DOI (digital object identifier) is an identification code for a journal article or other published works. The code was developed and introduced in 2000 by the International DOI Foundation (IDF) and is assigned by the publisher. A DOI is generated by a registration agency (Crossref) that contains an alphanumeric string beginning with “10” and a prefix of four or more numbers. The prefix is followed by a slash (/) and a suffix. The suffix is assigned by the publisher.