[jinnko]

> All URLs or URNs are URIs, but not all URIs are URLs or URNs.

[yuchi]

And only very malformed URNs may be actually valid URLs

[jwilk]

RFC 3986 ("Uniform Resource Identifier (URI): Generic Syntax") has a wider definition of URNs, according to which URLs and URNs are not disjoint.

https://www.rfc-editor.org/rfc/rfc3986.html#section-1.1.3