A URN (Uniform Resource Name) is an Internet resource with a name that, unlike a URL, has persistent significance - that is, the owner of the URN can expect that someone else (or a program) will always be able to find the resource. A frequent problem in using the Web is that Web content is sometimes moved to a new site or a new page on the same site. Since links are made using Uniform Resource Locators (URLs), they no longer work when content is moved.
A URN looks something like a URL. For example, here's a hypothetical URN:
where "def://" might indicate an agency or an accessible directory of all dictionaries, glossaries, and encyclopedias on the Internet and "blue laser" was the name of a term. The result of using the agency could be the "best definition," the "longest definition," or even all definitions that the agency could find of "blue laser."
A comparable URL would need to specify one specific location for a definition such as:
In this case, the user has to know where the resource is located as well as how to spell the file name and suffix. With a URN, the user only needs to know the name of a resource. One or more agencies will presumably be able to locate the nearest copy of the resource and the user is freed from understanding where resources are located or relocated to.
Both URN and URL are types of a concept called the Uniform Resource Identifier (URI). A URN is associated with another concept called Uniform Resource Characteristics (URC), which allows descriptive information to be associated with a URN, such as author, date, length, and so forth.
It is possible to have a name that includes an address so, in some cases, a URN may also be a URL...but it doesn't have to be.
The URN is still being developed by members of the Internet Engineering Task Force (IETF).