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> NAT isn't needed anymore.

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.

View on HN

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.