Hardening the network you already run. Articles tagged Network Security cover control-plane protection, access control, segmentation, and the attacks each countermeasure is actually stopping.
Version detection tells you what software a port is running. OS detection (-O) tries to answer a different question: what operating system is the whole host running, even on ports that give nothing away. It does this without logging in
A port number tells you almost nothing on its own. "Port 80 open" could be nginx, Apache, an embedded management interface, or a printer's web UI. "Port 22 open" could be OpenSSH, a Cisco
Nmap does not have one way to scan a TCP port, it has a whole family of them, and each one sets different flags to provoke a different reaction from the target. Knowing which scan type to reach for is
Once you know which hosts are alive, the real work is finding out what each one is listening on. Port scanning is how you build a service inventory, confirm a firewall change did what you expected, and catch the forgotten
Before you scan a single port, you need to know which addresses are actually alive. Blasting a full port scan at every host in a /24 wastes time on dead space and lights up any IDS in the path. Host