The 100.64.0.0/10 address block is not private address space; it is shared address space. This is spelled out in RFC 6598, IANA-Reserved IPv4 Prefix for Shared Address SpaceRFC 6598, IANA-Reserved IPv4 Prefix for Shared Address Space (I highlighted the relevant verbiage):
- Introduction
IPv4 address space is nearly exhausted. However, ISPs must continue to support IPv4 growth until IPv6 is fully deployed. To that end, many ISPs will deploy a Carrier-Grade NAT (CGN) device, such as that described in [RFC6264RFC6264]. Because CGNs are used on networks where public address space is expected, and currently available private address space causes operational issues when used in this context, ISPs require a new IPv4 /10 address block. This address block will be called the "Shared Address Space" and will be used to number the interfaces that connect CGN devices to Customer Premises Equipment (CPE).
Shared Address Space is similar to [RFC1918RFC1918] private address space in that it is not globally routable address space and can be used by multiple pieces of equipment. However, Shared Address Space has limitations in its use that the current [RFC1918RFC1918] private address space does not have. In particular, Shared Address Space can only be used in Service Provider networks or on routing equipment that is able to do address translation across router interfaces when the addresses are identical on two different interfaces.
This document requests the allocation of an IPv4 /10 address block to be used as Shared Address Space. In conversations with many ISPs, a /10 is the smallest block that will allow them to deploy CGNs on a regional basis without requiring nested CGNs. For instance, as described in [ISP-SHARED-ADDRISP-SHARED-ADDR], a /10 is sufficient to service Points of Presence in the Tokyo area.
This document details the allocation of an additional special-use IPv4 address block and updates [RFC5735RFC5735].
