An ARK is represented by a sequence of characters that contains the label, "ark:", optionally preceded by the protocol name ("http://") and hostname that begins every URL. That first part of the URL, or the "Name Mapping Authority" (NMA), is mutable and replaceable, as neither the web server itself nor the current web protocols are expected to last longer than the identified objects. The immutable, globally unique identifier follows the "ark:" label. This includes a "Name Assigning Authority Number" (NAAN) identifying the naming organization, followed by the name that it assigns to the object.
Here is a diagrammed example:
The ARK syntax can be summarized,
The NMA part, which makes the ARK actionable (clickable in a web browser), is in brackets to indicate that it is optional and replaceable. ARKs are intended to work with objects that last longer than the organizations that provide services for them, so when the provider changes it should not affect the object's identity. A different provider hosting the object would simply replace the NMA to reflect the new "home" of the object. For example,
Note that the ark:/NAAN/Name remains the same.
NAAN: Name Assigning Authority Number
The NAAN part, following the "ark:" label, uniquely identifies the organization that assigned the Name part of the ARK. Often the initial access provider (the first NMA) coincides with the original namer (represented by the NAAN), however, access may be provided by one or more different entities instead of or in addition to the original naming authority.
The NAAN used in the ARK anatomy diagram, 13030, represents the California Digital Library. As of 2012, roughly a hundred organizations have registered for ARK NAANs, including numerous universities, Google, the Internet Archive, WIPO, the British Library, and other national libraries.
UC3/CDL maintains a complete registry of all currently assigned NAANs, which is mirrored at the (U.S.) National Library of Medicine and the Bibliothèque nationale de France.