URIs are a standard for identifying documents using a short string of numbers, letters, and symbols. They are defined by RFC 3986 - Uniform Resource Identifier (URI): Generic Syntax. URLs, URNs, and URCs are all types of URI.
Contains information about how to fetch a resource from its location. For example:
/other/link.html (A relative URL, only useful in the context of another URL)
URLs always start with a protocol (
http) and usually contain information such as the network host name (
example.com) and often a document path (
/foo/mypage.html). URLs may have query parameters and fragment identifiers.
Identifies a resource by a unique and persistent name, but doesn't necessarily tell you how to locate it on the internet. It usually starts with the prefix
urn: For example:
urn:isbn:0451450523 to identify a book by its ISBN number.
urn:uuid:6e8bc430-9c3a-11d9-9669-0800200c9a66 a globally unique identifier
urn:publishing:book - An XML namespace that identifies the document as a type of book.
URNs can identify ideas and concepts. They are not restricted to identifying documents. When a URN does represent a document, it can be translated into a URL by a "resolver". The document can then be downloaded from the URL.
URC -- Uniform Resource Citation
Points to meta data about a document rather than to the document itself. An example of a URC is one that points to the HTML source code of a page like:
Rather than locating it on the internet, or naming it, data can be placed directly into a URI. An example would be
Frequently Asked Questions
I've heard that I shouldn't say URL anymore, why?
The W3 spec for HTML says that the
href of an anchor tag can contain a URI, not just a URL. You should be able to put in a URN such as
<a href="urn:isbn:0451450523">. Your browser would then resolve that URN to a URL and download the book for you.
Do any browsers actually know how to fetch documents by URN?
Not that I know of, but modern web browser do implement the data URI scheme.
Does the difference between URL and URI have anything to do with whether it is relative or absolute?
No. Both relative and absolute URLs are URLs (and URIs.)
Does the difference between URL and URI have anything to do with whether it has query parameters?
No. Both URLs with and without query parameters are URLs (and URIs.)
Does the difference between URL and URI have anything to do with whether it has a fragment identifier?
No. Both URLs with and without fragment identifiers are URLs (and URIs.)
Does the difference between URL and URI have anything to do with what characters are permitted?
No. URLs are defined to be a strict subset of URIs. If a parser allows a character in a URL but not in a URI, there is a bug in the parser. The specs go into great detail about which characters are allowed in which parts of URLs and URIs. Some characters may be allowed only in some parts of the URL, but characters alone are not a difference between URLs and URIs.
But doesn't the W3C now say that URLs and URIs are the same thing?
Yes. The W3C realized that there is a ton of confusion about this. They issued a URI clarification document that says that it is now OK to use the terms URL and URI interchangeably (to mean URI). It is no longer useful to strictly segment URIs into different types such as URL, URN, and URC.
Can a URI be both a URL and a URN?
The definition of URN is now looser than what I stated above. The latest RFC on URIs says that any URI can now be a URN (regardless of whether it starts with
urn:) as long as it has "the properties of a name." That is: It is globally unique and persistent even when the resource ceases to exist or becomes unavailable. An example: The URIs used in HTML doctypes such as
http://www.w3.org/TR/html4/strict.dtd. That URI would continue to name the HTML4 transitional doctype even if the page on the w3.org website were deleted.