The National Provider Identifier is the one identifier every HIPAA-covered provider and organization in the country carries: a 10-digit number issued by CMS through the National Plan and Provider Enumeration System (NPPES). Every prescription, claim, and payer enrollment keys on it.
The number is intelligence-free by design - no digits encode a state, a specialty, or a type. All of that lives in the registry record behind it: the provider's taxonomy code, practice locations, and enumeration history. That record is public, which is what makes point-of-need verification possible at all.
One caveat worth internalizing: NPPES is self-reported. The NPI and its active status are authoritative, but addresses and phone numbers are only as fresh as the provider's last update.