What are useful command-line network monitors on Linux

Network monitoring is a critical IT function for businesses of all sizes. The goal of network monitoring can vary. For example, the monitoring activity can be part of long-term network provisioning, security protection, performance troubleshooting, network usage accounting, and so on. Depending on its goal, network monitoring is done in many different ways, such as performing packet-level sniffing, collecting flow-level statistics, parsing server logs, actively injecting probes into the network, etc.