CGNAT
also: Carrier-Grade NAT, Large-Scale NAT, LSN
Carrier-Grade Network Address Translation (CGNAT) is a technique where an Internet Service Provider performs NAT on customer traffic, translating private IP addresses to shared public IPs, allowing ISPs to reduce their public IPv4 address consumption.
CGNAT operates at the ISP level rather than at individual routers. Multiple customers share the same public IP address, with the ISP's equipment tracking which traffic belongs to which customer using port ranges or other mechanisms.
This became necessary because IPv4 addresses are exhausted; CGNAT lets ISPs serve more customers with fewer public IPs. However, it creates problems: customers cannot run public servers, port forwarding becomes unreliable, and some peer-to-peer applications fail.
For example, an ISP might assign you the public address 203.0.113.5:5000-5999 while another customer gets 203.0.113.5:6000-6999, all sharing the same public IP.
CGNAT is distinct from standard consumer NAT—it's transparent to end users and controlled entirely by the ISP, making it difficult to work around.