NAT / PAT, Port Forwarding (DNAT) & Stateful Firewalls
How can dozens of devices inside your home or office (192.168.1.x) share a single Public IP Address without getting their traffic mixed up?
And how does a Stateful Firewall let replies back in while keeping hackers out?
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The Switchboard Extension Analogy:
Your office only has one public phone number (203.0.113.5).
When Laptop (.10) and Phone (.11) both call out at the same time, the PAT (Port Address Translation) switchboard tags Laptop's call with Extension :60001 and Phone's call with Extension :60002.
When the outside server replies to :60002, the switchboard knows immediately to route it to the Phone!