NAT Fundamentals — How and Why NAT Works

Why NAT Exists

NAT was never designed to be a security feature — it was created because IPv4 address space ran out. The idea was simple: let multiple devices share a single public IP address. But this "quick fix" became permanent infrastructure, creating both problems and accidental benefits.

How NAT Works

The Basic Translation Table

When a device behind NAT sends a packet:

1. Outbound packet: Source IP is private (e.g., 192.168.1.100:45678)
2. NAT device rewrites: Source IP to public IP, source port to unique port
3. Entry created: 192.168.1.100:45678 ↔ 203.0.113.5:61234
4. Return traffic: When response arrives at 203.0.113.5:61234, NAT looks up mapping and forwards to 192.168.1.100:45678

Private Network                    Internet
192.168.1.100:45678  ------>  203.0.113.5:61234 (rewritten)
                      <------  Response to 203.0.113.5:61234
                                           ↓
                              NAT table lookup → 192.168.1.100:45678

slug: "nat-fundamentals"

NAT Types (Critical for VoIP)

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