It provides a lot of improvements actually. Stating the obvious, NAT isn't needed anymore. Also with modern Firewalls rules need to be written only once. At this point I'm just surprised why it's not adopted
False.
The most obvious case is multi-homing (for redundancy, fail-over, and policy-routing reasons) without an AS available and thus without BGP. In other words, a typical case when a user has a fiber connection and LTE as a backup. Then it is the router who should pick the correct source address, according to the link which is up.
Another reason is to deal with dynamic addressing from the ISP. Let's suppose we have an ADSL PPPoE connection, with prefix delegation. The modem connects, gets a prefix, devices grab IPs from it. Then a rat chews upon the line, causing a disconnection and a reconnection - but the ISP now delegates a different prefix. Or worse - the modem crashes and reboots, also picking up a different prefix. Devices are still not picking up such unexpected renumberings well. So they continue using old addresses, which don't work. Using a layer of network prefix translation solves the problem, as now only the router needs to be aware of the renumbering that has just happened due to the rat.
> False.
"IPv6 Multihoming without Network Address Translation"
Network Address and Port Translation (NAPT) works well for conserving
global addresses and addressing multihoming requirements because an
IPv4 NAPT router implements three functions: source address
selection, next-hop resolution, and (optionally) DNS resolution. For
IPv6 hosts, one approach could be the use of IPv6-to-IPv6 Network
Prefix Translation (NPTv6). However, NAT and NPTv6 should be
avoided, if at all possible, to permit transparent end-to-end
connectivity. In this document, we analyze the use cases of
multihoming. We also describe functional requirements and possible
solutions for multihoming without the use of NAT in IPv6 for hosts
and small IPv6 networks that would otherwise be unable to meet
minimum IPv6-allocation criteria. We conclude that DHCPv6-based
solutions are suitable to solve the multihoming issues described in
this document, but NPTv6 may be required as an intermediate solution.
The client would still need some smart steering to select the correct route no? Does the gateway invalidate the ethernet address somehow? But with NAT you don't need to worry about it.
Also consider a case where there is an ADSL modem (with the ISP giving out /56 via prefix delegation) and a home lab with virtual machines, that are behind a virtualization host, which grabs a subprefix (let's say a /64, separate from the main home LAN /64 prefix) for its VMs from the modem via DHCPv6. While there is indeed a mechanism for flash renumbering over SLAAC, which may work for devices in the home LAN, there is also a need to invalidate the subprefix delegated for virtual machines via DHCPv6 through the virtualization host. Last time I checked, this is not implemented anywhere.
The popularity of uPnP for automatic port forwarding should be a clue, anything that uses that is blocked by cgnat.
At-home hosting would open up tons of applications. You could have at home video cameras that are actually private (no third party connection). You could share photos with family and friends from a home photo frame - directly. There could be tons of applications that normal people would be interested in.