Subnetting is the logical breakdown of a network address space into progressively smaller subnetworks. That’s it. Stop thinking and take it for what it is! Now, as you break down your address space into smaller subnetworks, you determine the numbers of network bits and host bits by the requirements of your network. Network bits and host bits are manipulated by the subnet mask. At this point I’m hoping you’re saying to yourself, “Oh yeah, I remember this stuff.” If not, please dig into the details on your own. We are looking at this topic in terms of how it will aid our effort as hackers.
Now that you grasp the basics of the subnet mask and how to use it to manipulate the address space, you can see how knowing a few IP addresses can give you a clue as to how an organization’s network is laid out. There’s more to come on this topic, but as a quick example, knowing a single internal IP address can give a hacker much insight into the company’s addressing scheme.