When writing blacklist automation for my network, is there a way to automatically determine the prefix size of the other party?

  • Actually, this question is not too broad. There is plenty of advice to give about standard prefix sizes, and in the RIPE region ISPs document the size of their delegations to customers in the RIPE database. Sep 24 '17 at 17:29
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    Everyone assumes /64, but that's a poor(at best) guess You'd have to find a listing of the netblock within some registry (ARIN, RIPE, etc.) to know the actual delegated network size. Even BGP tables won't necessarily show the entire block as people are free to announce less than the full block.
    – Ricky
    Sep 26 '17 at 5:10

No there isn't. For SMTP block lists people tend to use a /64.

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