Going back a year or so, I wrote a story on the passive recon, specifically the IPINFO API (https://isc.sans.edu/diary/28596). This API returns various information on an IP address: the registered owning organization and ASN, and a (usually reasonably accurate) approximation of where that IP might reside.
NMAP Scanning without Scanning (Part 2) – The ipinfo API, (Wed, May 22nd)
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