Used IP addresses

Anyone got some C code that loops over all used IP addresses?  Can you come up with any special optimizations?  (I need to handle them in the "%u.%u.%u.%u"-form; computations are independant and can be run in parallel)

Got 80.230.70.64 from the second day^[1], I'd say this pretty much confirms it.

_{[1] http://dis.4chan.org/read/sci/1172747610/20}