Nmap is widely regarded as the de facto standard tool for network exploration and security auditing. Network administrators use Nmap to identify devices running on their systems, discover available hosts and the services they offer, finding open ports and detecting security risks. While Nmap is often used for such security audits, many systems and network administrators find the tool helps with routine tasks such as network inventory, managing service upgrade schedules, and monitoring host or service uptime. This is the ninth in our series of articles highlighting essential system tools. These are small utilities, useful for system administrators as well as regular users of Linux based systems. The series examines both graphical and text based open source utilities.