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Networking Fundamentals · Intermediate

Carrier-Grade NAT (CGNAT)

Large-scale NAT performed by an ISP so many subscribers share a small pool of public IPv4 addresses.

Definition

CGNAT (also called NAT444 or large-scale NAT) translates subscriber traffic, often using 100.64.0.0/10 between the ISP and customer routers, which may themselves perform NAT. It conserves IPv4 addresses but breaks inbound port forwarding and complicates logging, geolocation and some applications. IPv6 deployment is the long-term alternative.

Example

A gamer cannot host a server because their ISP places them behind CGNAT.

Exam tip

If port forwarding on a home router fails and the WAN address is 100.64.x.x, CGNAT is the cause.

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