RFC 2068:Hypertext Transfer Protocol -- HTTP/1.1
RFC-Ref

URN


Click on the red underlined text to get to the source

... URL) [4] or name (URN) , for indicating the resource to which a method is to be applied. Messages are passed in a format similar to that used by ...


... combination of Uniform Resource Locators (URL) and Names (URN). As far as HTTP is concerned, Uniform Resource Identifiers ...



Google
Web
RFC-Ref