NAT & Port Forwarding

Also Known As

TL;DR

Explanation

NAT (Network Address Translation): a router with one public IP maps multiple private devices (192.168.x.x, 10.x.x.x) by tracking source IP:port to destination. Port forwarding: manually map public_ip:port → private_ip:port for inbound connections. NAT traversal: WebRTC uses STUN to discover public IP and TURN as a relay when direct P2P fails. For PHP developers: ngrok creates a tunnel bypassing NAT so Stripe can reach localhost:8000.

Common Misconception

Why It Matters

Common Mistakes

• Relying on NAT as a security boundary
• Not using ngrok for local webhook development
• Hard-coding private IPs in configuration — not routable on internet
• Docker: forgetting -p host:container to publish ports

Code Examples

// Stripe webhook URL: http://192.168.1.100:8000/webhook
// Problem: 192.168.1.100 is a private IP
// Stripe cannot reach this from the internet — webhook never fires

// Use ngrok for local webhook testing:
// Terminal 1: php -S localhost:8000
// Terminal 2: ngrok http 8000
// ngrok output: https://abc123.ngrok.io -> localhost:8000
// Stripe webhook URL: https://abc123.ngrok.io/webhook

// Docker: expose port to host:
// docker run -p 8000:80 myapp
// nginx on port 80 inside container → localhost:8000 on host

References

• ↗ https://www.cloudflare.com/learning/network-layer/what-is-nat/

Tags

Related Terms