Whether an ISRC really belongs to the right recording only shows up in a database. The lookup checks an ISRC, ISWC, UPC or EAN against MusicBrainz, the open music database: for an ISRC the recordings with their releases and linked work, for an ISWC the works with their writers, for a barcode the releases. Codes used more than once stand out at a glance.
On click, reCAPTCHA by Google checks the request. The server of this website sends only the code to MusicBrainz, not the IP address; for the limit of 20 lookups per hour it counts only the IP address. Neither the code nor the answers from MusicBrainz are stored.
ISRC: MusicBrainz first returns the recordings that carry the code. For the first three, the lookup loads their releases, all ISRCs of the recording and the linked work with its ISWC. MusicBrainz returns at most 25 releases per lookup.
ISWC: MusicBrainz returns the works with this ISWC, the linked people with their role and, where entered, the work identifiers of collecting societies.
UPC and EAN: MusicBrainz finds barcodes only through its search. Only releases whose barcode matches exactly are counted; a UPC-A is also searched with a leading zero.
Structure, IFPI prefix list and check digits in detail: ISRC, UPC/EAN and ISWC Validator
Authoritative databases of the issuing bodies: ISRC Search (IFPI) · ISWC Network (CISAC)
Sources: MusicBrainz API · MusicBrainz API: Rate Limiting · MusicBrainz Database (core data, CC0) · MusicBrainz Style: Recording · ISRC FAQ (IFPI) · ISRC Handbook, 4th Edition (IFPI) · The ISWC (CISAC)
MusicBrainz is an open database maintained by volunteers; its core data is released under CC0. A code missing there may still have been issued, and entries can contain mistakes. The authoritative sources are the issuing ISRC agency, the ISWC agency and the distributor.