A Digital Object Identifier (DOI) is a persistent alphanumeric handle used to uniquely identify a document in an online environment. The DOI assigned to the object will never change. The International Organization of Standardization (ISO) standardizes DOIs, which should be thought of as digital identifiers of objects rather than identifiers of digital objects.
All DOIs begin with a 10, which is a distinguishing characteristic in the DOI, and contain a prefix number assigned to organizations and a suffix number assigned by the organization (prefix/suffix). Some examples:
10.1007/s00138-017-0825-7 Prefix 10.1007 Suffix s00138-017-0825-7
10.1371/journal.pone.0195878 Prefix 10.1371 Suffix journal.pone.0195878
10.1080/01930826.2018.1436778 Prefix 10.1080 Suffix 01930826.2018.1436778
10.1016/j.acalib.2017.09.003 Prefix 10.1016 Suffix j.acalib.2017.09.003
The 4 numbers after the 10 identify the registrant. The suffix is the item ID.