Carrier-grade NAT
NAT run by the ISP between customers and the internet — on top of the home's regular NAT. Result: the customer has a private IP, shares one public IP with thousands of others. A workaround for IPv4 exhaustion before IPv6 adoption.
Uses the 100.64.0.0/10 space (RFC 6598), not 10/8 — so it doesn't collide with the customer's own LAN range. Mobile 4G/5G is almost always CGNAT, much fiber ISP too. Consequences: no port forwarding possible (hosting a server from home doesn't work), online-gaming peer-to-peer NAT traversal harder, IP-based geo-IP or bans hit several customers. Worked around with IPv6 (where available) or a VPN with a dedicated IP. Symptom: whatismyip shows a different IP than ip addr on the router.