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Turning the Tables: How to Make Spammers Reveal Their Own IP Address
(codefoundry.de)
This is a most excellent place for technology news and articles.
What you've described is often referred to as a rainbow table and is generally not considered to be GDPR compliant:
https://skymonitor.com/why-hash-dont-anonimize-an-ip-address-and-what-this-affects-gdpr/