NAT (Network Address Translation) is a method used in networking to allow multiple devices on a local private network to access external networks (such as the internet) using a single public IP address. NAT translation works by modifying the IP address information in IP packet headers as they pass through a router or firewall device. This process ensures proper routing of traffic between internal hosts and external networks while preserving limited IPv4 address space.
How NAT translation works
Basic concept of address translation
NAT operates by translating the source or destination IP address (and optionally port numbers) of packets as they traverse between private and public networks. On outbound communication, NAT replaces the private IP address of the source device with the router’s public IP address. When a response comes back, NAT reverses the translation and forwards the data to the correct internal device.
This method allows multiple devices in a LAN (using private IP ranges like 192.168.x.x or 10.x.x.x) to share a single public IP on the internet, reducing the need for a unique public IP for every host.
Types of NAT translation
There are several types of NAT based on how translation is managed:
- Static NAT: Maps one private IP to one public IP. This is a fixed one-to-one mapping. It is used when a specific internal server (e.g., a web server) needs to be reachable from the outside.
- Dynamic NAT: Maps a private IP to a public IP selected from a pool of available public IPs. It uses a temporary mapping and is typically used when a large number of users share a limited set of public IP addresses.
- PAT (Port Address Translation): Also known as NAT Overload. It allows multiple devices to share a single public IP address by assigning a unique source port number to each session. This is the most common form used in home and small office routers.
Translation method and table management
Translation table
NAT devices maintain a translation table to keep track of which internal device is associated with which public IP and port combination. This table includes:
- Private IP and source port
- Translated public IP and port
- Destination IP and port
- Timeouts for session entries
When a packet is sent from a private device, the NAT router creates or updates an entry in this table, performs the IP and port translation, and forwards the packet. When the response comes back, NAT uses the table to map the destination back to the correct private IP and port.
Example of PAT in action
Imagine two devices on a LAN—192.168.1.10 and 192.168.1.11—accessing a website. The router might map their sessions like this:
- 192.168.1.10:12345 → 203.0.113.5:54321
- 192.168.1.11:23456 → 203.0.113.5:65432
Both share the same public IP (203.0.113.5), but different ports. The NAT device uses the port mapping to send returning packets to the correct internal host.
Advantages and limitations of NAT
NAT provides several benefits:
- Conserves public IP addresses by allowing many devices to share one.
- Adds a layer of security by masking internal IP addresses from the public internet.
- Allows for flexible internal addressing that doesn’t depend on ISP-assigned IPs.
However, it also introduces challenges:
- Some applications that require end-to-end IP transparency (like VoIP or certain VPNs) may not work correctly without NAT traversal support.
- Breaks traditional IP-to-IP communication, making protocols like IPsec harder to implement.
In modern networks, NAT is ubiquitous, especially with IPv4. However, with the gradual transition to IPv6, where every device can have a unique global IP, the reliance on NAT is expected to decrease.