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A persistent URL (PURL) is a uniform resource locator (URL) that redirects requests to the proper location of a user’s requested Web resource. It is meant to remain constant despite an ever-changing Web infrastructure that can cause websites to change servers or hosts, as it does not directly point toward the Web resource but, rather, a type of Web service known as an intermediate resolution service, which resolves the PURL with the resource's actual current address and then redirects the request.
A PURL is like a PO box with an address that is constant, even if there are changes to the owner's address. When an address changes, the postal service receives forwarding information, so there is no need for the owner to inform others; the postal service simply forwards mail to the owner's new address.
A PURL is also like a public phone number that a subscriber uses for most contacts. In addition, the subscriber may have a private phone number that is linked to the public number and set up to receive forwarded calls. All public contacts can continue using the public number, even if the user changes his or her personal number.
PURLs provide a level of indirection to Web resources, allowing them to change over time without affecting any dependent resource or system. This adds a certain permanence to Web resources that may migrate due to technical, social or other similar reasons.