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)

>>44
How do you crack ID?

>>45
With great difficulty.

>>46
Explain.

>>47
You utilize great difficulty as a part of the cracking process.