RFC 6598
For implications on Internet governance, see CGNAT.
RFC 6598, IANA-Reserved IPv4 Prefix for Shared Address Space, is an IETF RFC document that reserves the IPv4 block "100.64.0.0/10" as "Shared Address Space" for use mainly in CGNAT deployments by service providers.[1] It is part of BCP 153 and extends the registry of special-use IPv4 addresses.[2]
Background[edit | edit source]
By the time of RFC 6598, routable IPv4 space was close to exhaustion, while ISPs still needed to grow IPv4 access networks and could not move customers to IPv6 quickly. Operators were already using RFC 1918 private space between customer-premises equipment and carrier networks, which created frequent address collisions with the same private ranges used inside home or enterprise networks.[1]
Some providers requested extra non-routed private space from RIRs, particularly ARIN. ARIN instead returned a /10 block to IANA and pointed to the IETF as the forum to standardize a dedicated carrier-only range, which led to the Shared Address Space allocation.[3]
[edit | edit source]
RFC 6598 defines "100.64.0.0/10" (100.64.0.0–100.127.255.255) as Shared Address Space, providing 4,194,304 IPv4 addresses intended to be reused by many ISPs in their infrastructure.[1][3]
The space:
- Is not globally routable and is registered as special-purpose address space;[1][4]
- Is meant for service-provider infrastructure, primarily for the interfaces between CPEs and CGN devices in NAT444-style topologies;[1][5]
- Must not be forwarded across provider boundaries, except in controlled business arrangements.
DNS usage is constrained. Shared Address Space addresses must not appear in public-facing zone files, and reverse DNS queries for this /10 should not be sent to the public DNS.[1] The intent is to keep this addressing confined to carrier networks and avoid leakage.
Deployment and Operational Use[edit | edit source]
In practice, many ISPs assign 100.64.0.0/10 addresses to subscriber WAN interfaces and translate them at CGN gateways to small pools of public IPv4 addresses, allowing large subscriber bases to share limited public space.[1][6]
References[edit | edit source]
- ↑ 1.0 1.1 1.2 1.3 1.4 1.5 1.6 J. Weil et al., "IANA-Reserved IPv4 Prefix for Shared Address Space", RFC 6598, IETF, April 2012.
- ↑ IETF Datatracker, "BCP 153: IANA-Reserved IPv4 Prefix for Shared Address Space" (accessed 2025-11-20).
- ↑ RFC Editor, "Information on RFC 6598" (accessed 2025-11-20).
- ↑ "Carrier-grade NAT", Wikipedia (accessed 2025-11-20).
- ↑ Simplificando Redes, "What is CGNAT?" (accessed 2025-11-20).
ICANNWiki resources: Special Pages | Content Guide | Documentation | Development || Maintenance: Articles needing attention | Candidates for deletion || Projects: Internet & Digital Governance Library