**HDL** (Handle) can be expressed as URIs by prefixing the resolver URL http://hdl.handle.net/: http://hdl.handle.net/2027/mdp.39015000000482 **ISSN**s (International Standard Serial Number) can be expressed as URIs in the urn scheme according to https://tools.ietf.org/html/rfc3044: urn:ISSN:1534-0481 **ISNI**s (International Standard Name Identifiers) can be expressed as URIs by prefixing the resolver URL http://isni.org/: http://isni.org/000000012150090X **ARK**s (Archival Resource Keys) can be expressed as URIs by prefixing the Name Mapping Authority Hostport (NMAH) part as described in https://tools.ietf.org/html/draft-kunze-ark-18: [http://gallica.bnf.fr/ark:/12148/bpt6k107371t] [ NMAH / ark ] **DOI**s (Digital Object Identifiers ISO 26324) can be expressed as URIs by prefixing the resolver URL http://dx.doi.org/: http://dx.doi.org/10.1594/PANGAEA.726855 **ISBN**s (International Standard Book Numbers ISO 2108:2005) can be expressed as URIs in two different ways (which might complicate comparison of identifiers). Example of an ISBN in the urn scheme according to (https://tools.ietf.org/html/rfc3187): URN:ISBN:978-3-7657-1538-9 It is possible to use the ISBN as an actionable ISBN (ISBN-A https://www.doi.org/factsheets/ISBN-A.html and https://www.isbn-international.org/content/other-identifiers ) within the resolvable DOI namespace (prefix 10.978 and 10.979 are reserved within the DOI namespace). The URIs are defined to be unique but not neccessarily resolvable, if not registered: http://dx.doi.org/10.978.37657/15389 For other published identifiers (including URNs as URIs) use a community accepted (and adopted) good pratice depending on the scope and potential use. In Germany, for example, cultural heritage institutions might be identified by an **ISIL** (International Standard Identifier for Libraries and Related Organizations, ISO 15511). A good practice in that case is to use the resolver URI http://ld.zdb-services.de/resource/organisations/*ISIL* (e.g., http://ld.zdb-services.de/resource/organisations/DE-1). [Based on [Request for Comments: 3650 – Handle System Overview](http://www.rfc-editor.org/rfc/rfc3650.txt)]
Universal Resource Identifiers (URIs) can be used to establish relationships between resources from disparate source systems. Identifiers, such as Digital Object Identifier (DOI), Handle (HDL), International Standard Book Number (ISBN), International Standard Music Number (ISMN), International Standard Name Identifier (ISNI), International Standard Recording Code (ISRC), International Standard Serials Number (ISSN), should be expressed as Universal Resource Identifiers (URIs). See examples for recommendations. Due to the lack of discovery options for resolvers for the various Uniform Resource Name (URN) schemes it is recommended to express URNs as URLs including the resolver address. It is highly recommended to express URIs as HTTP URIs whenever applicable.
URI
Uniform Resource Identifier
**URI**, as a value for the type attribute of any of the LIDO identifier elements, such as *Actor Identifier*, *Concept Identifier*, *Event Identifier*, designates a globally unique assigned code that conforms to the syntax as defined in [IETF RFC3986](https://www.ietf.org/rfc/rfc3986.txt "Internet Engineering Task Force: Request for Comments 3986 – Uniform Resource Identifier (URI): Generic Syntax"). An URI may have meaning outside of the local system. [Based on [Request for Comments: 3986 – Uniform Resource Identifier (URI): Generic Syntax](https://www.ietf.org/rfc/rfc3986.txt)]