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> How could I reliably tell if I'm behind a CGNAT or not? If I poll one of those "what is my IP" websites, and the IP my router has assigned from the wan matche

by tomxor 3y ago

> How could I reliably tell if I'm behind a CGNAT or not? If I poll one of those "what is my IP" websites, and the IP my router has assigned from the wan matches the what is my IP then I'm fine, I'm not behind one, right? My layman understanding is that the WAN IP on your router will match the "what is my ip" website, but the problem is that it will also match many other people's routers. Basically imagine your home router and NAT has been moved into the ISP and is serving thousands of end users as well as you... you can't just configure the ISP's CGNAT to forward port 80 to you alone. I'm not sure if there is a definitive way to know if you are behind CGNAT, however its definitely the case on all mobile networks.

deleted 3y ago
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hunter2_ 3y ago
> the WAN IP on your router will match the "what is my ip" website, but the problem is that it will also match many other people's routers That's interesting. So the ISP is directing packets at the various customer routers using some other identifier than the WAN IP the router sees? MAC address or something?
- dwattttt 3y ago
  I don't specifically know, but typical NAT can use stateful flow information, i.e. "a UDP packet originated from inside the NAT to external IP/port X, packets from that external IP/port should be routed to the originating NAT/customer" (see https://en.m.wikipedia.org/wiki/UDP_hole_punching https://en.m.wikipedia.org/wiki/UDP_hole_punching)
- LilBytes 3y ago
  Some ISPs use MAC address filtering. HFC (Hybrid Fiber Coaxial) and FttN (Fiber to the Node) are such examples. The rest of them if you don't have a static IP or an internet routable address, use NAT (Network Address Translation) and in some even more exotic environments they use Port Address Translating (PAT), but it's been many years since I've seen PAT in any way at commercial, residential or enterprise setups in ISP space. But PAT is quite common in the reverse and forward proxy space.

by tomxor 3y ago

deleted 3y ago

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hunter2_ 3y ago

> the WAN IP on your router will match the "what is my ip" website, but the problem is that it will also match many other people's routers That's interesting. So the ISP is directing packets at the various customer routers using some other identifier than the WAN IP the router sees? MAC address or something?

dwattttt 3y ago
I don't specifically know, but typical NAT can use stateful flow information, i.e. "a UDP packet originated from inside the NAT to external IP/port X, packets from that external IP/port should be routed to the originating NAT/customer" (see https://en.m.wikipedia.org/wiki/UDP_hole_punching https://en.m.wikipedia.org/wiki/UDP_hole_punching)
LilBytes 3y ago
Some ISPs use MAC address filtering. HFC (Hybrid Fiber Coaxial) and FttN (Fiber to the Node) are such examples. The rest of them if you don't have a static IP or an internet routable address, use NAT (Network Address Translation) and in some even more exotic environments they use Port Address Translating (PAT), but it's been many years since I've seen PAT in any way at commercial, residential or enterprise setups in ISP space. But PAT is quite common in the reverse and forward proxy space.