What is the difference between a URI, a URL and a URN?

Question

People talk about URLs, URIs, and URNs as if they\'re different things, but they look the same to the naked eye.

W

Answer 1

After reading through the posts, I find some very relevant comments. In short, the confusion between the URL and URI definitions is based in part on which definition depends on which and also informal use of the word URI in software development.

By definition URL is a subset of URI [RFC2396]. URI contain URN and URL. Both URI and URL each have their own specific syntax that confers upon them the status of being either URI or URL. URN are for uniquely identifying a resource while URL are for locating a resource. Note that a resource can have more than one URL but only a single URN.[RFC2611]

As web developers and programmers we will almost always be concerned with URL and therefore URI. Now a URL is specifically defined to have all the parts scheme:scheme-specific-part, like for example https://stackoverflow.com/questions. This is a URL and it is also a URI. Now consider a relative link embedded in the page such as ../index.html. This is no longer a URL by definition. It is still what is referred to as a "URI-reference" [RFC2396].

I believe that when the word URI is used to refer to relative paths, "URI-reference" is actually what is being thought of. So informally, software systems use URI to refer to relative pathing and URL for the absolute address. So in this sense, a relative path is no longer a URL but still URI.