What about a small business with Fibre, and a cellular backup? Both IPv6. How do I ensure my PCs only use the Fibre link until its not working, and only then us

What about a small business with Fibre, and a cellular backup? Both IPv6. How do I ensure my PCs only use the Fibre link until its not working, and only then use Cellular? Right now that just works out of the box with IPv4 and NAT. IPv6 also makes it difficult to plug and play new devices and install new networks. Example: The business needs to separate their Office PCs and their payment terminals (PCI DSS). Easiest way is to plug a firewall behind the existing LAN, and hang the terminals off the new firewall. You can't do that with IPv6 without the original router supporting DHCP6-PD, which it likely won't; or the ISP hasn't routed a large enough subnet to it to delegate smaller chunks. This is a mom and pop business that knows next to nothing about IT and shouldn't be penalised for that. In an ideal world where everyone has full knowledge of their systems and networks and can work with their providers to precisely accommodate their networking needs, IPv6 would work. I don't believe with the limited time, resources, and money we have that it is possible. Ultimately it's down to who caves in first. If the ideal solution requires £$€3000/yr more than another solution, and the other solution solves all the issues the business cares about, why would they spend the money to remove NAT?