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CGNAT Detection

Detect whether your ISP has placed you behind Carrier-Grade NAT. Critical for self-hosting and port forwarding.

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What is CGNAT and why should you care?

Carrier-Grade NAT (also called Large-Scale NAT or NAT444) is when your ISP shares one public IPv4 address across many customers. Instead of giving you a real public IP, they put you behind a giant NAT box, similar to what your home router does for your devices — but at the ISP scale.

The IPv4 range 100.64.0.0/10 (RFC 6598) is reserved specifically for ISPs to use for CGNAT. If your "public" IP starts with 100.64 through 100.127, you're behind CGNAT.

Symptoms of being behind CGNAT

  • Port forwarding doesn't work — you don't have a unique public IP, so there's nothing to forward from. Self-hosting (game servers, home security cameras, NAS access, VPN servers) is broken.
  • VoIP/SIP issues — NAT traversal can fail or be unreliable.
  • Some games have issues — peer-to-peer matchmaking, console NAT type warnings.
  • Remote desktop / SSH from outside — won't work directly without a tunnel.
  • You appear to share an IP with hundreds of others — captchas, IP-based rate limiting, getting blocked from services for someone else's behavior.

What to do if you're behind CGNAT

  1. Ask your ISP for a real public IP. Usually free if you call; sometimes a small extra fee. Some ISPs require business-class service.
  2. Use IPv6. CGNAT only applies to IPv4. If your ISP offers IPv6 (most do now), services that support IPv6 work normally.
  3. Tunnel out. Services like Tailscale, ZeroTier, Cloudflare Tunnel, or a VPS+WireGuard let you reach your home network without needing a public IP on your end.
  4. Switch ISPs. Not every ISP uses CGNAT. Smaller fiber providers usually give real public IPs.
Cellular networks are almost always CGNAT

If you tether to your phone or use a cellular hotspot, you're almost certainly behind CGNAT. T-Mobile, Verizon, AT&T, and basically every mobile carrier uses CGNAT for cellular data. Don't try to self-host over a phone hotspot.