>>xvecto+W4
For ipv4 is there a difference between storing IPs and storing their hash with a nonce? You can calculate the hash of every IP address in reasonable time, so it's reversible.
Only benefit I can think of is you can forget the nonce and now the data is securely useless, if the nonce was secure, but that doesn't seem that useful really.
>>kadoba+K5
I think if we use a difficult enough hash function it should be okay? With 4 billion IPv4 addresses it would take 120+ years to iterate through all of them. You could probably rotate the nonce periodically, making it effectively worthless to pre-compute any table. But this gets complicated fast.
>>xvecto+Ra
You could try to do a more difficult hash or something (bcrypt maybe?) but I don't know if it's a very good idea. I think you'd spike your latency, open yourself to DoS attacks or only minorly inconvenience anyone reversing the hashes, or some combination of those.