A URN identifier is a unique and persistent identifier of an electronic document. Once given to a document, the identifier remains unaltered as long as the name and the contents of the document remain intact. The same identifier is never assigned to two separate documents. The URN identifier can be exploited as a persistent web address through registering.
The National Library of Finland's URN register works as a decentralised service for Finnish universities and other institutions. Currently private persons cannot register URN identifiers.
Assigned URN identifiers and corresponding URLs must be registered at the National Library. A persistent web address comprises the URN identifier prefixed with the address of the resolving service. For example:
The resolving server redirects to the current location of the document. When the document is moved, the change will be registered and thus the persistent address remains functional.
The publisher may register URN identifiers manually, but usually the data are updated automatically from the publisher’s system, for example, the institutional repository.
If a document is eligible for an ISBN or an ISSN number, it will be assigned one, and the URN identifier is based on it.
If a document cannot be given an ISBN or an ISSN number, it will be assigned an NBN number.
Identical copies of a document recieve the same URN identifier. Therefore, a URN based web address may resolve successfully to several URLs.
If a document is edited, a new URN identifier is only assigned if the contents change significantly.
NBN numbers may be obtained from the URN generator maintained by the National Library. A publisher may apply for a NBN sub-namespace and generate numbers independently.
URN:ISBN and URN:ISSN identifiers must be both displayed in writing in the body of the document (if applicable) and recorded in its metadata.
The manner of writing a URN identifier is as follows:
The identifier should also be recorded in the embedded metadata, too. For example, in an HTML document, it should be entered in the HEAD section:
META NAME="Identifier" SCHEME="URN:ISBN" CONTENT="951-611-4415"
META NAME="Identifier" SCHEME="URN:ISSN" CONTENT="1234-5678"
The URN:NBN identifier should be displayed in the same way, if possible. If a URN is assigned to a document after it is finished, it is not strictly obligatory to add it to the document body.
The URN:NBN is expressed in the following ways:
In creating metadata, one can use the Dublin Core metadata generator.
The URN identifier is specified in the Internet standards of the IETF (Internet Engineering Task Force).
Using ISBN numbers as part of a URN identifier is specified in RFC 3187, ISSN numbers in RFC 3044, and NBN numbers in RFC 3188. URN syntax is specified in RFC 2141.