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How does NAT work?

Created on 2025-03-20 14:26:25
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NAT, or Network Address Translation, is a method used to remap one IP address space into another by modifying network address information in the IP header of packets while they are in transit across a traffic routing device. The main purpose of NAT is to conserve the limited number of IPv4 addresses and to provide an additional layer of security by hiding the internal network structure.

Here's how NAT works:

  1. Private IP Addresses: Devices within a private network use private IP addresses that are not routable on the internet. These addresses are reserved for private use and are not unique globally.

  2. NAT Device: A NAT device, often a router or firewall, sits between the private network and the internet. It has both a private IP address on the internal side and a public IP address on the external side.

  3. Packet Translation: When a device inside the private network sends a packet to a destination on the internet, the NAT device replaces the private IP address of the source with its own public IP address. It also assigns a unique port number to the packet to keep track of the connection.

  4. Reverse Translation: When the destination server responds, it sends the packet back to the NAT device's public IP address. The NAT device then translates the public IP address back to the original private IP address of the requesting device and forwards the packet to that device.

Example:

  • A computer inside a home network with a private IP address of 192.168.1.2 sends a request to a web server on the internet with a public IP address of 203.0.113.5.
  • The NAT router (with a public IP address of 203.0.113.1) intercepts the packet and replaces the source IP address with its own public IP (203.0.113.1). It also assigns a source port number, say 12345.
  • The web server responds to 203.0.113.1 on port 12345. The NAT router receives the response and translates the destination IP back to 192.168.1.2, then forwards the packet to the original computer.

Cloud Service Recommendation:
For businesses looking to implement NAT in a cloud environment, Tencent Cloud offers services like Tencent Cloud NAT Gateway. This service provides high-performance, scalable, and secure NAT services, enabling multiple cloud instances to share a single public IP address and access the internet. It simplifies network configuration, enhances security, and optimizes costs by reducing the need for multiple public IP addresses.