I have the standard setup with computers behind a netgear router that has the "auto config" ipv6 option enabled. As I understand it, this means that it assigns local ipv6 addresses on LAN and when ipv6 is available on WAN, it will assign ipv6 addresses to all of the computers on the LAN such that they are all accessible on WAN ("the internet"). The ISP does not currently support ipv6 but will enable it in the future.
Also currently, there are consumer devices that are connected to the network (a printer, a file server, computers with open network shares). This is "fine" because there are ~10 devices connected to the LAN, and they all trust each other (you can access the files on other computers and/or print as long as you are connected to the LAN).
What interests me is that in theory, when the ISP enables ipv6, all of the devices that are connected to the LAN will be accessible on WAN (with the default netgear consumer router settings). Does this mean that anyone connected to the internet can now print and/or access files on computers connected to the LAN? I'm not unfamiliar with networking protocols, so it does seem possible in theory.
If so, this would seem to contradict the "plug and play" model where computers can automatically discover the printers on LAN. And honestly, making the LAN "trustless" is kind of scary.
This question is really about looking at this question within the context of the typical modem + wireless router setup. Everyone has assumed that they are safe because of NAT + ipv4, but I'm not so sure about that assumption anymore.
Note: this is the only "firewall" specified in the manual. It really does seem that consumer routers do not provide a firewall outside of NAT.