The four NAT types

Not all NAT is equal. Your router (or your ISP's carrier-grade NAT) falls into one of these:

1. Full Cone NAT — the friendly one. Once an internal address/port is mapped, anyone on the internet can send packets to it. P2P just works.
2. Address-Restricted Cone — inbound traffic only from IPs you've already contacted. Fine for most things.
3. Port-Restricted Cone — stricter: inbound only from the exact IP and port you've contacted. This is where some P2P starts failing.
4. Symmetric NAT — the strict one. Every outbound connection gets a different external port mapping. Direct inbound connections are essentially impossible, which is why two symmetric-NAT peers can almost never establish a direct P2P link.

If you've ever seen "NAT Type: Strict" on a console, you're usually looking at symmetric or port-restricted NAT.

Why it matters beyond gaming

• Remote NAS access — reaching your files from outside without relay services
• Self-hosted services — anything that needs inbound connections
• VoIP / video calls — WebRTC hole-punching success rates depend heavily on NAT combinations

Test yours in seconds

I built a free tool that detects your NAT type using multiple STUN nodes: https://17nas.com/nat-test.php?lang=en-US

It evaluates your public port-mapping behavior across several geographically distributed STUN servers and gives you a high-confidence NAT type result, plus guidance on what it means for P2P, remote access, and gaming. No sign-up needed.

If the result is symmetric NAT and you're on a home connection, it's worth checking whether your ISP uses CGNAT — in that case no amount of port forwarding on your own router will help, and IPv6 (if available) becomes your best path to real end-to-end connectivity.

Disclosure: I built this free tool. It's part of 17NAS, a small collection of free network utilities (IPv6 connectivity test, NAT type test, and a browser/IP timezone checker).

Two different data sources

The browser timezone comes from the operating system:

Intl.DateTimeFormat().resolvedOptions().timeZone
// e.g. "America/New_York"

The browser just reads the OS setting. It knows nothing about the network.

The IP timezone comes from a geolocation database. Your server looks up the user's IP, finds the registered country/region, and maps that to a timezone. It's an estimate about where the network exit is, not where the person is.

Why they disagree

• VPNs and proxies — the IP exit is in another country
• Corporate networks — the gateway sits at HQ, possibly on another continent
• Cloud VMs and remote desktops — the machine's region differs from the user's location
• ISP registration quirks — an IP block's registered address doesn't always match where it's actually used
• Manually changed OS timezone — only the browser side changes

None of this is broken. The two values answer different questions: "what did the user set?" vs "where does this IP appear to be?" Treating them as interchangeable is what causes wrong timestamps, confused scheduling UIs, and false positives in fraud detection.

A free tool that shows both side by side

I built a small free tool that puts both answers on one screen, so you can see the mismatch directly instead of guessing:

What Is My Timezone — Time Zone Checker | 17NAS

It shows:

• Browser timezone vs IP-derived timezone, side by side
• UTC offset and daylight saving status for each
• A converter for any two IANA timezones (DST-aware), plus a country comparison table
• No signup, runs entirely in the browser

Handy when you're debugging a timezone complaint ("the site shows the wrong time!") — check whether the user's browser and IP even agree before digging into your own code.

(Disclosure: I built this free tool.)