NAT

NAT (Network Address Translation) is the mechanism a router uses to let every device on a private network — laptops, phones, smart TVs — share a single public IP address when talking to the internet, the same sharing mechanism Cisco’s own NAT FAQ walks through. The router keeps a translation table mapping each internal device’s private address and port to a public port, rewriting packets in both directions so replies reach the right device. Without NAT, every device would need its own public IPv4 address, which the internet ran out of long ago — the RIPE NCC, the regional internet registry for Europe, the Middle East and parts of Central Asia, exhausted its remaining IPv4 pool in November 2019. Home routers do one layer of NAT; some ISPs add a second layer on top, called CGNAT, which shares one public address across multiple customers’ routers rather than just one household’s devices. Opening a specific inbound connection through NAT usually means configuring port forwarding manually; a related but distinct translation, NAT64, translates between IPv6-only and IPv4-only networks rather than sharing one IPv4 address among IPv4 hosts. The private address ranges NAT translates from are reserved in RFC 1918 (10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16), and also appear as “Private-Use” entries in IANA’s IPv4 Special-Purpose Address Registry; the translation behavior itself is described in RFC 3022, the original Traditional IP Network Address Translator specification.

NAT is part of AdminKit’s IP & network cluster; the full AdminKit glossary lists other related terms.