CGNAT (carrier-grade NAT)

Carrier-grade NAT (CGNAT) is the technique carriers use to put many subscribers behind a single shared public IP address, translating between that public address and the private addresses used inside their own network. Your phone thinks it has an address; the internet only ever sees the carrier's.

Why carriers do it

There are roughly 4.3 billion IPv4 addresses in existence and far more devices than that. Rather than give every handset its own public address, carriers share one address across a large group of subscribers and keep track of which connection belongs to whom. See IPv4 vs IPv6 for the underlying shortage.

Why it matters when you buy proxies

CGNAT is the single mechanic that makes a mobile proxy worth paying for. Because a carrier address is genuinely shared with ordinary customers, a site that blocks it also blocks paying users of that carrier — so most will throttle or challenge instead of banning outright. It also means a mobile IP is a poor fit for anything that needs inbound connections, since there is no way for the outside world to reach through the carrier's translation to your device.

A concrete example

Two people on the same carrier in the same city can show the same public IP to a website at the same moment. A site that treats one shared address as one user will get that wrong constantly, which is exactly why anti-fraud systems weight carrier addresses differently from a hosting range they can safely take out with a subnet ban.