Unlocking NAT: How Private Networks Share a Single Public IP

Public IP Addresses and Private IP Addresses

Public (Internet) IP addresses are globally unique and routable, while private IP addresses (reserved in the A, B, and C classes) are used only within internal networks and cannot be routed on the public Internet.

A‑class private range: 10.0.0.0 – 10.255.255.255 B‑class private range: 172.16.0.0 – 172.31.255.255 C‑class private range: 192.168.0.0 –

192.168.255.255

NAT Types

Static NAT, Dynamic NAT, NAPT, and Easy IP enable private networks to access the public Internet.

Static NAT

Static NAT maps each private address to a fixed public address on the router, establishing a one‑to‑one correspondence without using an address pool.

Dynamic NAT

Dynamic NAT uses an address pool of public IPs; the router assigns a free public address to a private host when it initiates traffic, releasing it when the session ends.

NAPT

Network Address and Port Translation (NAPT) extends NAT by also translating transport‑layer ports, allowing many private hosts to share a few public IP addresses through one‑to‑many mappings, thereby conserving public IP space.

Easy IP

Easy IP works like NAPT but without a dedicated address pool; the router’s own interface address is used as the public IP for translation, suitable for connections that obtain dynamic IPs via DHCP or PPPoE.

NAT Server (Port Mapping)

A NAT server (port mapping) links a public IP : port pair to a private IP : port pair, enabling external users to reach internal services.

Map the internal web server’s IP : port (e.g., 192.168.0.2:80) to the router’s public IP : port (e.g., 12.xx8:43333).

External clients connect to the public IP : port; the router forwards traffic to the internal server.

The router translates destination addresses and ports according to the NAT table.

This allows Internet users to access services hosted inside the private network.

Advantages and Disadvantages of NAT

Advantages

1. Reduces consumption of scarce public IP addresses by allowing many private hosts to share a few public IPs, especially with NAPT.

2. Simplifies ISP changes; internal addressing remains unchanged.

3. Enhances internal network security by hiding private addresses from the public Internet.

Disadvantages

1. NAT processing adds latency and consumes router resources because packets must be rewritten at both network and transport layers.

2. Source IP translation obscures original private addresses, making end‑to‑end tracing difficult.

3. Public networks cannot initiate connections to private hosts without explicit port mapping.

4. Certain applications (e.g., IPsec) may not function correctly through NAT.