this post was submitted on 16 Aug 2024
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IPv6 has two main types of non-broadcast addresses to think about: link-local (fe80::) and public.
A device can self-assign a link-local address, but it only provides direct access to other devices connected to the same physical network. This would be used for peer discovery, such as asking every device if they are capable of acting as a router.
Once it finds the router, there are two ways it can get an IP address that can reach the wider internet: SLAAC and DHCPv6. SLAAC involves the device picking its own unique address from the block of addresses the router advertises itself as owning, which is likely what you're concerned about. One option for ensuring a device can't just pick a different address and pretend to be a new device is by giving it a subset of the router's full public address space to work with, so no matter what address it picks, it always picks something within a range exclusively assigned to it.
Edit: I butchered the explanation by tying to simplify it. Rewrote it to try again.
In most cases, the router advertises the prefix, and the devices choose their own IPv6. Unless you run DHCPv6 (which really no-one does in reality, I don't even think android will use it if present).
It doesn't allow firewall bypass though, as the other commenter noted.
Yeah, I butchered my answer by trying to simplify the process. I rewrote it in a hopefully more accurate but still simple to understand way.
Yep, it's all good. In my opinion, IPv6 routers should just be dropping incoming connections by default. If you want to run services you give your machine a static IPv6 and open ports on that IP/port specifically. It's actually easier than NAT because you don't need to translate ports and each IP can use the same ports (multiple web servers on 80/443).
I do agree that the average joe is going to expect NAT level security by default and that would provide that.
I absolutely agree with you on all points here.
From a security perspective, allowing all incoming connections by default is unnecessarily exposing devices to a hostile environment. The average Joe isn't going to understand the risk unless somebody explained it as "it's like posting your home address on 4chan and hoping nobody manages to pick your front door lock," and they're likely never going to take advantage of the benefits that come from having their device be globally reachable.
Another benefit to not having to deal with NAT is that you can actually host services using the same protocol (e.g. HTTP) on multiple machines without having to resort to alternate port numbers or using a proxy with virtual host support.