DOI DEFINITION
A DOI, or Digital Object Identifier, is a unique string of numbers and letters that permanently identify the location of an article, or other work, on the web. A DOI is a permanent or persistent link.
PRESENTED as ACTIVE or INACTIVE
When you locate an article, DOIs may or may not be listed as active links.
To make a DOI active, simply add http://dx.doi.org/ before the DOI. You can also locate the article from an non-active link by pasting the DOI into a DOI resolver. https://dx.doi.org/
DOIs are NOT UNIVERSAL
Not all articles have a DOI.