Port Address Translation (PAT) is a specific type of Network Address Translation (NAT) used in networking, especially with IPv4 addresses. It allows multiple devices on a private network to share a single public IP address when accessing the internet.
Understanding PAT
PAT is crucial because it helps conserve IPv4 addresses, which are becoming scarce. Instead of assigning a unique public IP to each device on a private network, PAT allows them to share one.
How PAT Works
- Private IP Addresses: Devices within a local network use private IP addresses, which are not routable on the public internet.
- Public IP Address: A router with PAT is assigned a single, public IP address that is visible on the internet.
- Port Mapping: When a device on the local network sends a request to the internet, the PAT-enabled router replaces the private IP address with its public IP address. Additionally, it assigns a unique port number from its pool of available ports to keep track of different connections. This is how multiple connections using the same public IP address can be differentiated.
- Reverse Translation: When data returns from the internet, the router uses the port number to identify the original device on the local network and forwards the traffic.
Key Differences Between NAT and PAT
While PAT is a form of NAT, there are key differences:
| Feature | NAT | PAT |
|---|---|---|
| IP Addresses | 1-to-1 mapping of private to public | Many-to-1 mapping of private to public |
| Use Case | Assigning public IPs to few internal hosts | Sharing one public IP among many internal hosts |
| Port Use | No port translation | Uses port numbers to distinguish connections |
Practical Insights
- Home Routers: Home routers commonly use PAT to connect multiple devices like computers, smartphones, and gaming consoles to the internet via one public IP address assigned by the Internet Service Provider.
- Cost Savings: By allowing multiple devices to share a single public IP, PAT significantly reduces the need for public IP addresses, which can be costly.
- Security: PAT provides a basic level of security by hiding the private IP addresses of internal devices from the public internet.
Examples
- Imagine a home network with multiple computers, all using private IP addresses (e.g., 192.168.1.x). The router has a single public IP address assigned by the ISP. When these devices access websites, the router uses PAT to translate the internal addresses to its public IP, along with different port numbers for each device's request, enabling all of them to browse the internet simultaneously.
