IANA-Reserved IPv4 Prefix for Shared Address Space
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J. Weil, V. Kuarsingh, C. Donley, C. Liljenstolpe, M. Azinger · 2012-04
Abstract
RFC 6598 reserves the 100.64.0.0/10 address block for use in carrier-grade NAT (CGN) deployments, specifically for the interface between a CGN device and its upstream provider. This Shared Address Space avoids conflicts with private addresses (RFC 1918) that may be in use on the customer side of the CGN, and must not be routed on the public Internet.
Why This RFC Matters
As IPv4 addresses became exhausted, many ISPs deployed Carrier-Grade NAT (CGN, also called LSN) to share a single public IPv4 address among hundreds of subscribers. Using RFC 1918 addresses between the CGN and subscribers caused conflicts with customer home networks using the same ranges. RFC 6598's 100.64.0.0/10 provides a clean, dedicated range for this CGN interface, and has been widely deployed by mobile carriers and broadband ISPs worldwide as a transitional measure while IPv6 adoption grows.