If you want really fast, are only dealing with IPv4 addresses and have half a gig of diskspace to spare, then you can have your lookups run at 22,000/second if the ip has already been seen and 120/second for those that haven't.

This by using a single binary file 512MB in size that uses 1 bit to represent each of 4GB possible IPs. Write contention only occurs if two or more new IPs within the same 8 value range, connect at exactly the same time. This is probably a very rare occurance and can be dealt with by a re-read, back-off and retry scheme (think Nagal).

However, the contention risk can be reduce to zero if you are using an OS that supports byte-range file locking. This does slow things down a little, but not hugely.