Host Discovery with nmap

As a Computer Security Engineer that regularly conducts external penetration tests, a recurring challenge seems to arise when assessing organizations with a large allocation of IP address space. What does one do when faced with multiple class B’s, a few class C’s, and a limited amount of time? Do you stick all of the address space in your favorite scanner and hit the Go button, wait till it’s done and hope the results are accurate? How can you be sure that your scanner found all the hosts that are accessible? Do you even know the method your scanner uses to discover which hosts are alive?

This document attempts to answer the above questions, and will illustrate (at a very technical level) the methodology that I use to accurately discover which hosts are accessible prior to conducting port scanning or a vulnerability assessment.

Download the paper in PDF format here.