6MAN G. Huston Internet-Draft APNIC Updates: 4291, 4007 (if approved) W. Kumari Intended status: Standards Track Google, Inc. Expires: 12 April 2027 9 October 2026 The IPv6 Loopback Address Prefix draft-ietf-6man-loopback-01 Abstract This document updates the IP Version 6 Address Architecture to expand the size of the IPv6 loopback space from a single address to a /48 prefix, formally defined as Node-Local Unicast space. This change allows for a much larger number of loopback addresses and internal virtual networks within an IPv6 host, which can be used for inter-process communication, local virtualization, container networking, and diagnostics. This document updates RFC 4291 to define the prefix and its functional semantics, and updates RFC 4007 to specify how the prefix is integrated into the scoped address architecture. It also updates the IANA IPv6 Address registry, the IPv6 Special- Purpose Address registry, and the Locally-Served DNS Zone Registry. About This Document This note is to be removed before publishing as an RFC. The latest revision of this draft can be found at https://wkumari.github.io/draft-kumari-ipv6-loopback/draft-ietf-6man- loopback.html. Status information for this document may be found at https://datatracker.ietf.org/doc/draft-ietf-6man-loopback/. Source for this draft and an issue tracker can be found at https://github.com/wkumari/draft-kumari-ipv6-loopback. Status of This Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Huston & Kumari Expires 12 April 2027 [Page 1] Internet-Draft IPv6 Loopback Prefix October 2026 Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at https://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on 12 April 2027. Copyright Notice Copyright (c) 2026 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/ license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Revised BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Revised BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1. The Need for Expanded Host-Internal Space . . . . . . . . 3 1.2. Terminology and Functional Semantics . . . . . . . . . . 4 2. Conventions and Definitions . . . . . . . . . . . . . . . . . 4 3. Loopback address background . . . . . . . . . . . . . . . . . 4 4. The IPv6 Loopback / Node-Local Unicast Prefix . . . . . . . . 5 4.1. Update to RFC 4291 . . . . . . . . . . . . . . . . . . . 5 4.2. Update to RFC 4007 (IPv6 Scoped Address Architecture) . . 6 5. Security Considerations . . . . . . . . . . . . . . . . . . . 6 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6 7. References . . . . . . . . . . . . . . . . . . . . . . . . . 7 7.1. Normative References . . . . . . . . . . . . . . . . . . 7 7.2. Informative References . . . . . . . . . . . . . . . . . 7 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 8 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8 Huston & Kumari Expires 12 April 2027 [Page 2] Internet-Draft IPv6 Loopback Prefix October 2026 1. Introduction In the IPv4 addressing architecture, the entire 127.0.0.0/8 block is reserved for loopback routing purposes. This generous allocation allows developers and system administrators to utilize over 16 million distinct host-internal addresses. While historically viewed as a byproduct of classful network design, this large local address space has become fundamental to modern network operations, enabling complex local testing, containerization, and inter-process communication without port exhaustion. By contrast, the IPv6 Addressing Architecture allocates only a single address, ::1/128, for local loopback. While sufficient for basic localhost identification, this strict limitation creates significant operational friction in modern IPv6-only and dual-stack environments. 1.1. The Need for Expanded Host-Internal Space As application architectures have evolved, the restriction of a single IPv6 loopback address has become a tangible bottleneck. Below are some examples of use cases which would benefit from an expanded loopback space: * Application Testing and Containerization: Developers frequently run multiple instances of a service locally. In IPv4, these instances can bind to the same port on different 127.x.x.x addresses. In IPv6, developers are forced to modify application port numbers, which breaks environment parity and complicates test scaffolding. * Local Virtual Networks and Container Routing: Modern containerized runtimes and microservice setups require running multiple virtual network segments within a single machine. Utilizing a /48 prefix enables the creation of multiple internal subnetworks and simulated network topologies completely within a single physical node. * Local Proxying and Service Meshes: Complex local routing paradigms (such as sidecar proxies) often require distinct IP assignments to securely isolate and route traffic locally without exposing services to the external network. * Controlled Interruption and Name Collisions: Global infrastructure services occasionally rely on localized sinkholes to safely manage deprecation or name collisions. For example, ICANN has historically utilized 127.0.53.53 for name collision controlled interruption. Replicating this fail-safe behavior in IPv6 requires a dedicated, local-only prefix. Huston & Kumari Expires 12 April 2027 [Page 3] Internet-Draft IPv6 Loopback Prefix October 2026 1.2. Terminology and Functional Semantics While historically referred to as "loopback" space, the functional requirement described in this document is a dedicated address block for host-internal virtual interfaces, referred to here as a *Node- Local Unicast Prefix*. The core semantic of this proposed space is strict isolation. Implementations MUST ensure that: * Addresses from this block can be assigned to multiple internal virtual interfaces and virtual bridge networks simultaneously. * Packets with a source or destination address drawn from this block MUST NOT be forwarded to any physical network interface. * These packets MUST never be routed off the local host. If a router or switch receives a packet on a physical interface bearing one of these addresses, the packet MUST be dropped. To support these operational realities, this document requests the allocation of a new, dedicated IPv6 prefix (e.g., a /48 drawn from the IANA IPv6 Special-Purpose Address Registry) to serve as expanded Node-Local Unicast space. This block will operate with the same fundamental constraints as the primary ::1/128 loopback address, without overlapping with the Unspecified Address (::/128). 2. Conventions and Definitions 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. Loopback address background The IPv4 network 127.0.0.0/8 was reserved by the IANA in [RFC791] where the class-based address architecture was described. It is understood that it was the IANA's policy at the time to reserve the first and last network of each class, and the address prefixes 0.0.0.0/8 and 127.0.0.0/8 from the Class A space were reserved in accordance with this practice. [RFC990] listed the 127.0.0.0/8 address prefix as being used by the loopback function, and this function was listed as a requirement for all Internet hosts in [RFC1122]. Huston & Kumari Expires 12 April 2027 [Page 4] Internet-Draft IPv6 Loopback Prefix October 2026 The "loopback" function is defined such that an outbound packet whose destination address triggers this loopback function should loop the packet back to the packet ingress queue for processing by the same host. No packet that is addressed to a loopback address should ever be passed to any physical network. [RFC1884], the original IPv6 Addressing Architecture document, allocates a single local loopback address, ::1. This single address allocation has been preserved in all subsequent revisions to the IPv6 addressing specification ([RFC2373], [RFC3513], [RFC4291]). Loopback addresses enable localhost communication, network diagnostics, and inter-process connections, making them essential for various local functions. Multiple loopback addresses can increase the number of distinct sockets that can be used for inter-process communication within a host. A larger Node-Local Unicast prefix in IPv6 can permit large numbers of distinct concurrent loopback TCP connections and complete virtualized subnets within a single host, which is comparable to and extends the functionality supported by the IPv4 loopback address prefix. 4. The IPv6 Loopback / Node-Local Unicast Prefix The IANA IPv6 Address registry denotes the address prefix ::/8 as being reserved by the IETF in [RFC3513] [RFC4291]. This range of addresses has been partially allocated with the prefix ::FFFF:0:0/96 being used in the context of an IPv6 transition technology to map IPv4 addresses into IPv6 addresses. This document expands the set of IPv6 loopback addresses by adding an additional Node-Local Unicast prefix: TBD/48. 4.1. Update to RFC 4291 This RFC replaces section 2.5.3 of [RFC4291] as follows: The unicast addresses 0:0:0:0:0:0:0:1 and the prefix TBD/48 are defined for loopback and node-local unicast functions. These may be used by a node to send IPv6 packets to itself, or to communicate across local virtual interfaces within the same host. They must not be assigned to any physical interface. They are treated as having Link-Local scope, and may be thought of as the Link-Local unicast addresses of a virtual interface (typically called the "loopback interface" or local virtual bridges) to an imaginary link that goes nowhere. Huston & Kumari Expires 12 April 2027 [Page 5] Internet-Draft IPv6 Loopback Prefix October 2026 The loopback address and addresses within the TBD/48 prefix must not be used as the source address in IPv6 packets that are sent outside of a single node. An IPv6 packet with a destination address in this prefix must never be sent outside of a single node and must never be forwarded by an IPv6 router. A packet received on an interface with a destination address of loopback or within the TBD/48 prefix must be dropped. 4.2. Update to RFC 4007 (IPv6 Scoped Address Architecture) Section 11.1 of [RFC4007] ("Non-Global Addresses") specifies the zone-id treatment of scoped addresses. It explicitly dictates that the loopback address ::1 does not require and must not be qualified with a zone identifier. This document updates Section 11.1 of [RFC4007] to extend this rule to the entire Node-Local Unicast prefix (TBD/48). Because addresses drawn from the TBD/48 prefix are strictly host- internal and do not associate with physical links, they MUST NOT be qualified with a zone identifier in user interfaces or socket APIs. Node implementations MUST treat the entire TBD/48 prefix as belonging to the default node-local loopback zone. 5. Security Considerations IPv6 addressing documents do not have any direct impact on Internet infrastructure security. However, system implementations MUST ensure strict isolation. Packets containing destination or source addresses from the TBD/48 block MUST be dropped if they appear on physical media, preventing leakage of host-internal IPC or virtual network communications to the public Internet, and mitigating any potential external spoofing or scanning vectors. 6. IANA Considerations The IANA is requested to assign a new IPv6 address prefix, TBD/48, to be used for the loopback and node-local unicast functions as described in this document. This prefix should be allocated from the IANA IPv6 Special-Purpose Address registry. The IANA is requested to amend the IPv6 Address registry and the IPv6 Special Purpose Address registry to record the designation of this address prefix. Huston & Kumari Expires 12 April 2027 [Page 6] Internet-Draft IPv6 Loopback Prefix October 2026 The IANA is also requested to add an entry to the IPv6 Locally-Served DNS Zone Registry for the new prefix, TBD/48, to ensure that reverse DNS lookups for addresses within this prefix are properly handled. 7. References 7.1. 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, . [RFC4007] Deering, S., Haberman, B., Jinmei, T., Nordmark, E., and B. Zill, "IPv6 Scoped Address Architecture", RFC 4007, DOI 10.17487/RFC4007, March 2005, . [RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing Architecture", RFC 4291, DOI 10.17487/RFC4291, February 2006, . [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, . 7.2. Informative References [RFC791] Postel, J., "Internet Protocol", STD 5, RFC 791, DOI 10.17487/RFC791, September 1981, . [RFC990] Reynolds, J. and J. Postel, "Assigned numbers", RFC 990, DOI 10.17487/RFC990, November 1986, . [RFC1122] Braden, R., Ed., "Requirements for Internet Hosts - Communication Layers", STD 3, RFC 1122, DOI 10.17487/RFC1122, October 1989, . [RFC1884] Hinden, R., Ed. and S. Deering, Ed., "IP Version 6 Addressing Architecture", RFC 1884, DOI 10.17487/RFC1884, December 1995, . [RFC2373] Hinden, R. and S. Deering, "IP Version 6 Addressing Architecture", RFC 2373, DOI 10.17487/RFC2373, July 1998, . Huston & Kumari Expires 12 April 2027 [Page 7] Internet-Draft IPv6 Loopback Prefix October 2026 [RFC3513] Hinden, R. and S. Deering, "Internet Protocol Version 6 (IPv6) Addressing Architecture", RFC 3513, DOI 10.17487/RFC3513, April 2003, . Acknowledgments The authors would like to thank Alejandro Acosta, Brian Carpenter, Antonis Chariton, Owen DeLong, Gert Doering, Jeremy Duncan, Lorenzo Colitti, David Farmer, Steinar Haug, Gábor Lencse, Michael Richardson, Terry Sweetser, Ole Trøan, and Maciej Żenczykowski for their comments, discussions, and suggestions on this topic. Additional thanks to John Heasley for submitting Pull Requests. In addition we would like to thank Jen Linkova for presenting the proposal at IETF 125, as the authors were participating in other sessions at the time. We would also like to specifically thank Mark Smith for an earlier (2013) effort: draft-smith-v6ops-larger-ipv6-loopback-prefix-04, which proposed a /32 designation. The need for a loopback address prefix has long been a topic of discussion in various forums, and we would like to acknowledge the contributions of many individuals who have participated in these discussions over the years. Unfortunately, at least one of the authors has a terrible memory, and has lost track of all those who have contributed to this topic over the years, and will be more than happy to acknowledge their input if reminded of this :-) Authors' Addresses Geoff Huston APNIC Email: gih@apnic.net W. Kumari Google, Inc. Email: warren@kumari.net Huston & Kumari Expires 12 April 2027 [Page 8]