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[ZiNgA BuRgA]

ip2long doesn't encrypt or hash the IP, so yes, it's completely reversable.

Hashing functions would mainly be md5() and sha1().  As these are long, you may wish to truncate the hashes a bit.  Be sure to add in a "salt" or static unquifier unknown to the user, otherwise these hashes will also be reversable.

Some sites just hide the last two octets of the IP address.  That may also be a solution which may work a bit better with dynamic IPs, at the cost of being less unique.