So it's not really about NAT although it ends up being a consequence—it's about having a truly private network "air gapped" from the public internet.
The person I replied to said that they only get a single v6 address. If that's true, it doesn't matter whether they have NAT or not; their network isn't going to have publicly-routable addresses either way.
If your network is air-gapped then no connections will be happening at all, in or out... and if you connect a router to both the Internet and to your network, and enable routing on it, then it's not air-gapped any more.
Well no shit. The NAT is a requirement for devices without a publicly routable IP because if my router just sends packets out with a source address being my 192.168.1.101 local IP, my ISP is most likely just going to drop the packets.
You know this, I'm sure, so I'm really unsure what point you're trying to make.
> The person I replied to said that they only get a single v6 address. If that's true, it doesn't matter whether they have NAT or not; their network isn't going to have publicly-routable addresses either way.
Correction: It will have ONE publicly-routable IP, and if I assign it to my router, but don't use NAT, then none of my devices on the network will be able to talk to the Internet, either in or out.
> Correction: It will have ONE publicly-routable IP, and if I assign it to my router, but don't use NAT, then none of my devices on the network will be able to talk to the Internet, either in or out.
Right, and then if you add NAT you'll be able to make outbound connections, but inbound connections will be unaffected and will still not work. So what is NAT doing here to prevent inbound connections, given that the exact same connections already didn't work before you were NATing?
NAT in its customary usage is a bit of a historical accident that as a side effect happens to make it basically impossible for non-technical people to expose their devices.
In the post I was replying to, the hosts were already unreachable (or... mostly unreachable, not completely unreachable) before NAT was even in the picture.
If I want all the computer on my network to have Internet access, I have two options: Each gets a publicly routable IP, which results in all computers being exposed to incoming connections unless I have a firewall, or I get a single IP which gets assigned to my router, use NAT, and all my devices are no longer exposed to incoming connections unless I go out of my way to configure port forwarding on the router.
So when I talk about the "side effect of using NAT", I really mean "side effect of using NAT instead of assigning public IPs to each computer on my network".
Does that help clear things up?