Nat Pat

NAT and PAT — IP Services (10%) NAT and PAT are tested on virtually every CCNA exam — expect questions on address terminology, the three NAT types, and PAT configuration syntax. Getting the four address terms wrong is one of the most common ways candidates lose easy points in this domain. --- ## Why NAT Exists IPv4 address exhaustion created a real-world problem: organizations have far more devices than public IP addresses. Network Address Translation (NAT) solves this by allowing private (RFC 1918) addresses inside a network to be mapped to one or more public addresses when communicating with the internet. Without NAT, every device would need a globally routable address — practically impossible at scale. --- ## The Four NAT Address Terms This is the #1 exam vocabulary trap. Cisco uses a precise four-term system. Every term is tested. | Term | Meaning | Think of it as… | |---|---|---| | Inside Local | Private IP of the internal host (before translation) | The host's *actual* address | | Inside Global | Public IP representing the internal host (after translation) | The host's *translated* address seen by the internet | | Outside Global | Public IP of the external destination (the internet server) | The server's *real* address | | Outside Local | How the external destination appears to the inside network | Usually the same as Outside Global (unless doing double NAT) | Memory trick: "Inside/Outside" describes *which side of the router* the address belongs to. "Local/Global" describes whether the address is *private (local)* or *public (global)*. --- ## The Three NAT Types ### Static NAT A one-to-one permanent mapping between one inside local address and one…