IPv6-Only vs IPv6-Mostly: Appropriate Use Cases | RIPE Labs

Jordi Palet Martinez

IPv6-Only vs IPv6-Mostly: Appropriate Use Cases

Jordi Palet Martinez(community contributor)

Jordi Palet Martinez

Based in Spain

Jordi Palet Martínez has been working with computers, networking and technology since he was 8 years old. The last 30 years, as CEO/CTO at "The IPv6 Company", devoted most of his time to IPv6 R&D, standardization, training and consultancy, having worked already with hundreds of IPv6 customers in more than … More

9 min read — 24 Jun 2026

ipv6

use case

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Not all IPv6 deployment models belong in the same flock. IPv6-Only and IPv6-Mostly are often discussed as if they were interchangeable, but each was designed for different environments. This article explores why matching the right approach to the right use case matters.

IETF terminology is not always clearly defined, which can lead to confusion and, in some cases, the use of mechanisms outside the scenarios for which they were originally intended, potentially affecting the quality of service offered to users.<br>The terms IPv6-Only and IPv6-Mostly are a clear example of this. But despite the difficulty of reaching consensus on definitions, v6ops is making progress on this front through draft-palet-v6ops-ipv6-only.<br>One of the key points emerging from this work is that terms such as IPv6-Only and IPv6-Mostly only make sense when their scope is clearly defined. For example, saying that a network is IPv6-Only is probably incorrect, because it would imply that IPv4 is neither configured nor used anywhere in the network.<br>This is not a common scenario today, as services, applications, and internal or external content may still require IPv4 connectivity. If instead we say "IPv6-Only Access Network", we make it clear that only the access portion of the network is IPv6-only. The CPE LAN and other parts of the operator's network may still operate in dual-stack mode.<br>Understanding where these terms apply in practice helps explain why IPv6-Only and IPv6-Mostly should not be treated as interchangeable.

IPv6-Only+IPv4aaS<br>This is the most common scenario today in mobile access networks, using 464XLAT, and is a growing trend in residential access networks. It is one of the five IPv6-Only + IPv4aaS ("IPv4 as a Service") transition mechanisms for access networks . It allows the WAN portion to be IPv6-Only while maintaining dual-stack functionality on the user network, preventing any device or application from ceasing to function. Relevant documents include: RFC6877, RFC8585, RFC8683, RFC9313, draft-ietf-v6ops-rfc6146-bis, and draft-ietf-v6ops-rfc7084bis.<br>These five mechanisms, and especially 464XLAT (which is the only one supported in mobile networks), have a clear use case: unmanaged networks . Users of these networks do not need to perform special configurations, regardless of whether the devices they have on their home networks or tethering networks (when the mobile phone becomes a router to provide access to other devices) use IPv4, IPv6, or both, since the CPE incorporates a stateless NAT46 mechanism (CLAT or equivalent) to facilitate communication between IPv4 and IPv6, without local communication traffic having to leave the local network or be translated .

In recent years, IPv6 deployment has progressed globally, especially in mobile and residential networks. This is reflected in statistics from Akamai, APNIC, Facebook, Google, among others, which indicate IPv6 adoption rates exceeding 50%.<br>This figure doesn't even include traffic from China (which has legal IPv6-Only obligations for ISPs), likely placing that percentage closer to 70-75% and growing rapidly in that country. However, while major content providers have deployed IPv6-Only data centres, organisations (both public and private) generally have low levels of IPv6 adoption.

IPv6 adoption as seen by Google

IPv6-Mostly<br>To address deployment challenges in managed networks, the IETF has taken steps to facilitate deployment in managed networks , i.e., corporate networks, with mechanisms to achieve the “IPv6-Mostly” model (draft-ietf-v6ops-6mops).<br>IPv6-Mostly, compared to the dual-stack model in corporate networks, has several objectives:<br>Reduce the consumption of private IPv4 addresses and DHCPv4 resources.<br>Avoid masking IPv6 problems (due to HappyEyeballs' fallback to IPv4), facilitating the debugging of IPv6 errors.<br>Enable a gradual, host-by-host transition to IPv6-Only, eliminating the need for dedicated IPv6-Only segments and simplifying operations, thus increasing scalability.<br>Discover and prevent incompatibilities in a future IPv6-Only environment.<br>Reduce NAT4 usage.<br>IPv6-Mostly is based on the premise of RFC8925: a dual-stack host can signal, using a new DHCPv4 option (108), that it does not need to use IPv4 on its interface. The goal is...