I think using MD5 (as i demonstrated above) sort-of solves this irreversibility-problem. MD5 is a one-way hashing algorithm, so in principle you'll only be able to get back from rgb-value to IP if you have a full table of IP to rgb-value's (a 'brute force' attack). MD5 is not the strongest of one-way hashing algoritms, but it's widely used and widely available.