[fadmmatt]

I hope that's SHA1 plus a secret salt.

Otherwise, I can just build a reverse map of SHA1'd ips.

for ip = 0 to 2^32: unhash[SHA1(ip)] = ip.

[drm237]

Yes it is. But first you'd have to get all of the results from my database, which is cleared every 24 hours (this is not a challenge!).