NAT

 

NAT (Network Address Translation) is a networking technique that modifies the IP address information in packet headers while the packets pass through a router or firewall.

Its primary purpose is to map private (internal) IP addresses to public (external) IP addresses, and vice versa.

 

 

Real-World Example

Suppose two employees (e.g., Manager Hongcheol and Deputy Gayoung) each have private IP addresses like 192.168.0.x.

Using a NAT device, both can access the internet through a single public IP address, such as 121.165.151.200.

• From the internet's perspective, all requests appear to come from the same public IP.
• NAT keeps track of which internal device initiated each request using port numbers and translation tables.

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Key Functions of NAT

1. Use of Private IP Addresses
   •     Allows internal networks to use private IP ranges (e.g., 192.168.x.x, 10.x.x.x, 172.16.x.x ~ 172.31.x.x)
   •     Private IPs are not routable on the public internet and must be translated before leaving the local network.
2. IP Address Translation
   •     NAT converts private IPs to public IPs when data is sent out.
   •     During response, NAT maps the public IP back to the correct private IP.
3. Port Mapping (PAT: Port Address Translation)
   •     NAT uses port numbers to distinguish between multiple devices using the same public IP.
   •     Enables multiple hosts to share a single public IP address.
4. Security Benefits
   •     NAT prevents direct access from external hosts to internal devices.
   •     This acts as a basic firewall, providing an extra layer of security.

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Why NAT Is Widely Used

• NAT effectively addresses IPv4 address exhaustion by enabling many devices to share a single public IP.
• It improves security by isolating internal hosts from the public internet.
• As a result, NAT has become one of the most commonly used IP management techniques in modern networking.

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Summary:

NAT allows private devices to communicate externally using a single public IP, while offering address conservation, scalability, and basic security.