All doctors, suppliers, and healthcare organizations covered by the Portability and Transparency Act for Health Insurance (HIPAA) are expected to apply for a National Provider Identifier (NPI). NPI is a unique identification is an industry-standard for providers, health plans, prescribers, healthcare clearinghouses, business associates, and more to submit or process health insurance claim. The reason NPI was brought to improve the efficiency and effectiveness of the electronic transmission of health information.
NPI Type 1 and Type 2 Providers
Generally there are two types of NPI commonly known as Type1 and Type2.
Type1 NPI is for healthcare providers who are individuals, including physicians, dentists, and all sole proprietors. Only one NPI is given to an individual throughout his lifetime.
Type 2 NPI is for healthcare providers who are organizations, including physician groups, hospitals, nursing homes, and the corporations.

How to Lookup and Verify Providers and Healthcare Organizations
NPI Registry
The NPPES NPI Registry is a free directory offered by the CMS to search for all active information about the National Provider Identifier. CMS also offers an active NPI list as a file download, or via an application programming interface, in addition to an online searchable database (API).
It is fairly easy to look up a physician or healthcare agency on the NPPES NPI List, but a slight learning curve may occur when referencing any database for the first time. For NPI validation, you will need to quickly associate either a person or an entity with which form of provider you are trying to reference. Next, it is helpful to provide a summary of the taxonomy to help provide more contexts for search results. The NPI Registry will then deliver a few results once you have entered all of your data. CMS publishes only publicly-relevant portions of an NPI record, including the name of the provider, specialty (taxonomy), and practice address. The https://npidb.org help find NPI Registry for any healthcare provider including doctor. You can search on it by group or individual name or with NPI number.