Shared address space (100.64.0.0/10)
termThe IPv4 range reserved for the middle layer of carrier-grade NAT, between the subscriber's own private network and the provider's public addresses.
It exists because providers were previously borrowing ordinary private ranges for this purpose, which collided with the ranges customers were already using at home and produced faults nobody could diagnose from either end. Treat an address from this range as a signal rather than a destination: it means the customer sits behind provider translation, so inbound connections will not work and the address does not identify a household.
RFC 6598 (April 2012) reserved 100.64.0.0/10, 4,194,304 addresses from 100.64.0.0 to 100.127.255.255, because providers said a /10 was the smallest block that let them deploy CGNs regionally without nesting them. RFC 1918 space remains safe on the CGN-to-CPE link only if the provider knows the CPE copes with the same block on both sides, or that subscribers do not use it, which an unmanaged service cannot know. Numbering the link from someone else's public space, squat space, is forbidden.
The rules are MUSTs: packets with these addresses must not cross provider boundaries, the range must stay out of external DNS zones, and reverse queries for it must not reach the global DNS. The IANA registry marks it forwardable but not globally reachable. One behaviour differs from RFC 1918 space: a CPE given a shared or public WAN address may try 6to4, which then fails.
On a router or Linux host, ip address show to 100.64.0.0/10 finds a WAN address in the range, and a traceroute shows it as an early hop after the home router. The SSRF URL classifier flags such addresses as CGNAT.
Also known as: 100.64/10, rfc 6598, nat444 middle layer