This method is quiet useful for inverse mapping the targets for host alive checks.The point will be proved through the scapy via packet crafting.In this we will discuss about inverse mapping with or without padding to compare the results.We know with some tools the response like host unreachable or no response with tool like hping.Lets play with scapy a bit: Link:http://zeroknock.blogspot.com/2007/02/inverse-mapping-via-packet-crafting.html