You could encrypt instead of hash, and then have some policy (e.g. the decryption library/service/piece will only allow decrypting ciphertext newer than 30 days).
If you need the ability to group ciphertexts without decrypting them, you could create a scheme which will make cryptographers cringe, but could be justified in this specific case.
For large sites, you also have the risk that you might be able to say something statistically useful about the plaintext.
For instance, you can probably assert things like which IP blocks are likely to comprise most of the entries in the table or which IP blocks or addresses cannot be in the table.
If you need the ability to group ciphertexts without decrypting them, you could create a scheme which will make cryptographers cringe, but could be justified in this specific case.