[TeMPOraL]

No matter how complex your scheme is, if IP address is the only input, it's a (mathematical) function of f: IP → hash. Since IP(v4) space is 32bit (in practice, slightly less), if you know the function f, you can trivially enumerate all inputs.

From security point of view, if you use a fixed (unrelated to input) salt, the attacker will have a harder time to discover the function f (unless you store the salt next to your IP hashes). But from privacy point of view, in relationship between me (user) and you (service provider), you are the attacker. And you know your function f. Hashing IPv4 addresses, salt or not, gives me no privacy protection, since you can trivially reverse the hash - just due to small domain size. With IPv6, this problem will resolve itself somewhat; till then, I'd prefer if you encrypted those IPs with keys that have finite and short life time, in a way that a third party could audit if need be.