To understand how packets—which are the bits of information that travel from one machine to another—run networks, it's a good idea to have a basic idea of the Open Systems Interconnection (OSI) model. This is a conceptual way to think about how networks work on different layers. It's easy to understand that physically, networks are just pieces of hardware with electrical signals running across them; it's actually much more difficult to conceptualize the logical networks that developers and network operations staff (as well as computers) work with.

The simplest explanation is that different protocols and bits of information work on different layers of the model, and work off of each other. The following diagram briefly explains which layer performs which role, so that we can understand how service banners and ports work later in this chapter.

It isn't necessary to understand all the intricacies of the OSI model or low-level networking in order to use Nmap effectively, and to interpret...