Uniform Resource Names (URNs)
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Overview of URNs
In recognition of this issue, an alternative identification mechanism for Internet resources was developed, called Uniform Resource Names (URNs). The basic standard describing URNs is RFC 1737, Functional Requirements for Uniform Resource Names, which was published in 1994. In 1997, RFC 2141 was published, which specifies the syntax of URNs.
As you can probably tell from that term, a URN is intended to label a resource based on its actual identity, rather than where it can be found. So, where a URL is like Joe Zachariah's address, a URN would be his name. Or, as I gave as an example in the overview of URIs, a URN would be identifying a book based on its ISBN number rather than specifying what bookshelf it is on in a building.
To be useful in identifying a particular resource, it is necessary that a URN be globally unique, and that's not always as simple as it may at first appear. Consider human names, for example. Even though there is probably only one Charles Marlin Kozierok in the entire world, if your name is John Paul Smith or José Garcia, you likely share that name with thousands of others. This means using common names may not be sufficient for identifying human resources and some other method might need to be devised.
There are many types of resources that URNs are intended to identify on the Internet, each of which may require a different form of naming. To allow URNs to represent many kinds of resources, numerous URN namespaces are defined. A namespace is referenced using a unique string that tells the person or computer interpreting the URN what type of resource the URN identifies. The namespace also ensures the uniqueness of URNs, when a particular identifier might exist in more than one context. For example, both North American telephone numbers and ISBN numbers consist of ten digits, so a particular number such as 4167819249 could represent both a telephone number and a book number; the namespace identifier tells us what the number means when it is encountered in a URN.
The general syntax of a URN is:
For example, a book with the ISBN number 0-679-73669-7 could be represented as:
This string identifies that particular book uniquely, wherever it might happen to be in the world. Many other namespaces have also been defined to specify the URNs for other types of resources, such as documents on the Internet.
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