Network Address Translation
Many networks can use the same private address blocks because the outside world never sees them. When data leaves the private networks its source address is changed to a unique address. This is called Network Address Translation, or NAT.
- NAT was originally developed to help companies change Internet Service Providers without having to manually renumber all their computers.
- NAT has never been standardized by the IETF or another Standards Development Organization, so each equipment vendor implements it in a slightly different way.
Devices behind a NAT cannot be directly contacted from the internet as their private IPv4 address can only be routed internally. That means the internal devices have to initiate connections out to devices on the internet. And the NAT has to track the flow of data, so that return packets get to the right local network device.
- 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