Free tool
CGNAT Checker
Find out instantly if your ISP is hiding you behind Carrier-Grade NAT — and what that means for your home server.
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Background
Everything you need to know about CGNAT
What is CGNAT?
Carrier-Grade NAT (RFC 6598) lets an ISP share a single public IPv4 address across many customers. Your router gets an IP in the 100.64.0.0/10 range — a private address space reserved for ISPs — instead of a real public IP.
Why it breaks home servers
When you port-forward on your router, that rule only works between your LAN and your router's WAN IP. Under CGNAT, that WAN IP is inside the ISP's network and unreachable from the open internet. Traffic destined for your home never gets past the ISP's NAT gateway.
What you can do
Ask your ISP for a public IP (sometimes free, sometimes a small fee). Alternatively, use a tunnel service such as Cloudflare Tunnel or Tailscale Funnel that proxies traffic from a public endpoint to your device — no port forwarding needed.
IPv6 is the long-term fix
IPv6 provides enough addresses that every device gets a direct public address — no NAT, no CGNAT. If your ISP offers IPv6 and your router supports it, IPv6 services are reachable from the internet just like any server.
Reference
How CGNAT is detected
IANA reserved the 100.64.0.0/10 block (100.64.0.0 – 100.127.255.255) specifically for ISP carrier-grade NAT, as defined in RFC 6598. If your public-facing IP falls in this range, your ISP is definitely using CGNAT.
| Range | Type | Implication |
|---|---|---|
| 100.64.0.0 – 100.127.255.255 | CGNAT (RFC 6598) | ISP carrier-grade NAT — no port forwarding possible |
| 10.0.0.0 – 10.255.255.255 | Private (RFC 1918) | Behind a router or VPN — port forwarding works on your router |
| 172.16.0.0 – 172.31.255.255 | Private (RFC 1918) | Behind a router or VPN — port forwarding works on your router |
| 192.168.0.0 – 192.168.255.255 | Private (RFC 1918) | Behind a router or VPN — port forwarding works on your router |
| Everything else (IPv4) | Public | Direct public IP — DDNS works, port forwarding works |
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