Are your servers available on the wider Internet? If so, I strongly encourage you to use DNS names for all connections to those servers. This has a bunch of benefits:
it avoids any weird edge cases with ATS
it make IPv6 compatibility easier
it enables VPN On Demand
Share and Enjoy
Quinn “The Eskimo!”
Apple Developer Relations, Developer Technical Support, Core OS/Hardware
let myEmail = "eskimo" + "1" + "@apple.com"
My servers are available on wider Internet. I know the benefits using domains instead of IPs for the connections. But our business is to provide basic SDK dealing with domain name resolution to other developers. So we have to provide services accessed using IP to avoid services of ourselves being domain hijacked.
So in this case, as you said, our services connections accessed through IP address will use HTTPS in the near future.