CGNAT
acronymISP & telecom
Carrier-Grade NAT: the ISP shares one public IPv4 address among many subscribers, translating at provider scale because the addresses ran out.
Breaks inbound connections, port forwarding, and some games; the strongest everyday argument for IPv6.
Carrier-grade NAT is what internet providers deployed when public addresses ran out: subscribers receive a private address, and the provider translates many of them onto each public address it still holds. It bought the industry years of extra runway and pushed a set of costs onto users who never chose it.
The visible effects are all consequences of sharing. Inbound connections are impossible, so hosting anything from home, or running peer-to-peer applications and some game consoles, breaks or requires relays. Reputation becomes collective: when one subscriber behind a shared address misbehaves, the block or CAPTCHA lands on everyone sharing it. And attributing activity to a subscriber now requires the provider's translation logs with precise timestamps and port ranges, which is why legal requests moved from asking about an address to asking about an address, a port and a moment.
The clean exit is IPv6, where the address shortage that motivated all of this simply does not exist. Providers running dual stack let native IPv6 bypass the translation entirely, which is why enabling it often fixes the exact symptoms people blame on their connection.