Some document identifier: a brief description

How an ISBN is issued? How many versions of ISSN are available? What is a DOI?
Here a very brief description of the 3 most used document/object identifiers.

The International Standard Book Number is an ISO numeric commercial identifier for books. Since 2007 ISBNs have contained 13 digits, a format that is compatible with EAN (European Article Number). You can find the ISBN on the back cover of the books or on the verso of title page. 
Here an example of ISBN:

978: EAN
0: Group identifier (0-1 are for English-speaking countroes)
7334: Publisher identifier
2609: Title identifier
4: Check digit

Books do not have ISBN when: 
- they have been published before 1970 (year of the publication of the standard);
- athors have published them privatly;
- editors do not have followed the correct procedure in order to obtain an ISBN.
If you want to learn more and have fun with algorithms, see the related Wikipedia page

The International Standard Serial Number is an 8-digit number that identify periodical publications and their media type. It's essential to disambiguate journals with same title and to facilitate orders, legal deposit and interlibrary loan services.
ISSN variants:
- p-ISSN, that identify the content of the print version of a journal;
- e-ISSN, that identify the content of the electronic version of a journa;
- ISSN-L is a unique identifier for all versions of the periodical containing the same content across different media.
ISSNs are assigned by a network of national centres. The Italian national centre for the creation and assignment of ISSN is the library of CNR G. Marconi in Rome. 

The Digital Object Identifier is a character string that identify a specific electronic document. The difference between the DOI and the e-ISSN is that the second one identify the electronic version of a journal that will include a number of articles each of them identified with a DOI. Metadata of objects are stored in their DOI, that it contains, for example, the URL of the document. DOI are fundamental for persistence citations in scholarly journals.
Is possible to retrieve the link of a document by its DOI, from the website of the International DOI Foundation:
IMT Library joined the DOI project two years ago, in order to identify each record of our  PhD theses to make them more easily citable.