A digital object identifier (DOI) is a unique alphanumeric string assigned by a registration agency (the International DOI Foundation) to identify digital content and provide a perfect & persistent link to its location on the Internet. Publisher need to assigns a DOI to any research or review article to give that article a digital address.
Any DOI numbers begin with a 10 and contain a prefix and a suffix separated by a slash. The prefix is a unique number of four or more digits assigned to publishers; the suffix is assigned by the publisher and was designed to be flexible with publisher identification standards.
The DOI must be written in print or electronic media of article. It must be written on first page as well as it must be added in citation. The redirection page must be clear so that if someone clicks on DOI link then it must be redirected to that page.