100.64.0.0/10

This is shared address space for CGN (Carrier-Grade Network Address Translation). Internet providers use it between customer routers and the provider's own NAT (Network Address Translation) equipment when many customers share a smaller pool of public IPv4 addresses.

This block is not publicly routable. It is meant to stay inside the provider network, so seeing it does not identify a unique host on the public Internet.

A system administrator is likely to encounter it on the WAN (wide area network) side of a home or branch router, in traceroute output inside an ISP (Internet service provider) network, or when troubleshooting why inbound port forwarding does not work. If the router's "public" address is in 100.64.0.0/10, there is usually another NAT layer upstream.

Firewall tip: 100.64.0.0/10 is normally listed with addresses that cannot be routed globally. At the edge of an ordinary customer network, packets claiming to come from this range from the public Internet are usually dropped. If your provider actually uses CGN on your uplink, allow it only on the expected provider-facing side.