A quick note here that if what you’re talking to is Fly.io apps talking to other Fly.io apps in your organization, you can use fly-app-name.internal to find their 6PN private IPv6 addresses. A query for just fly-app-name.internal will give you the address of every instance in your deployment; you can narrow it down with region.fly-app-name.internal (like nrt.foo.internal), or with top3.nearest.of.foo.internal (or top1, or top10).
This only works for communication inside of Fly.io; you can’t use it to connect to your app from a random host on the Internet.