I'm thinking about building a very anonymous forum, where people can comment and upvote each other's comments anonymously. People are supposed to remain anonymous even if the forum's database contents becomes publicly available. As one step to prevent vote fraud, I'm thinking about storing parts of the IP address. Only parts of it — if I were to store the whole IP, the forum would no longer be so very anonymous.
IP addresses are like
111.222.333.444. Which parts of the IP address would you recommend that I store? (If any?)
And what about IPv6, which parts of an IPv6 number?
As far as I know, the initial part of the IP address identifies the Internet Service Provider. And the last digits identify the computer within the ISP's network. What about storing parts _22 and _44,
(Hashing the IP doesn't seem like an option, since there are only 4e9 IPs and it therefore would be relatively easy to crack the hash, i.e. test all 4e9 IPs and find out which one results in a given hash. And hashing a secret value + the IP also won't work, since people are supposed to remain anonymous even if the secret value is exposed.)
(Bloom filters is an option, but I'm thinking they'd require rather much storage space, something like 5KB per comment.)