PlantUML DiagramPAT Flow
A structured lesson workspace with readable content, hands-on examples, and a clean path to completion.
| NAT type | Mapping | Use case |
|---|---|---|
| Static NAT | 1 private → 1 public, fixed | Servers that must be reachable from outside |
| Dynamic NAT | 1 private → 1 public, from a pool | Rarely used; similar to static but pooled |
| PAT / NAT overload | Many private → 1 public, differentiated by port | Every home router, most enterprise networks |
| Static PAT | 1 private:port → 1 public:port | Port forwarding, service publishing |
PAT is the workhorse. One public IP can support tens of thousands of simultaneous connections because each is differentiated by source port. That's how an entire home network or a small office shares one public address without conflict.
For most networks, Outside Local and Outside Global are the same (NAT doesn't rewrite the destination). Inside Local and Inside Global differ whenever traffic crosses the NAT boundary. Knowing the terms helps when you're reading `show ip nat translations` output.
NAT breaks the end-to-end principle. It hides internal topology, breaks protocols that embed IPs in payloads (SIP, FTP, some games), and makes inbound connections require explicit configuration (port forwarding). IPv6 was designed without NAT for exactly these reasons — but NAT is so entrenched that even IPv6 deployments sometimes use it (NAT66).
On your home router, look at the NAT table (`show ip nat translations` on Cisco or the web UI). You'll see every active connection — browser tabs, phone apps, streaming services — each with its own port mapping. That's PAT making the internet work for a dozen devices on one address.
Finish the lesson once you have worked through the material. This awards ★ 15 XP.