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🧱 CGNAT Explained

CGNAT and Synology VPN Why Inbound VPN Fails and Your Honest Options

Some connections simply cannot receive visitors. If yours is one of them, every hour spent 'fixing the VPN' is wasted — here is how to know, and what actually works instead.

✍️ Technical Anshu🌐 Internet Connection⏱️ 8 minute read
🎥 Watch the full Synology VPN setup in the video, step by step

This page exists to save you a weekend. CGNAT is invisible, extremely common, and produces a perfectly configured VPN that never connects. First the two-minute test, then an honest explanation of why the wall is not yours to open, then the three real exits — ranked by effort, not by hype.

Router · WAN Status Page
Router WAN address100.72.14.9
Your public IP online49.207.x.x
Two addresses, not equalCGNAT
Port forwardingCannot work
When these two numbers disagree, the internet cannot reach your router at all — no setting on the NAS changes that.
1

The Two-Minute Test That Settles It

  1. Log in to your router and find its WAN or Internet IP address on the status page
  2. On any device, search the web for "what is my ip" and note the address shown
  3. Same address: you have a real public IP and normal VPN setup will work
  4. Different addresses — or the WAN address starts with 100.64 to 100.127, 10. or 192.168. — you are behind CGNAT

CGNAT means your provider shares one public IP among many customers and keeps a second router of its own between you and the internet. Many Indian broadband and fibre plans, and almost all mobile connections, work exactly like this.

2

Why No Setting Can Fix It

Port forwarding is an instruction on your router. With CGNAT, the first wall is the provider's router, which you cannot configure and which has no rule for you. Your phone's knock never arrives at your house at all. This is also why the failure is total: every forwarded service fails, not just the VPN.

  1. DDNS still works — and faithfully publishes an address that cannot be reached
  2. Changing VPN protocols, ports or apps changes nothing
  3. Anyone selling you a "CGNAT bypass port trick" for inbound hosting is selling you the provider's own wall
3

Option One — Ask the Provider for a Real Address

The clean fix lives with your ISP, and it is worth one phone call:

  1. Ask for a public IP or removal from CGNAT. Some providers do it free on request; some sell a static IP add-on for a small monthly fee
  2. Static IP has a bonus: the address never changes, so DDNS becomes a backup instead of a necessity
  3. If support does not understand "CGNAT", ask for "a public IP for my CCTV" — it is the same request and a very common one
4

Option Two — Use Roads That Travel Outward

If the provider will not give you a public IP, stop pushing inward and use connections your NAS starts itself — outbound connections pass CGNAT without any problem.

  1. QuickConnect reaches DSM and Synology apps through Synology's relay. It is not a VPN and your phone's other apps cannot use it, but for files and photos it honestly works
  2. Outbound tunnel services (the Tailscale class of tools) have the NAS dial out to a meeting point and your phone meet it there. This is a different architecture from VPN Server — judge it on its own terms, including its own accounts and limits
  3. A small rented server (VPS) as a meeting point is the advanced version of the same idea — real cost, real maintenance, full control
🔴 Say it plainly. Behind CGNAT, Synology VPN Server cannot accept connections from the internet. That is not a Synology defect and not your mistake — it is the shape of your connection. Pick the provider fix or an outward road, and stop re-checking settings that were never wrong.

🎥 Watch the Complete Process in the Video

Before rewatching any setup step, run the test on this page — the video's setup is correct, and CGNAT is the one thing it cannot override.

▶️ Watch Full Video
❓ Frequently Asked Questions

How do I know if my internet is behind CGNAT?

Compare your router's WAN IP with the public IP a website shows you. If they differ, or the WAN IP starts with 100.64–100.127, 10., or 192.168., you are behind CGNAT (or double NAT, which your own second router can fix).

Will a static IP from my provider fix the VPN?

Yes. A public static IP removes CGNAT from your connection entirely, and port forwarding works normally again. It also stops your address from changing, which makes remote access simpler in every way.

Does QuickConnect work behind CGNAT?

Yes. QuickConnect is an outbound relay connection, so CGNAT does not block it. It gives you DSM and Synology app access, but it is not a VPN — it does not put your phone inside your home network for other apps and devices.

This guide is based on the Technical Anshu Synology NAS VPN video • Stuck on any step? Ask in the video comments — you will get a reply there 🙏