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Maxmind (and others probably) regularly publish lists of IP blocks mapped to the owning ASN. Basically geoip but instead of mapping to geographical regions, maps it to ASN. If you want to roll your own, you can probably start from a public route reflector and go from there.

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whois -h whois.radb.net -- -i origin AS20473 | /usr/bin/egrep '^route' | awk '{print $2}'

This, fastly makes this easy to do as well (I don't work for them, just use the product).

Maxmind dbs are... awkward to get access to. They are expensive as hell if you buy them directly but usually built into edge service products. F5 had them as well I think.

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