NAT firewall: What is it, and how does it improve network security? (2024)

How does a NAT firewall work?

To understand network address translation (NAT) firewalls, we first need to understand what a firewall is and what it does. We might use a simple analogy to explain. Imagine your computer is a busy CEO. That would make a firewall the CEO’s secretary. The secretary sorts the mail and screens calls and makes sure that the only mail and messages that get through are those that the CEO actually wants to receive.

Similarly, when you browse online or send emails, your firewall stands between your local network and the internet, allowing only the information you requested to enter while blocking unrecognized or potentially harmful internet traffic.

A network address translation (NAT) firewall operates on a router to protect a private network. It works by only allowing internet traffic to pass through if a device on the private network requests it. A NAT firewall protects the identity of a network and doesn’t show internal IP addresses to the internet.

When your router connects to the internet, it’s assigned a single public IP address. This address is necessary for communicating with web servers, while each device connected to the router has a private IP address that can’t directly interact with external servers. NAT bridges this gap by managing the flow of traffic.

1. Your device sends a request to a web server through data packets. These packets include information such as the sender and destination IP address, a port number, and the requested information.
2. The request passes through the router’s NAT firewall, which replaces the private IP with the router’s public IP and logs the change.
3. Data packets reach the web server and get the necessary information.
4. The information travels back to the router. Now it’s NAT’s job to send the information back to the device that requested it. Otherwise, every connected device would receive the same information. NAT uses its forwarding table to determine who requested this data.
5. NAT changes the data packet’s public IP to its previous private IP and sends it to the requested device.

For more information, check out our YouTube video explaining how NAT firewalls work:

Types of NAT firewall configurations

NAT firewalls come in three main types, each with its own purpose – static NAT, dynamic NAT, and port address translation. Let’s explore each of them in more detail.

• Static NAT. With this type of NAT, every internal private IP address is linked to a unique external public IP address. This process is also called one-to-one mapping. It ensures that every internal device always uses the same public IP address. Static NAT is often used for services that need consistent external access, such as web hosting or email servers.
• Dynamic NAT. With this type of NAT, several private IP addresses are mapped to a set of public IP addresses. Unlike static NAT, instead of having a fixed public IP for each internal device, each device gets a different public IP when it connects to the internet. This setup works well when you have a known number of users who will be online at a specific time, but the particular devices might change.
• Port address translation (PAT), or NAT overload. It lets many internal IP addresses share a single public IP address but with different port numbers. This way, the devices share one IP address, but the sessions are still unique for each device. This method is mostly used in home networks.

Setting up a NAT firewall

Setting up a NAT firewall can improve the security and performance of your home or business network. Here are general guidelines for how you can do so:

1. Access your router’s configuration page. Open the web browser and enter your router’s IP address in the address bar. Log in with your admin credentials.
2. Find the NAT settings. Go to the firewall or NAT section in your router’s settings, usually found under “Advanced settings” or “Network.” Enable the NAT firewall.
3. Set up port forwarding rules. Define the devices and ports that need specific configurations. This step ensures that traffic is properly routed to the correct devices on your network.
4. Save your changes. After configuring your NAT settings and port forwarding rules, save the changes. If necessary, restart your router to apply the new settings.
5. Test connectivity. Check both external and internal devices to ensure that everything is working as expected.

Note: The process may vary depending on your router model and platform, so check your router’s instructions if you encounter any issues.

Advantages and disadvantages of using a NAT firewall

Using a NAT firewall provides several benefits for network security and management, but it also comes with certain limitations. The table below breaks down the main advantages and disadvantages of using a NAT firewall.

Common issues and troubleshooting

When using a NAT firewall, you might encounter some issues. Here’s a look at the most common problems and how to troubleshoot them:

• NAT is configured incorrectly. If you fail to set up sessions, it might be because the NAT settings are wrong. Double-check your NAT rules and make sure you configured them correctly.
• The NAT gateway fails to connect. If the NAT gateway (the device that connects your network to the internet) can’t access external sites, it may not have a route to follow. Make sure the gateway has the right paths to connect externally.
• The network access control list (ACL) is configured incorrectly. ACLs are rules that control what traffic is allowed on your network. If they’re set up incorrectly, traffic might be blocked. Review these rules to ensure the necessary traffic is allowed.
• An internal host can’t connect to the NAT gateway. If a device on your network can’t connect to the NAT gateway, there might be a setup problem. Check the network settings on both the device and the gateway.
• The application layer gateway (ALG) is disabled. ALG is a feature that helps certain applications (like VoIP or online games) work correctly with NAT. If it’s turned off, these apps might not function properly. If needed, enable ALG in your firewall settings.

You can also ensure that your router or firewall’s firmware is up to date. Updates often include fixes for NAT-related issues, and regularly checking for and applying them can prevent many common problems.

NAT and VPNs

Some argue that a VPN shouldn’t be used with NAT. Why? A VPN encrypts your traffic before it reaches the internet, making it indecipherable. The NAT needs to receive some information about that traffic to do its job. Some older or obsolete VPN protocols, like PPTP and IPsec, interfere with NAT because they don’t forward enough information and can be blocked as a result. To solve this problem, your router needs a VPN passthrough.

The good news is that most routers have built-in VPN passthroughs. Even if they don’t, most popular VPN providers offer more advanced protocols that do not require passthroughs because they are designed to work smoothly with NAT. NordVPN, for example, no longer uses these outdated protocols and even uses built-in stateful and NAT firewalls on its servers.