If you just want to build your own IP-to-ASN table, you can download dumps of "RIS Raw Data" [0] from RIPE and parse them if you don't yourself run BGP.

I'm a network engineer at an ISP and it's pretty common to use something like this for analyzing traffic network when considering peering sessions, for example. Even if you don't run BGP, you could use it for answering questions like "how much traffic do we send to/receive from Facebook?" and such.

RIPE's RIS dumps are performed every five minutes from more than a dozen different "vantage points" across the Internet.

ARIN used to provide an "originAS" file [1] but it looks like they quit doing that a few years ago. You may be able to find some interesting stuff browsing around /pub on their FTP server, though [2].

[0]: https://www.ripe.net/analyse/internet-measurements/routing-i...

[1]: ftp://ftp.arin.net/pub/originAS/

[2]: ftp://ftp.arin.net/pub/