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this post was submitted on 19 Apr 2024
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You're thinking of a firewall. NAT is just the thing that makes a connection appear to come from an IP on the internet when it's really coming from your router, and it's not needed with IPv6. But you would not see any difference with IPv6 without it.
That connection only "appears to come from" if I explicitly put a rule in my NAT table directing it to my computer behind the router doing the NAT-ing.
Otherwise all connections through NAT are started from internal->external network requests and the state table in NAT keeps track of which internal IP is talking to which external IP and directs traffic as necessary.
So OP is correct, it does apply a measure of security. Port scanning someone behind NAT isn't possible, you just end up port scanning their crappy NAT router provided by their ISP unless they have specifically opened up some ports and directed them to their internal IP address.
Compare this to IPV6 where you get a slice of the public address space to place your devices in and they are all directly addressable. In that case your crappy ISP router also is a "proper" firewall. Strangely enough it usually is a "stateful" firewall with default deny-all rules that tracks network connections and looks and performs almost exactly like the NAT version, just without address translation.
You end up just port scanning their crappy router on IPv6 as well because ports that are not opened are stuck at the firewall either way, no matter if you use IPv4 or IPv6.
Just because every device gets a public IP does not mean that IP is publicly accessible.
An advantage that IPv6 has against port scanning is the absurdly large network sizes. For example, my ISP gives me a /56 prefix, that is 4,722,366,482,869,645,213,696 IPv6 addresses. Good luck finding the used ones with the port open you need.
Even with just a /64 prefix you get 18,446,744,073,709,551,616 addresses, way outside the feasibility of port scanning.
realistically, it wouldnt surprise me if ISPs started NATing on residential IPV6 networks, just for the simplicity, but still allowed end users to assign their own IPs if they so pleased. Given the surge in shitty IOT devices, that's probably a good thing for most people. Though a firewall would also accomplish this as well.