   *  Filtering the IPv4 ethertype (0x0800): Some hardware platforms may
      allow for low level filtering of ethertype 0x0800 preventing IPv4
      from passing into or out of a given switchport.

   The ACL-based approach has some scalability implications and
   increases operational complexity, therefore it could only be
   recommended as a stopgap solution.

4.3.  Access to IPv4-only Destinations

4.3.1.  NAT64

   IPv6-only endpoints require NAT64 to access IPv4-only destinations.
   Quite often operators choose to combine NAT44 and NAT64 functions,
   deploying NAT64 at the Internet edge.  However, if not all internal
   services are IPv6-enabled, then NAT64 might need to allow accessing
   internal destinations as well as external ones.  In that case it
   might be beneficial to deploy NAT64 closer to the users, instead of
   the network Internet edge, in order to simplify routing/firewall
   rules and reduce latency.

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   If IPv4-only internal destinations are using [RFC1918] address space
   and IPv6-only nodes are expected to communicate with those IPv4-only
   destinations, then the operator MUST NOT use the well-known prefix
   64:ff9b::/96 for NAT64 (see section 3.1 of [RFC6052]).  In that
   scenario the operator is required to allocate a network-specific
   NAT64 prefix from the operator's address space. ).

   Discussion: reports from the field indicate that it's very common for
   NAT64 and CLAT implementations to ignore the requirement from section
   3.1 of [RFC6052].  Such implementations successfully translate
   [RFC1918] IPv4 addresses using the well-known NAT64 prefix.

4.3.2.  464XLAT

   Unless the usage of IPv4-only network sockets is prohibited or
   mitigated in the operating system, enabling CLAT (Customer-side
   translator) on endpoints is essential for seamless operation of
   IPv4-only applications in IPv6-only environments.  CLAT provides an
   IPv4 address and IPv4 default route, ensuring functionality even
   without a native IPv4 address from the network.  Without CLAT or
   similar compatibility layer inside the operating system, IPv4-only
   applications would fail, negatively impacting user experience and
   increasing support overhead.

   Recommendations for Network Administrators controlling the endpoints:

   *  CLAT + DHCPv4 Option 108: If the network administrator can control
      endpoint configuration, CLAT SHOULD be enabled on endpoints
      sending DHCPv4 Option 108.  This streamlines the transition.

   *  Option 108 Without CLAT MAY be enabled if the administrator is
      willing to identify and fix IPv4-only systems/applications, or if
      all applications are confirmed to work in IPv6-only mode.

4.3.3.  Signalling NAT64 Prefix to Hosts

   Hosts running 464XLAT, translating IPv4 literal to an IPv6 literals
   (Section 7.1 of [RFC8305]) or performing local DNS64 functions need
   to discover the PREF64 (the IPv6 prefix used by NAT64).  The network
   administrator SHOULD configure the first-hop routers to include
   PREF64 information in Router Advertisements as per ([RFC8781] ) even
   if the network provides DNS64 (so hosts can use DNS64-based prefix
   discovery, [RFC7050]).  [I-D.ietf-v6ops-prefer8781] discusses this
   recommendation in more details.

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4.3.4.  DNS vs DNS64

   Traditionally, DNS64 (with NAT64) is used to enable IPv6-only
   endpoints to access IPv4-only destinations.  However, using DNS64 has
   a number of drawbacks, such as:

   *  DNSSEC Incompatibility: DNS64 responses fail DNSSEC validation.

   *  Custom Resolvers: Endpoints or applications configured with custom
      resolvers or running recursive resolvers locally can not benefit
      from the DNS64 provided by the network.  Such systems would need
      to rely on CLAT or local synthesis instead (Section 7.4.8
      discusses this scenario in more details).

   *  Additional requirements for application: to benefit from DNS64,
      applications need to be IPv6-enabled, use DNS (do not use IPv4
      literals).  Many applications do not satisfy those requirements
      and therefore fail if the endpoint does not have an IPv4 address/
      native IPv4 connectivity.  Existence of such applications is the
      main reason why transition to IPv6-only must be incremental (via
      IPv6-mostly phase) and requires the vast majority of endpoints to
      support CLAT.

   Those concerns make DNS64 is suboptimal and undesirtable solution
   long-term.  To eliminate the needs for DNS64, the network shall
   signal PREF64 to endpoints (see Section 4.3.3), and the endpoints
   need to use the obtained PREF64 for performing local synthesis and
   for CLAT.  It should be noted that not every application can benefit
   from local synthesis performed by the operating system, as it would
   require the application to use DNS (not IP literals).  Therefore
   DNS64 is only required to support the following cases:

   *  Systems and applications which perform local synthesis but do not
      support [RFC8781] for prefix discovery, and can only discover the
      NAT64 prefix using via DNS64 [RFC7050].

   *  IPv6-only devices without CLAT (unless those endpoints are
      guaranteed never to need IPv4-only destinations, e.g. in case of a
      specialized network segment communicating solely with IPv6-capable
      destinations).

   Using DNS64 however bypasses CLAT for IPv6-capable applications
   communicating with IPv4-only destinations, making the communication
   slightly more efficient.  For that reason it might be beneficial to
   run DNS64 even if it is not strictly necessary, provided the the
   above mentioned drawbacks are adressed.

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5.  Solution Analysis

5.1.  IPv6-Only Compared to Dual-Stack

   IPv6-Mostly networks offer significant advantages over traditional
   dual-stack models where endpoints have both IPv4 and IPv6 addresses:

   *  Drastically Reduced IPv4 Consumption: Dual-stack deployments don't
      solve the core problem of IPv4 address exhaustion.  IPv6-Mostly
      allows to significantly reduce IPv4 consumption, as well as to
      reclaim IPv4 space.  This reduction depends on endpoint
      capabilities (DHCPv4 Option 108 and CLAT support).  In real-world
      scenarios, like conference Wi-Fi, 60-70% of endpoints may support
      IPv6-only operation, potentially allowing 2-4 times smaller IPv4
      subnets.

   *  Controlled and incremental phase-out of IPv4: IPv6-Mostly allows
      for the controlled phase out IPv4 from many endpoints,
      streamlining operations and improving overall network reliability
      at a measured and operator-controlled pace.

   *  Reduced Dependency on DHCPv4: As more devices operate seamlessly
      in IPv6-only mode, the criticality of DHCPv4 service diminishes
      significantly.  This allows operators to scale down DHCPv4
      infrastructure or operate it with less stringent service level
      objectives (SLOs), optimizing costs and resource allocation.

   *  Simplified troubleshooting due reduced impact of Happy Eyeballs:
      when both the client and the server are dual-stack, communication
      can happen either over IPv6 or over IPv4 and the protocol choice
      can change over time.  This may obscure network issues or make
      them intermittent, complicating the troubleshooting.  In
      IPv6-Mostly network IPv6-only hosts are much less affected by such
      an issue.  Although modern versions of Happy Eyeballs algorithm
      support falling back to IPv4 even in case of IPv6-only network,
      the delay before switching to IPv4 is so long that the fallback
      does not happen during regular operations.  It should be noted,
      however, that when an IPv6-only client communicates with an
      IPv4-only destination, the traffic may traverse CLAT or be sent as
      IPv6 towards the NAT64, depending on how the specific application
      is written.

   The introduction of NAT64 within the IPv6-Mostly model may appear as
   a drawback, as it inherits many of the limitations associated with
   NAT44, such as translating flows from different hosts to thesame
   public IPv4 address, or scalability challenges caused by stateful
   opeation.  However, it's important to recognize that most IPv4-only
   or dual-stack networks already rely on NAT44 due to IPv4 address

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   scarcity.  For these networks, transitioning to an IPv6-Mostly
   architecture would simply require enabling NAT64 functionality on
   existing NAT44 devices.

   In a dual-stack network, flows originating from IPv6-only capable
   endpoints to IPv4-only destinations are, in most cases, translated by
   NAT44.  Within an IPv6-Mostly environment, this role is fulfilled by
   NAT64.  Consequently, the overall load on the NAT infrastructure
   remains effectively unchanged, with NAT64 essentially replacing NAT44
   for a subset of traffic flows.

   It is also worth emphasizing that NAT64 is employed solely for
   facilitating access to IPv4 resources.  As the availability of
   IPv6-enabled resources continues to increase, the volume of traffic
   traversing NAT64 is expected to diminish proportionally.

   As discussed in Section 7.3 coexistence of IPv6-only and IPv4-only
   hosts on the same link might present challenges for onlink service
   discovery and onlink peer2peer communications.  It might be consider
   a disadvantge of IPv6-Moslty model for networks where onlink
   communication between IPv4-only and IPv6-only devices is desirable.

5.2.  IPv6-Mostly Compared to a Dedicated IPv6-Only Network

   Traditional IPv6-only adoption involves separate networks alongside
   dual-stack ones.  IPv6-Mostly approach offers significant
   improvements, such as:

   *  Enhanced Scalability: Separate IPv6-only networks double the
      number of SSIDs in wireless environments, causing channel
      congestion and degrading performance.  IPv6-Mostly doesn't require
      additional SSIDs.  Similarly, it allows IPv4 and IPv6-only devices
      to coexist on the same wired VLANs, eliminating the need of
      additional layer2 segments, VLAN IDs, and their respective layer3
      configurations.

   *  Operational Simplicity: Managing one network segment for all
      clients (regardless of IPv4 needs) simplifies operations, improves
      user experience (no more confusing SSID choices), and reduces
      support tickets related to mismatched connections.  For wired
      connections dynamic VLAN assignment becomes easier without device-
      specific IPv6 capability tracking.

   *  Optimized IPv4 Consumption: User-selected dual-stack networks
      often lead to unnecessary IPv4 use, as users often connect
      IPv6-only capable devices to a dual-stack network.  IPv6-Mostly
      network allocates IPv4 addresses only when devices don't advertise
      IPv6-only capability (DHCPv4 Option 108).

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   *  Improved Problem Visibility: User-selected fallback to dual-stack
      networks can mask issues with IPv6-only operation, hindering
      problem reporting and resolution.  IPv6-Mostly forces users to
      work through any issues, improving identification and enabling
      fixes for smoother long-term transition.

   *  Flexible, Incremental Transition: IPv6-Mostly allows for gradual
      migration on a per-segment or even on per-device basis.  Devices
      become IPv6-only only when deemed fully compatible with that mode.

6.  Incremental Rollout Considerations

   Migrating endpoints to IPv6-only fundamentally changes network
   dynamics by removing the IPv4 safety net.  This includes the masking
   effect of traditional Happy Eyeballs algorithm [RFC6555].  IPv6
   connectivity issues become far more prominent, including those
   previously hidden within dual-stack environments.  Operators should
   be prepared to discover and troubleshoot issues in both endpoints and
   network infrastructure, even if the dual-stack network appeared
   problem-free.

   Some rollout considerations are discussed in the following sections.

6.1.  Per-Device and Per-Subnet Incremental Rollout

   Limited control over endpoint configuration necessitates a per-subnet
   rollout, incrementally enabling first PREF64 signalling and then
   option 108 processing in DHCP.  Unless special circumstances like
   unicast router advertisements for each host, enabling PREF64
   signalling will happen per-subnet without per-endpoint opt out
   mechanism.  If endpoint control exists, per-device rollout is
   possible for option 108 processing (at least for OSes with
   configurable Option 108).  Note that some OSes unconditionally enable
   Option 108 support, becoming IPv6-only the moment it's activated on
   the server side.  The following approach is RECOMMENDED:

   *  PREF64 signalling: Enable PREF64 option for the router
      advertisement and/or DNS64 (see above).  Especially if DNS64 is
      used, this might affect behaviour of some devices as the path via
      NAT64 would get generally preferred over native IPv4 path.
      Rollback at this stage affects the entire subnet.

   *  DHCP Server-Side Activation: Enable Option 108 processing.  Some
      OSes automatically switch to IPv6-only.  Rollback at this stage
      affects the entire subnet.

   *  Controlled Endpoint Activation: Enable Option 108 on managed
      endpoints with per-device rollback possible.

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6.2.  Rollback Approach

   For quick rollback, the administrator SHOULD start with a minimal
   Option 108 value (300 seconds, Section 3.4 of [RFC8925]) and possibly
   increase this value as the IPv6-mostly network proves reliable,
   reducing the likelihood of full-scale rollback.  In most deployments,
   the load of DHCP infrastructure caused by each endpoint restarting
   the DHCP handshake every 300 seconds would be negligible.

6.3.  Opt-In and Opt-Out Modes

   Before user-facing deployment, the administrator SHOULD consider a
   dedicated IPv6-mostly proof-of-concept network for early adopters.
   While this temporarily sacrifices some IPv6-mostly benefits
   (Section 5.2 ), it provides valuable operational experience and early
   issue detection.

   *  Opt-In Phase: Invite tech-savvy early adopters to join an
      IPv6-mostly network and report issues.  While response rates may
      be low, dedicated participants provide valuable troubleshooting
      data.

   *  Opt-Out Phase: Incrementally enable PREF64 signalling as well as
      Option 108.  Allow selective disabling for problematic endpoints,
      in extreme case even by providing a separate network or rolling
      back IPv6-mostly in that particular subnet.  Opting out SHOULD NOT
      be possible without a problem reports to ensure issue tracking and
      resolution.

   In some scenarios (see Section 7.4.1 ) the administrator MAY keep a
   dual-stack network as a last resort fallback mechanism but SHOULD
   prevent usesrs from connecting to it accidentially (e.g. it should be
   a hidden SSID with authentication enabled).  For recurring temporary
   networks, for instance during a conference, it might be a good
   practice to change the network SSID of the last resort fallback
   network between each event.

7.  Operational Considerations

7.1.  Address Assignment Policy

   As outlined in Section 6.3 of [RFC6877] , CLAT requires either a
   dedicated IPv6 prefix or, if unavailable, a dedicated IPv6 address.
   Currently (2024), all implementations use SLAAC for CLAT address
   acquisition.  Therefore, to enable CLAT functionality within
   IPv6-mostly network segments, first-hop routers MUST be configured to
   advertise a Prefix Information Option (PIO, [RFC4861]) containing a
   globally routable SLAAC-suitable prefix with the 'Autonomous Address-

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   Configuration' (A) flag set to one.

7.2.  Extension Headers

   Being an IPv6-specific concept, IPv6 extension headers are often
   neglected or even explictly prohibited by security policies in dual-
   stack networks.  The issues caused by blockig extension headers might
   be masked by the presense of Happy Eyeballs but become highly visible
   when there is no IPv4 to fallback to.

   The network SHOULD permit at least the following extension headers:

   *  Fragment Header (Section 4.5 of [RFC8200] ).Section 7.4.5
      discusses the fragmentation in more details.

   *  ESP Header, which is used for IPSec traffic, such as VPN and Wi-Fi
      Calling.

7.3.  Onlink Communication and Service Discovery

   Shared link is often used for automatic discovery of neighboring
   devices and their services by means of different protocols like
   Multicast DNS [RFC6762].  Many devices and/or services are built on
   an assumption that devices on the same link also share the same set
   of address families or at least they have one common address family
   shared between all of them.

   In IPv6-Mostly, this assumption is not valid as there might be both
   IPv4-only and IPv6-only devices on the same link.  As per Section 20
   of [RFC6762], this means that the link has two unrelated ".local."
   zones, one for each address family.  Discovery of devices across
   different addres families is impossible, unless some sort of relay is
   deployed on the link.

   This concern, however, is only applicable if onlink communications
   are desired and permitted by security policies.  For networks where
   peer2peer onlink communications are explicitly denied (it's often a
   case for enterprise networks and public WiFi), this issue is not a
   concern.  If onlink peer2peer communication is required, the operator
   needs to ensure all participating devices are either dual-stack or
   IPv6-mostly.

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7.4.  Typical Issues

   IPv6-mostly networks expose hidden issues by removing the IPv4 safety
   net.  While implementation bugs vary greatly and are beyond the scope
   of this document, this section focus on common problems caused by
   configuration, topology, or design choices.  It's crucial to note
   that these issues likely pre-exist in dual-stack networks, but remain
   unnoticed due to IPv4 fallback.

7.4.1.  Hosts with Disabled or Disfunctional IPv6

   Historically, tech support often advised disabling IPv6 as a quick
   workaround, leading to devices with disabled IPv6.  Similarly,
   corporate IT may have disabled or filtered IPv6 under the assumption
   that it's not widely used.  Such endpoints requesting Option 108 will
   fail to connect in an IPv6-mostly network, as they won't receive IPv4
   addresses and IPv6 is disabled.

   Administrators controlling endpoints SHOULD ensure those endpoints
   have IPv6 enabled and operational before transitioning the network to
   IPv6-mostly mode.  This includes verifying protocol enablement as
   well as validation of any central or discreetly managed host based
   firewalls that could potentially interfere with proper IPv6 function
   that may be masked by the presence of IPv4, and therefor exp[osed
   with its removal.

7.4.2.  Network Extension

   IPv4's NAT44 allows endpoints to extend connectivity to downstream
   systems without upstream network awareness or permission.  This
   creates challenges in IPv6-mostly deployments where endpoints lack
   IPv4 addresses:

   Solutions and trade-offs:

   *  Using DHCPv6-PD to allocate prefixes to endpoints ([RFC9663]).
      Provides downstream systems with IPv6 addresses and native
      connectivity.

   *  Enabling the CLAT function on the endpoint.  This scenario is
      similar to the Wireline Network Architecture described in
      Section 4.1 of [RFC6877] . The downstream systems would receive
      IPv4 addresses and their IPv4 traffic would be translated to IPv6
      by the endpoint.  However this approach leads to the downstream
      systems using IPv4 only and not benefiting from end2end IPv6
      connectivity.  To enable IPv6 benefits, combine this with IPv6
      Prefix Delegation as discussed above.

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   *  Bridging and ND Proxy: The endpoint bridges IPv6 traffic and masks
      downstream devices behind its MAC address.  This can lead to
      scalability issues ( Section 7.4.3 ) due to the single MAC being
      mapped to many IPv6 addresses.

7.4.3.  Multiple Addresses per Device

   Unlike IPv4, where endpoints typically have a single IPv4 address per
   interface, IPv6 endpoints inherently use multiple addresses:

   *  Link-local address.

   *  Temporary address (common on mobile devices for privacy)

   *  Stable address (for long-term identification)

   *  CLAT address (in IPv6-mostly/IPv6-only networks)

   Endpoints with containers, namespaces, or ND proxy functions may have
   even more addresses.  This poses challenges for network
   infrastructure devices (SAVI switches, wireless access points, etc.)
   that map MAC addresses to IPv6 addresses, often with limits to
   prevent resource exhaustion or DoS attacks.  When the number of IPs
   per MAC limit is exceeded, infastructure devices behavior varies
   across implementations, leading to inconsistent connectivity loss and
   other unexpected behavior: while some systems drop new addresses,
   others delete older entries, causing previously functional addresses
   to lose connectivity.  In all those cases Endpoints and applications
   don't receive explicit signalling about the address becoming
   unusable.

   Allocating prefixes to endpoints via DHCP-PD ([RFC9663]) alows to
   eliminate the issue and to address the corresponding scalability
   concerns.  Operators of large-scale IPv6-mostly networks might
   benefit from using DHCPv6 prefix delegation for IPv6-only and
   IPv6-enabled endpoints.  In that case the administrator also need to
   ensure that the first-hop routers set the P-flag [RFC9762] in RAs.
   However, the deployment model described in [RFC9663] might not yet be
   supported by all endpoints.  The network administrator SHOULD ensure
   that the deployed network infastructure devices allow sufficient
   number of IPv6 addresses to be mapped to a client's MAC and SHOULD
   monitor for events, indicating that the limit has been reached (such
   as syslog messages, etc).

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7.4.4.  Host Mobility and Renumbering

   Networks employing dynamic VLAN assignment (e.g., based on 802.1x or
   MAC-based authentication) can cause endpoints to move between VLANs
   and IPv6 subnets.  As client operating systems do not always handle
   changes in link-layer state (e.g., VLAN changes) correctly, this
   mobility often leads to inconsistent IP stack behavior on operating
   systems, resulting in the persistence of old subnet addresses and
   potential connectivity issues due to incorrect source address
   selection.  In such networks, the administrator SHOULD configure the
   link-local addresses of the first-hop routers (such as link-local
   addresses assigned to router interfaces or the corresponding VRRPv3
   (Virtual Router Redundancy Protocol, [RFC9568]) virtual link-local IP
   address) to be unique within the mobility domain (the set of links
   the host can move between).  For example, if there are two VLANs
   using subnets 2001:db8:1:a::/64 and 2001:db8:1:b::/64, the
   administrator might configure the virtual router link-local addresses
   as 'fe80::2001:db8:1:a' and 'fe80::2001:db8:1:b', respectively.
   [I-D.link-6man-gulla] provides further analysis and discusses this
   approach in more detail.

7.4.5.  Fragmentation

   As the basic IPv6 header is 20 bytes longer than the IPv4 header,
   transating from IPv4 to IPv6 can result in packets exceeding the path
   MTU on the IPv6 side.  In that case NAT64 creates IPv6 packets with
   the Fragment Header (see Section 4 of [RFC6145] for more details.  As
   per [RFC6145] , by default the translator fragments IPv4 packets so
   that they fit in 1280-byte IPv6 packets.  It means that all IPv4
   packets larger than 1260 bytes are fragmented (or dropped if the DF
   bit is set).

   Administrators SHOULD maximize the path MTU on the IPv6 side (from
   the translator to IPv6-only hosts) to minimize fragmentation.  NAT64
   devices SHOULD be configured to use the actual path MTU on the IPv6
   side when fragmenting IPv4 packets.

   Another common case of IPv6 fragmentation is the use of protocols
   like DNS and RADIUS, where the server response needs to be sent as a
   single UDP datagram.  Network security policies MUST allow IPv6
   fragments for permitted UDP traffic (e.g., DNS, RADIUS) where single-
   datagram responses are required.  Allowing IPv6 fragments for
   permitted TCP traffic is RECOMMENDED unless the network
   infrastructure reliably performs TCP MSS clamping.

   To ensure reliable Path MTU Discovery (PMTUD, [RFC8201]), network
   security policies SHOULD permit the passage of ICMPv6 Type 2 (Packet
   Too Big, [RFC4443]) messages to and from all IPv6-enabled devices.

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   This recommendation is fundamentally no different from the
   established practice in IPv4 networks, where ICMP Type 3 Code 4
   (Fragmentation Needed and DF set) messages are essential for PMTUD.

7.4.6.  Representing IPv6 Addresses by CLAT

   Certain CLAT implementations face challenges when translating
   incoming IPv6 packets with native (non-synthesized) source addresses
   (e.g. ICMPv6 packets sent by intermediate hops on the path).  This
   lack of standardized translation mechanisms can lead to:

   *  Incomplete Traceroute: Omission of IPv6-only hops between the
      endpoint and NAT64 translator, hindering troubleshooting.

   *  Path MTU Discovery Issues: Potential disruptions in the PMTU
      discovery process.

   [I-D.ietf-v6ops-icmpext-xlat-v6only-source] proposes a solution for
   signalling the actual non-synthesized IPv6 source address while
   translating ICMPv6 error messages.

7.4.7.  IPv4-Dependencies in Network Admission Control

   Certain layer 2 network devices (e.g., wireless access points,
   controllers) may enforce a policy where a host's network access is
   contingent upon successful DHCP IPv4 address assignment or tied to
   results of DHCP snooping.  Consequently, hosts supporting Option 108
   may experience network access denial or disconnection, as they are
   not assigned IPv4 addresses.

7.4.8.  Custom DNS Configuration on Endpoints

   In IPv6-mostly networks without PREF64 in RAs, hosts rely on DNS64
   ([RFC7050] to discover the NAT64 prefix for CLAT operation.
   [RFC8880] requires that queries for AAAA resource records of
   "ipv4only.arpa."  MUST be sent to the recursive resolvers provided by
   the network, not to the resolvers configured manually, local
   recursive resolvers etc.  However some implementations do not comply
   with [RFC8880] and, when configured with custom DNS resolvers (e.g.,
   public or corporate DNS) may bypass the network-provided DNS64,
   preventing NAT64 prefix discovery and hindering CLAT functionality.

   Where feasible, administrators SHOULD include PREF64 in RAs within
   IPv6-mostly networks to minimize reliance on DNS64.  Administrators
   need to be aware of the potential for CLAT failures when endpoints
   use custom resolvers in environments lacking PREF64.

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8.  Security Considerations

   The proposed deployment scenario inherits security considerations of
   IPv6 (see [RFC9099]).  As IPv6-only device can not fall back to IPv4
   if IPv6 connectivity is not available or impacted by a malicious
   actor.  Therefore any attack affecting IPv6 connectivity would have
   much more drastic outcome, comparing to dual-stack networks.

   By signalling a rogue NAT64 prefix to a host, a malicious actor can:

   *  cause a DoS attack, if the network does not provide NAT64
      functions (for that prefix or at all);

   *  implement MitM attack by intercepting traffic to the rogue prefix.

   To countermeasure this attack vector, the network administrators
   SHOULD configure the first-hop routers to include PREF64 information
   in Router Advertisements as per [RFC8781] and ensure that layer 2
   security measures such as RA-Guard ([RFC6105]) are in place.
   Section 7 of [RFC8781] discusses this topic in more details.
   Additionally, [I-D.ietf-v6ops-claton] discusses security implications
   of enabling CLAT if native IPv4 connectivity is available and
   recommends disabling CLAT in that case.

   Security considerations of using DHCP Option 108 are documented in
   Section 6 of [RFC8925].

9.  Privacy Considerations

   This document does not introduce any new privacy considerations.
   IPv6-mostly networks have the same privacy considerations as other
   Dual-stacked or IPv6-only networks.

10.  IANA Considerations

   This memo does not introduce any requests to IANA.

11.  References

11.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

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   [RFC6105]  Levy-Abegnoli, E., Van de Velde, G., Popoviciu, C., and J.
              Mohacsi, "IPv6 Router Advertisement Guard", RFC 6105,
              DOI 10.17487/RFC6105, February 2011,
              <https://www.rfc-editor.org/info/rfc6105>.

   [RFC6146]  Bagnulo, M., Matthews, P., and I. van Beijnum, "Stateful
              NAT64: Network Address and Protocol Translation from IPv6
              Clients to IPv4 Servers", RFC 6146, DOI 10.17487/RFC6146,
              April 2011, <https://www.rfc-editor.org/info/rfc6146>.

   [RFC6333]  Durand, A., Droms, R., Woodyatt, J., and Y. Lee, "Dual-
              Stack Lite Broadband Deployments Following IPv4
              Exhaustion", RFC 6333, DOI 10.17487/RFC6333, August 2011,
              <https://www.rfc-editor.org/info/rfc6333>.

   [RFC6555]  Wing, D. and A. Yourtchenko, "Happy Eyeballs: Success with
              Dual-Stack Hosts", RFC 6555, DOI 10.17487/RFC6555, April
              2012, <https://www.rfc-editor.org/info/rfc6555>.

   [RFC6877]  Mawatari, M., Kawashima, M., and C. Byrne, "464XLAT:
              Combination of Stateful and Stateless Translation",
              RFC 6877, DOI 10.17487/RFC6877, April 2013,
              <https://www.rfc-editor.org/info/rfc6877>.

   [RFC7050]  Savolainen, T., Korhonen, J., and D. Wing, "Discovery of
              the IPv6 Prefix Used for IPv6 Address Synthesis",
              RFC 7050, DOI 10.17487/RFC7050, November 2013,
              <https://www.rfc-editor.org/info/rfc7050>.

   [RFC7335]  Byrne, C., "IPv4 Service Continuity Prefix", RFC 7335,
              DOI 10.17487/RFC7335, August 2014,
              <https://www.rfc-editor.org/info/rfc7335>.

   [RFC8200]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", STD 86, RFC 8200,
              DOI 10.17487/RFC8200, July 2017,
              <https://www.rfc-editor.org/info/rfc8200>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8781]  Colitti, L. and J. Linkova, "Discovering PREF64 in Router
              Advertisements", RFC 8781, DOI 10.17487/RFC8781, April
              2020, <https://www.rfc-editor.org/info/rfc8781>.

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   [RFC8585]  Palet Martinez, J., Liu, H. M.-H., and M. Kawashima,
              "Requirements for IPv6 Customer Edge Routers to Support
              IPv4-as-a-Service", RFC 8585, DOI 10.17487/RFC8585, May
              2019, <https://www.rfc-editor.org/info/rfc8585>.

   [RFC8925]  Colitti, L., Linkova, J., Richardson, M., and T.
              Mrugalski, "IPv6-Only Preferred Option for DHCPv4",
              RFC 8925, DOI 10.17487/RFC8925, October 2020,
              <https://www.rfc-editor.org/info/rfc8925>.

   [RFC9099]  Vyncke, É., Chittimaneni, K., Kaeo, M., and E. Rey,
              "Operational Security Considerations for IPv6 Networks",
              RFC 9099, DOI 10.17487/RFC9099, August 2021,
              <https://www.rfc-editor.org/info/rfc9099>.

11.2.  Informative References

   [RFC1918]  Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G.
              J., and E. Lear, "Address Allocation for Private
              Internets", BCP 5, RFC 1918, DOI 10.17487/RFC1918,
              February 1996, <https://www.rfc-editor.org/info/rfc1918>.

   [RFC4443]  Conta, A., Deering, S., and M. Gupta, Ed., "Internet
              Control Message Protocol (ICMPv6) for the Internet
              Protocol Version 6 (IPv6) Specification", STD 89,
              RFC 4443, DOI 10.17487/RFC4443, March 2006,
              <https://www.rfc-editor.org/info/rfc4443>.

   [RFC4861]  Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
              "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
              DOI 10.17487/RFC4861, September 2007,
              <https://www.rfc-editor.org/info/rfc4861>.

   [RFC4862]  Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
              Address Autoconfiguration", RFC 4862,
              DOI 10.17487/RFC4862, September 2007,
              <https://www.rfc-editor.org/info/rfc4862>.

   [RFC6052]  Bao, C., Huitema, C., Bagnulo, M., Boucadair, M., and X.
              Li, "IPv6 Addressing of IPv4/IPv6 Translators", RFC 6052,
              DOI 10.17487/RFC6052, October 2010,
              <https://www.rfc-editor.org/info/rfc6052>.

   [RFC6145]  Li, X., Bao, C., and F. Baker, "IP/ICMP Translation
              Algorithm", RFC 6145, DOI 10.17487/RFC6145, April 2011,
              <https://www.rfc-editor.org/info/rfc6145>.

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   [RFC6147]  Bagnulo, M., Sullivan, A., Matthews, P., and I. van
              Beijnum, "DNS64: DNS Extensions for Network Address
              Translation from IPv6 Clients to IPv4 Servers", RFC 6147,
              DOI 10.17487/RFC6147, April 2011,
              <https://www.rfc-editor.org/info/rfc6147>.

   [RFC6762]  Cheshire, S. and M. Krochmal, "Multicast DNS", RFC 6762,
              DOI 10.17487/RFC6762, February 2013,
              <https://www.rfc-editor.org/info/rfc6762>.

   [RFC7225]  Boucadair, M., "Discovering NAT64 IPv6 Prefixes Using the
              Port Control Protocol (PCP)", RFC 7225,
              DOI 10.17487/RFC7225, May 2014,
              <https://www.rfc-editor.org/info/rfc7225>.

   [RFC8201]  McCann, J., Deering, S., Mogul, J., and R. Hinden, Ed.,
              "Path MTU Discovery for IP version 6", STD 87, RFC 8201,
              DOI 10.17487/RFC8201, July 2017,
              <https://www.rfc-editor.org/info/rfc8201>.

   [RFC8305]  Schinazi, D. and T. Pauly, "Happy Eyeballs Version 2:
              Better Connectivity Using Concurrency", RFC 8305,
              DOI 10.17487/RFC8305, December 2017,
              <https://www.rfc-editor.org/info/rfc8305>.

   [RFC8880]  Cheshire, S. and D. Schinazi, "Special Use Domain Name
              'ipv4only.arpa'", RFC 8880, DOI 10.17487/RFC8880, August
              2020, <https://www.rfc-editor.org/info/rfc8880>.

   [RFC9568]  Lindem, A. and A. Dogra, "Virtual Router Redundancy
              Protocol (VRRP) Version 3 for IPv4 and IPv6", RFC 9568,
              DOI 10.17487/RFC9568, April 2024,
              <https://www.rfc-editor.org/info/rfc9568>.

   [RFC9663]  Colitti, L., Linkova, J., Ed., and X. Ma, Ed., "Using
              DHCPv6 Prefix Delegation (DHCPv6-PD) to Allocate Unique
              IPv6 Prefixes per Client in Large Broadcast Networks",
              RFC 9663, DOI 10.17487/RFC9663, October 2024,
              <https://www.rfc-editor.org/info/rfc9663>.

   [RFC9762]  Colitti, L., Linkova, J., Ma, X., Ed., and D. Lamparter,
              "Using Router Advertisements to Signal the Availability of
              DHCPv6 Prefix Delegation to Clients", RFC 9762,
              DOI 10.17487/RFC9762, June 2025,
              <https://www.rfc-editor.org/info/rfc9762>.

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   [I-D.ietf-v6ops-claton]
              Linkova, J. and T. Jensen, "464XLAT Customer-side
              Translator (CLAT): Node Recommendations", Work in
              Progress, Internet-Draft, draft-ietf-v6ops-claton-07, 25
              August 2025, <https://datatracker.ietf.org/doc/html/draft-
              ietf-v6ops-claton-07>.

   [I-D.ietf-v6ops-prefer8781]
              Buraglio, N., Jensen, T., and J. Linkova, "Recommendations
              for Discovering IPv6 Prefix Used for IPv6 Address
              Synthesis", Work in Progress, Internet-Draft, draft-ietf-
              v6ops-prefer8781-07, 7 August 2025,
              <https://datatracker.ietf.org/doc/html/draft-ietf-v6ops-
              prefer8781-07>.

   [I-D.link-6man-gulla]
              Linkova, J., "Using Prefix-Specific Link-Local Addresses
              to Improve SLAAC Robustness", Work in Progress, Internet-
              Draft, draft-link-6man-gulla-01, 3 March 2025,
              <https://datatracker.ietf.org/doc/html/draft-link-6man-
              gulla-01>.

   [I-D.ietf-v6ops-icmpext-xlat-v6only-source]
              Lamparter, D. and J. Linkova, "Using Dummy IPv4 Address
              and Node Identification Extensions for IP/ICMP translators
              (XLATs)", Work in Progress, Internet-Draft, draft-ietf-
              v6ops-icmpext-xlat-v6only-source-00, 4 July 2025,
              <https://datatracker.ietf.org/doc/html/draft-ietf-v6ops-
              icmpext-xlat-v6only-source-00>.

Acknowledgements

   Thanks to Brian Carpenter, Stuart Cheshire, Lorenzo Colitti, Jeremy
   Duncan, David Farmer, Jordi Palet, Michael Richardson, Matsuzaki
   Yoshinobu for the discussions, the feedback, and all contribution.

Authors' Addresses

   Nick Buraglio
   Energy Sciences Network
   IL
   United States of America
   Email: buraglio@forwardingplane.net, buraglio@es.net

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   Ondrej Caletka
   RIPE NCC
   Stationsplein 11
   Amsterdam
   Netherlands
   Email: Ondrej.Caletka@ripe.net

   Jen Linkova
   Google
   1 Darling Island Rd
   Pyrmont NSW 2009
   Australia
   Email: furry13@gmail.com, furry@google.com

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