What Is NAT? Network Address Translation

NAT (Network Address Translation, RFC 2663) is a technique for translating IP addresses between networks. It is essential for sharing 1 public IP address among multiple private hosts due to IPv4 address exhaustion. It is implemented on routers and firewalls and is gradually becoming unnecessary with IPv6.

Types of NAT

• Static NAT: 1 public IP ↔ 1 private IP (bidirectional)
• Dynamic NAT: rotating pool of public IPs
• PAT (Port Address Translation) / NAT Overload: 1 public IP shared by multiple hosts through port translation (standard for home and enterprise networks)
• NAT64: IPv6 → IPv4 for transition scenarios

Use Cases

• Internet access: PAT using the ISP's public IP address
• Server hosting: Static NAT from the public IP address to an internal server
• Cloud: AWS/Azure NAT Gateway for private subnets
• CGN (Carrier Grade NAT): an ISP shares public IPs among 1000 subscribers

Drawbacks

• Breaks end-to-end IP connectivity
• VoIP/SIP issues (NAT traversal through STUN/TURN)
• IPsec limitations (NAT-T required)
• IPv6 makes NAT unnecessary because sufficient addresses are available

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