Symmetric NAT: Why It Breaks Hole Punching

Symmetric NAT is the network address translation behavior that breaks peer-to-peer connections that rely on hole punching. If you are reading this because a game, VoIP call, or file transfer fails to connect even though both sides have internet access, the cause is often that at least one side is behind a symmetric NAT. This page explains what that means, how to confirm it, and what your options are—including the case where no fix exists because the network operator controls the NAT.

What Symmetric NAT Does

Network address translation (NAT) maps the private IP addresses and ports of devices on a local network to a single public IP address and a set of public ports. When a device sends a packet out, the NAT creates a mapping between the internal address/port and an external address/port. The behavior of that mapping is what distinguishes NAT types.

A symmetric NAT assigns a unique external port for each destination IP and port. If your device sends two packets to two different destinations, the NAT creates two separate mappings with different external ports. If it sends two packets to the same destination, it reuses the same mapping. This is in contrast to a cone NAT (full cone, restricted cone, or port-restricted cone), where the mapping is based only on the source address and port, regardless of destination.

The consequence: a symmetric NAT will only accept incoming traffic from a destination that the device has already sent a packet to, and only on the exact port that was assigned for that destination. Any other source is dropped.

Why Hole Punching Fails

Hole punching is a technique used by peer-to-peer applications to establish a direct connection between two devices behind NATs without a relay. The process works like this:

1. Each device connects to a public server (the rendezvous server) and learns its own public IP and port as seen by that server.
2. Each device sends a packet to the other’s public address, attempting to open a hole in its own NAT and to reach the other’s NAT.
3. If both NATs are cone types, the packets get through and a direct connection is established.

With a symmetric NAT, step 2 fails. When device A sends a packet to device B’s public address, the NAT creates a mapping that is only valid for B’s IP and port. But device B’s public address, as reported by the rendezvous server, is the address of B’s NAT, not the specific port that B will use for the direct connection. Because B’s NAT is also symmetric, it assigns a different port for the connection to A than it did for the rendezvous server. So A’s packet arrives at B’s NAT on a port that B never opened for A, and it is dropped. The same happens in reverse. The hole punching attempt fails, and the application falls back to a relay server, which adds latency and may be blocked or overloaded.

How to Detect a Symmetric NAT

The most reliable way is to use a STUN (Session Traversal Utilities for NAT) test. STUN servers are public and designed to reveal NAT behavior. Many online tools and applications (like WebRTC tests) perform this automatically. You can also run a manual test using a tool like stunclient or the stun command if you have it installed.

A simpler, though less precise, indicator is to check the NAT type reported by your router or by an application that displays it (e.g., some games show “Strict” or “Type 3”). However, that is not definitive, because the NAT type can be influenced by multiple layers—your router, your ISP’s carrier-grade NAT (CGNAT), or even a VPN.

If you want to be certain, run a STUN test from a device on your network. The test sends packets to multiple STUN servers and observes the external ports. If the external port differs for different destinations, you have a symmetric NAT.

What Can Be Done

If You Control the Router

If your router is the one performing symmetric NAT, you may be able to change its behavior. Many consumer routers offer a setting called “NAT type” or “Port Forwarding” that can be adjusted. Look for options like:

• Full cone NAT or Open NAT – This makes the NAT behave like a cone NAT, allowing hole punching.
• Port forwarding – Manually forward a specific port to your device. This bypasses the NAT’s mapping logic for that port, but it only works for a single port, and you must configure it for each application that needs it. It also requires that you have a public IP address (not behind CGNAT) and that your router supports it.
• UPnP (Universal Plug and Play) – Some applications can automatically request port mappings via UPnP. If your router has UPnP enabled, it may create a port forwarding rule automatically, but this does not change the NAT type itself; it just works around it.

Changing the NAT type may require disabling features like “Symmetric NAT” or “Strict NAT” if present. However, not all routers expose this setting. If you cannot find it, consult the router’s manual or the manufacturer’s support site.

If Your ISP Uses CGNAT

Carrier-grade NAT (CGNAT) is a large-scale NAT deployed by ISPs to share a limited number of public IPv4 addresses among many customers. If you are behind CGNAT, your router’s WAN IP is a private address (like 100.64.0.0/10), and the ISP’s NAT performs the translation. In that case, you have no control over the NAT behavior—it is symmetric by design, and you cannot change it.

To check if you are behind CGNAT, look at the WAN IP on your router. If it falls into the ranges 10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16, or 100.64.0.0/10, you are likely behind CGNAT. The 100.64.0.0/10 range is specifically reserved for CGNAT.

If you are behind CGNAT, your options are limited:

• Request a public IP from your ISP – Some ISPs offer a static or dynamic public IPv4 address for an extra fee. This gives you control over your own NAT.
• Use IPv6 – If your ISP provides IPv6, many applications can use it to bypass NAT entirely. However, not all applications support IPv6, and some networks still rely on IPv4.
• Use a relay service – Applications that fail to hole punch will often fall back to a relay. This adds latency but may be the only option if you cannot change the NAT.
• Use a VPN – A VPN can give you a public IP address (the VPN server’s), and the VPN provider’s NAT may be cone type. However, this adds overhead and may not be suitable for all applications.

When Nothing Can Be Done

If your ISP uses CGNAT and does not offer a public IP, and you cannot use IPv6 or a VPN, then there is no way to fix symmetric NAT. The NAT is owned by the carrier, and you have no control over it. The only workaround is to use applications that can work through a relay, or to accept the limitation.

Common Misconceptions

A frequent mistake is to blame the router when the real culprit is the ISP’s CGNAT. Many people spend time configuring their router, only to find that the problem persists because the NAT is happening upstream. This is a natural mistake because the router is the most visible network device, and its configuration interface often shows NAT settings. But if your WAN IP is a private address, the router’s NAT type is irrelevant—the ISP’s NAT is the one that matters.

Another misconception is that enabling UPnP will fix the problem. UPnP can create port mappings on your router, but if the ISP’s CGNAT is symmetric, those mappings are still behind a symmetric NAT, and hole punching will still fail. UPnP only helps if the router is the only NAT in the path.

Comparison Table

What People Get Wrong

A common error is to assume that symmetric NAT is a router setting that can be toggled off. Some routers do have a “symmetric NAT” option, but many do not, and even if they do, the term may not be used. The mistake is natural because the term “NAT” is often associated with the router, and users expect a configuration option. However, symmetric NAT is a behavior, not a feature that can be disabled on all devices. In many cases, the NAT is implemented in firmware and cannot be changed.

Another error is to try to fix the problem by forwarding ports on the router when the device is behind CGNAT. Port forwarding on your router only affects your router’s NAT; it does nothing for the ISP’s NAT. This is a natural mistake because port forwarding is a common solution for NAT issues, but it only works when you have a public IP.

Testing Your NAT Type

To confirm whether you have a symmetric NAT, you can use a STUN test. Here are a few ways:

• Use an online STUN test tool in a web browser (search for “STUN test”).
• Use the stunclient command-line tool if you have it installed. For example:

stunclient stun.l.google.com:19302

This will output your external IP and port. Run it multiple times with different STUN servers. If the external port changes for different servers, you have a symmetric NAT.

• On Windows, you can use PowerShell with the Test-NetConnection cmdlet to check your public IP, but it does not test NAT behavior. For a definitive test, use a STUN tool.

Conclusion

Symmetric NAT is a fundamental obstacle to peer-to-peer communication. Understanding how it works allows you to diagnose connection failures and determine whether a fix is possible. If you control the NAT, you may be able to change its behavior. If your ISP controls it, your options are limited, and in some cases, no fix exists. Knowing which situation you are in is the first step.