Carrier Grade NAT

Some large access networks don’t have a unique IPv4 address to give each subscriber. They cannot number their access network with private IPv4 addresses because those are already used on millions of private networks, like home Wi-Fi networks. So, they need to share a small pool of unique addresses with a large number of subscribers.
- About 4 million IPv4 addresses have been reserved as Shared Address Space for use in CGNAT.
- CGNAT subscribers don’t have the full range of ports available to them. Subscribers with a unique IPv4 address have 64,512 ports available to them. But CGNAT subscribers might only have 1,024 ports.
Carrier Grade Network Address Translation (CGNAT), or Large Scale Network Address Translation (LSNAT) is a way to do this. Often each subscriber is assigned a range of IP ports. There are 65,536 ports, which are a bit like the channels on a radio.
Subscribers on a CGNAT network will generally experience two layers of network address translation. Their device gets a private address and its packets are translated to the shared address. This then gets translated to the unique address by the access provider’s infrastructure.
- 192.168.0.0/16 is a block of 65,536 addresses in the former Class C space.
- 172.16.0.0/12 is a block of about 1 million addresses in the former Class B space
- 10.0.0.0/8 is a block of about 16 million addresses in the former Class A space