I was about to disagree with you and say that it indirectly says to use DNS through RFC 3986. But you're right! RFC 3986 says to (unless otherwise directed by

I was about to disagree with you and say that it indirectly says to use DNS through RFC 3986. But you're right! RFC 3986 says to (unless otherwise directed by the URI scheme) to use the operating system's registered name resolution mechanism: Instead, it delegates the issue of registered name syntax conformance to the operating system of each application performing URI resolution, and that operating system decides what it will allow for the purpose of host identification. A URI resolution implementation might use DNS, host tables, yellow pages, NetInfo, WINS, or any other system for lookup of registered names. It's just a convention that all modern operating systems (primarily) use DNS!