[Shish2k]

I was considering something related for my own software -- giving some level of differentiation to "Anonymous" users in a discussion, to reduce samefagging; users on the same subnet would get similar colours, etc. The main downside with that is that it's then plausible to look at somebody's colour and work out their IP address :P

[Udo]

Just hash the actual IP address. For example, take the IP and apply MD5; then take three bytes each to make, say, 3-colored "flags". That would totally work and yet be reasonably anonymous. This would also take care of the fact that colors come as three byte values and not four.

[dllthomas]

That would not work, as stated. The search space is way too small. Simply exhaustively building a table mapping hash to IP address is tractable.

This could be ameliorated by including a site-specific secret in the hashed value; I'm not comfortable calling that "secure" but it does address this particular flaw.

[Dylan16807]

Yes, but given a single hash you are only able to reduce to 200 candidate IPs.

[ryan-c]

If a site specific secret is present you would not be able to do that - unless the site will act as an oracle for you and doesn't care about you making several billion requests.