CGNAT Carrier-Grade NAT
NAT operated by the ISP — multiple customers share one public IPv4. A consequence of IPv4 exhaustion.
If you're behind CGNAT you have neither a public IP nor a stable outbound port mapping. Many mobile operators and fibre ISPs run it. Consequences: no inbound traffic works (port forwarding, hosting from home, certain P2P games, BitTorrent); STUN/TURN is required for WebRTC. RFC 6598 reserved 100.64.0.0/10 as address space specifically for CGNAT. Fix: IPv6 (but adoption is dragging).