> If anyone's interested in trying to implement something similar, there's a regularly updated database of ASN <-> IP ranges announced here: https://github.com/ipverse/asn-ip

What exactly is the source of these mappings? Never heard about ipverse before, seems to be a semi-anonymous GitHub organization and their website has had a failing certificate for more than a year by now.

whois (delegation files) according to the embedded blog post, eg https://ftp.arin.net/pub/stats/arin/delegated-arin-extended-...