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Host uses your default system resolver to do a reverse-DNS lookup. This means it queries the nameserver(s) in /etc/resolv.conf and/or your /etc/hosts file(s). It has no (direct) connection to whether a system with the given IP actually exists (but due to the nature of DNS, it should be accurate). If you want to use the ARIN (or some other) database, use whois instead of host.
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