DOIs are persistent and unique identification numbers allocated to articles, books and/or book chapters that will help you find them online.
Unlike a URL, which points to one specific location online (where the content may change), DOIs point to a specific piece of work regardless of where it is located online or how many different places it is located online.
It is important to note that not all online resources will have a DOI as there is a fee involved with purchasing the DOI. Smaller publishing journals will often save money by not having a DOI assigned to their articles.
A DOI can usually be found on the front or last page of an article and will look like the below image.
To find an article via its DOI number you can use the guest query form located in Crossref.
You must use a service offered by a DOI Registration Agency (RA). You do not need to be a member of the International DOI Foundation in order to work with an RA.
The cost of registering new DOI names depends on the services you purchase. Contact a DOI Registration Agency to discuss your options. Each RA is different, and each is free to offer its own business model. The IDF does not determine the costs charged to end users. Existing DOI names can be resolved by users free of charge.
The IDF does not have any rules on this. Individual RAs adopt appropriate rules for their community and application. As a general rule, if the change is substantial and/or it is necessary to identify both the original and the changed material, assign a new DOI name.