Now that we understand the very basics of how networks, ports, TCP, and UDP work, we can start to learn the intricacies of Nmap—a powerful tool that leverages various different elements of how computers and networks communicate, to help give us useful information about what services various different computers are running.

The most common use of Nmap—and its original design—was a simple port scanner. A port scanner is simply a piece of software that attempts to connect to each specific target port and see if that port is open—determining whether or not a TCP three-way handshake can be established.

A TCP three-way handshake is a simple way to establish a network-based connection before applications begin to communicate with one another. The structure is very simple—and don't worry if these flags don't mean much to you right now. The three-way handshake, as you might expect, consists of three steps between two speakers (let's call them Alice and Bob). The handshake works as follows...