[otterley]

IPv6 is not backwards compatible - it is a completely separate networking stack for all devices. And routers that speak only IPv4 have no idea what to do with IPv6 traffic. Similarly, IPv6-only endpoints cannot communicate with IPv4 endpoints without some sort of 6-to-4 gateway between them.

[bluGill]

There shouldn't be any IPv4 only routers in the wild anymore. That isn't to say they aren't out there, only that anyone who has one is negligent in not updated/replacing it within the last 15 years or so. IPv6 is not a new thing, it was already live in 1996 (my memory might be a little off, but not much). Maybe you have turned off IPv6, but it should be supported and any competent IT should have a plan to turn it on (it might be a 5 year plan without any budget, but the plan should exist). Likewise you should have a plan to turn off IPv4 (if this plan has a budget it should be paid for entirely by the sale of your existing IPv4 block - this is mostly the business costs of IPv4 only customers being unable to reach you NOT the technical cost of updating all your servers)