NAT “firewall”: Everything you need to know about network security

NAT (Network Address Translation) is a feature built into most routers, helping to manage traffic between your devices and the internet. It rewrites private IP addresses into a public one, making sure data gets to the right place.

While sometimes referred to as a “NAT firewall,” NAT doesn’t inspect or block traffic the way a traditional firewall does. Instead, it allows responses to requests from inside your network and ignores unexpected incoming data—creating a kind of natural barrier that limits direct access from the outside.

It also helps protect your privacy, because it means your router uses one public IP for all your devices. NAT then transmits incoming data from the public IP to the correct device inside your network. This means your private IP isn’t exposed to the internet.

In this guide, you’ll learn all about how NAT works. We’ll cover its benefits, limitations, and the setup process—whether for home, business, or gaming networks. Plus, we’ll explore NAT’s compatibility with VPNs and offer troubleshooting tips to help you optimize security and performance.

What is a NAT “firewall”?

NAT isn’t really a firewall—it doesn’t scan or filter traffic based on rules. Rather, it’s a system that helps route internet traffic to the right devices on your network and, as a result, prevents unsolicited external traffic from reaching them.

When you search for something online, your device sends a request using its private IP address. Since that address doesn’t make sense to servers on the internet, NAT steps in to translate it into your router’s public IP—something the internet can understand.

NAT also helps solve the IPv4 address shortage by allowing multiple devices on one network to share a single public IP address. An IPv4 address is a unique number—like 192.168.0.1—that identifies a device on the internet. But there’s a limited supply of these addresses (fewer than 4.3 billion), which has become an issue because of the rapid growth in internet-connected devices.

With NAT, each device on the network uses a unique internal IP address, but they all appear as the same public IP to the outside world, reducing the need for multiple public IP addresses.

How does a NAT “firewall” work?

NAT allows all your private devices to share one public IP address. This prevents those devices from being directly exposed to the open internet. That means outside sources can’t initiate a connection to your devices unless your device initiated contact first.

Imagine your home network is a company office. Inside, you’ve got different employees (your devices), each with their own phone extensions (private IP addresses). But the office only has one main phone line to the outside world (a public IP address).

NAT is like a receptionist. When someone inside makes a call to the outside (sending a request online), the receptionist notes who it came from and sends it out using the company’s main number. When a call comes back (like a reply from a website), NAT checks who requested it and directs it to the right extension.

Here’s how it works:

1. Making the request: Your device sends a request to a website, which includes your private IP address and a port number—information that identifies your device inside your local network.
2. Processing by NAT: Before sending the request to the internet, NAT replaces your private IP address with the router’s public IP address. It also records this translation so it knows how to route the response back to the correct device.
3. Website response: The website receives the request (now showing your public IP), processes it, and sends back the data—like a webpage or a file.
4. The router receives a response: When the response arrives, your router uses the saved record to determine which internal device requested it.
5. Delivering the data: NAT swaps the public IP address back to the private one and forwards the data to your device.

NAT effectively hides your internal IP addresses and drops unsolicited inbound traffic. However, it doesn’t actively inspect or filter data. That’s the role of a firewall, which analyzes both inbound and outbound traffic based on security rules.

In addition, traditional firewalls go further by monitoring connection states and enforcing broader security policies, offering more comprehensive control over network traffic. Learn how different types of firewalls work and what they protect against.

Types of NAT firewall configurations

There are several types of NAT, each suited for different network needs. Understanding how they differ can help you choose the right setup for performance, scalability, and security.

Here’s a breakdown of the differences between Static NAT, Dynamic NAT, and NAT Overload (PAT) configurations:

• Static NAT: Creates a fixed link between one private IP address and one public IP address. This means a specific device inside your network will always appear as the same public IP when it connects to the internet. It’s useful when you need external devices to reliably access internal services—like a web or email server.
  💡 Think of it like port forwarding all traffic from a specific public IP directly to one internal device.
• Dynamic NAT: Uses a shared pool of public IP addresses that are assigned to internal devices only when they need internet access. Like Static NAT, it creates one-to-one mappings—but these connections are temporary and change over time. This setup is useful when you expect multiple devices to be online but don’t need each to have a fixed public IP. It’s also the basis for how NAT was described earlier: the router dynamically remembers which request came from which device so it can route responses correctly.
• NAT Overload: A more advanced form of Dynamic NAT, NAT Overload lets multiple devices share a single public IP address by assigning each connection a unique port number. This ensures every device’s sessions remain distinct, even when using the same IP. It’s the most common NAT configuration, especially in home networks, where many devices connect to the internet through a single public IP.

What are Open, Moderate, and Strict NAT?

You might hear the terms Open, Moderate, and Strict NAT—especially in gaming. These labels describe how easily your device can connect to others online, based on how your network handles inbound traffic and port assignments.

• Open NAT (or Type A): You can connect easily to all other NAT types. This is ideal for gaming and voice chat because it allows smooth communication with other players. However, it offers less protection since more ports are open to the internet.
• Moderate NAT (or Type B): You can connect to Open and Moderate NAT users, but not Strict ones. It generally works fine, but you might run into occasional voice or game connection issues, and some ports may be closed, which can slightly limit performance.
• Strict NAT (or Type C or D): You can only connect to Open NAT users. This often leads to matchmaking problems, long wait times, or dropped connections. Strict NAT usually means your router is blocking ports or using more restrictive connection rules.

These terms are common on console networks like Xbox Network (previously Xbox Live), Nintendo Switch Online, or PlayStation Network. Behind the scenes, they often relate to how NAT is configured—whether it uses static or dynamic rules, or how it maps ports (full cone, restricted cone, or symmetric NAT).

How to set up NAT (step-by-step guide)

Below is a step-by-step guide on how to set up NAT on your router, create basic security rules, and test whether it’s working correctly.

Configuring a NAT “firewall” on a router

Most Wi-Fi routers come with built-in NAT settings. In this tutorial, I’ll walk you through the process using a Wi-Fi 6 TP-Link router, but the steps should be similar for other brands.

1. Open a web browser and enter your router’s IP address (e.g., 192.168.1.1 or 192.168.0.1) in the address bar. You can find this IP address in Network & Internet Settings on Windows or in System Settings on Mac.
   
2. Log in with your administrator username and password. If you’re having trouble finding your router IP info, we have a guide to help locate it.
3. Navigate to the Advanced/Settings section in your router’s configuration menu.
   
4. Enable NAT if it isn’t already active. Some models may not have a direct enable/disable toggle for NAT because it’s always on by default.
   
5. Save your settings.

Once you’ve finished configuring the NAT settings, reboot your router to apply the changes. Some routers may automatically restart after saving the settings.

Setting up security rules and traffic filtering

While NAT provides basic protection by hiding your internal IP addresses, it’s still important to set up additional security rules and traffic filtering to prevent unauthorized access to your network.

• Allow outgoing traffic only: Configure your firewall to allow only traffic that originates from inside your network. This ensures that responses to requests you send are allowed back in—but unsolicited requests are blocked.
• Block unsolicited inbound traffic: Explicitly set rules to drop any external data that doesn’t match an existing request.
• Configure port forwarding: If you run services like web or email servers that need to be reachable from outside your network, set up port forwarding. This tells your router to send specific types of traffic to the correct internal device, bypassing NAT’s default behavior of ignoring unsolicited requests.

How to test if NAT is working correctly

Here are a few simple ways to check that NAT is set up and functioning properly on your network.

• Check external IP: From any device on your network, open your browser and use our IP-checking tool to see your public IP. Then try the same on a second device. If both show the same public IP, your router is translating multiple private IPs into a shared public one—confirming NAT is active.
• Use ping and traceroute commands: While these tools won’t show your NAT configuration directly, they’re a quick way to confirm basic connectivity. If you’ve recently changed NAT settings, run ping or traceroute commands to make sure your device can still reach external servers.
• Firewall logs: Access your router’s admin dashboard (usually under Security, Advanced, or Logs) and review logs to verify that unsolicited inbound traffic is being dropped. This confirms your network is handling NAT and firewall rules as expected.

NAT network security

NAT provides a basic level of protection by hiding internal IP addresses and preventing unsolicited inbound connections.

How secure is NAT?

NAT improves your privacy by converting internal IP addresses into a single public IP, effectively shielding your devices from direct contact with the internet. Because inbound traffic must match a request from inside your network, it acts as a simple barrier against unexpected connections.

However, NAT isn’t a security solution. It doesn’t inspect incoming data or detect threats like phishing, malware, or other forms of attack. That’s why it should be used alongside additional security tools—like antivirus software, a VPN (such as ExpressVPN), and safe browsing practices—to keep your devices truly protected.

NAT vs. traditional firewalls: Security comparison

Compared to traditional firewalls, NAT focuses more on obscuring your internal network than actively blocking threats. A firewall inspects and filters traffic based on specific rules, often using deep packet inspection to block suspicious or malicious data.

NAT, by contrast, doesn’t analyze packet contents or apply security policies. Its main role is to prevent unsolicited inbound traffic by only allowing responses to requests that originated from within your network.

For full protection, NAT should be used alongside a dedicated firewall or intrusion detection system that can actively monitor, filter, and respond to security threats.

Common security risks of NAT firewalls and how to mitigate them

There are some risks associated with NAT firewalls. Here are some of the most common problems and how you can solve them.

NAT vs. VPN: What’s the difference?

NAT and VPNs serve very different purposes. NAT protects your devices by hiding internal IP addresses and blocking unrequested inbound traffic. It operates at the router level, allowing only responses to requests you initiate—like loading a webpage or downloading a file. Its main role is managing IP addresses and controlling how traffic flows within your local network.

A VPN, on the other hand, encrypts all your internet traffic and routes it through a secure server. It masks your IP address and location, protecting your online activity from websites, ISPs, hackers, and surveillance. VPNs are designed for privacy and security online.

Can NAT replace a VPN?

No, NAT can’t replace a VPN. While it helps limit unwanted connections, it doesn’t encrypt your data or protect your activity from being monitored. If you want true online privacy and security, a VPN is essential.

How to use NAT and VPN together for maximum security

For stronger online protection, use NAT and a VPN together. NAT helps protect your internal network by preventing unsolicited inbound traffic, while a VPN encrypts all internet traffic and hides your public IP address—keeping your data private and secure from outside observers.

Tip: If you want to learn more about the differences between a firewall and a VPN, read our comprehensive guide.

Some older VPN protocols—like IPsec—can run into issues with NAT, as they don’t always transmit enough connection metadata, which can lead to dropped or blocked connections. To avoid this, use NAT-friendly protocols like OpenVPN or IKEv2, both of which are supported by ExpressVPN and work reliably with NAT.

If switching protocols isn’t an option, look for a router that supports VPN pass-through, which allows VPN traffic to bypass NAT restrictions.

Common NAT-related issues and how to fix them

NAT is a crucial part of your network, but several issues can arise that impact performance and connectivity. Below are some common problems that might affect your network and how to address them.

• Incorrect NAT rules: If sessions aren’t being created properly, your NAT settings might be misconfigured. Open your router settings. Look for the NAT or port forwarding section and check if the correct devices and ports are listed. Make sure the internal device (like your PC or console) is mapped to the right port.
• NAT gateway can’t connect externally: If the device connecting your network to the internet can’t reach outside sites, it may be missing a route. Go into the router’s settings and make sure it has a “default route” or internet connection set up correctly. You can test by visiting a website or using the ping tool to check connectivity.
• Double NAT: Occurs when multiple NAT devices are used in a network, leading to complications with online gaming, VPN connections, and port forwarding. To resolve this, consider placing your networking devices in bridge mode or configuring them to avoid overlapping NAT functionalities.
• Internal device can’t reach NAT gateway: If a device on your local network can’t connect to the NAT gateway, check its network settings along with the gateway’s configuration to rule out setup problems. Make sure the device is getting an IP address automatically and that the “gateway” (usually your router’s IP) is correct. Reboot both the device and router if needed.
• Application Layer Gateway (ALG) is disabled: ALG helps certain services, such as VoIP or online games, work properly through NAT. If it’s disabled, those apps might not function as expected. In your router settings, look for a section called Application Layer Gateway. Turn on options like SIP ALG if you’re having issues with calling or gaming apps.
• Overlapping NAT configurations: When NAT settings conflict with existing IP addresses, connectivity issues can occur. Go to your NAT settings and make sure you’re not forwarding traffic to an IP that’s already in use by something else. It helps to use IPs like 192.168.1.x or 10.0.0.x to avoid overlap.

Is your NAT configuration blocking games or applications?

While NAT helps protect your network, it can sometimes interfere with online gaming—especially multiplayer or peer-to-peer connections.

If you’re experiencing lag, disconnection errors, or trouble connecting to other players, your NAT settings may be too restrictive. In this case, go to your router’s Advanced tab and look for NAT settings. Try enabling Universal Plug and Play (UPnP)—this feature automatically opens ports as needed, which can reduce NAT strictness and improve connectivity for games and apps.

How to fix port forwarding issues

If you’re having trouble with port forwarding, here are a few common steps to troubleshoot and resolve the issue.

Before you start: Make sure you’re not forwarding port 443—it’s used for secure web traffic and may be required to access your router’s admin panel. Forwarding it could block you from logging in.

• Verify port assignment: Log into your router’s admin panel and check that the correct internal IP address is entered for the device you’re forwarding to. Make sure the port numbers match the requirements of the app or service—some need the same external and internal ports, while others don’t.
• Review firewall settings: In the router’s firewall or security settings ,check for rules that might block the ports you’re trying to open. If needed, create a custom rule to allow traffic on the relevant ports.
• Disable conflicting settings: Features like DMZ, UPnP, or Double NAT can sometimes override or interfere with manual port forwarding. Try disabling these settings temporarily and test your connection again.
• Restart your router: After making changes, reboot your router to apply them. You can usually do this from the admin panel or by unplugging the router for 10 seconds and plugging it back in.

Does NAT slow down your internet?

NAT typically has minimal impact on internet speed. However, in some cases, it may cause slight delays—especially if the router is older or under heavy load. If you notice performance issues, check if your router has hardware acceleration enabled and consider upgrading to a more powerful model if needed.

NAT in business networks

NAT plays a crucial role in enterprise security by masking internal IP addresses and preventing direct access from external sources. It allows multiple devices to share a single public IP address, improving both network efficiency and security.

How does NAT protect enterprise networks?

By hiding internal IPs and blocking unsolicited inbound traffic, NAT reduces the risk of external attacks. It also helps enforce internal security policies by controlling how data flows between your internal network and the internet—making it an essential layer of defense in corporate environments.

NAT in VoIP communication: Benefits and challenges

NAT boosts VoIP security by blocking direct inbound access to internal VoIP devices. However, it can also create issues with voice or video call connections due to how it handles SIP (Session Initiation Protocol) traffic.

SIP is used to set up, manage, and end VoIP calls—and it often includes IP address information in the message itself. Since NAT rewrites IPs to protect your network, this mismatch can cause VoIP calls to fail or drop.

To fix this, you can:

• STUN (Session Traversal Utilities for NAT): Helps VoIP devices discover their public IP and determine the type of NAT in use, allowing direct connections when possible.
• TURN (Traversal Using Relays around NAT): If direct connections aren’t possible, TURN routes VoIP traffic through an external relay server to bypass strict NAT restrictions and ensure a stable connection.
• Port Forwarding: Manually opens specific ports for VoIP traffic, reducing connection issues while maintaining security.

By using STUN, TURN, or adjusting your port forwarding settings, you can ensure smooth VoIP calls without sacrificing security.

NAT best practices for home users and businesses

NAT is a key part of most networks and offers helpful protection by limiting unsolicited inbound traffic. To get the most out of it—and avoid common connectivity issues—follow these best practices:

For all users:

• Keep your router firmware up to date to patch vulnerabilities.
• Use strong, unique passwords for your router login and Wi-Fi network.
• Disable unnecessary remote access features like remote management unless absolutely needed.
• Review connected devices regularly and remove anything unfamiliar.

For home users:

• Use UPnP or port forwarding to resolve issues with gaming or video calls.
• Only forward the ports you need.
• Pair NAT with other tools like antivirus software and privacy-focused browsers.

For businesses:

• Segment your network to isolate critical systems and reduce exposure.
• Restrict outbound traffic where possible to limit unnecessary external connections.
• Monitor and log traffic to detect unusual behavior or misconfigurations.
• Use NAT with access controls and firewalls for stronger, policy-driven security.