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)

All right, I made some changes (most importantly I removed the overflow so it didn't loop forever on the inner loops, and I split the sprintf into four calls).  But I'm not really sure about what IP addresses I should check.  What are those LEGACY blocks?^[1]  My research has shown that they might indeed be in use.

_{[1] http://www.iana.org/assignments/ipv4-address-space/}