Once you create a VPC and launch instances, traffic begins to flow. But when you try to see "what's flowing right now," you suddenly feel lost. Operators ask three questions most frequently: ① Who is communicating with whom and how much, ② What is the actual content of suspicious packets, ③ Can this instance truly reach that RDS database. AWS provides a different tool for each question: Flow Logs, Traffic Mirroring, Reachability Analyzer. At first glance, the names seem similar, but their essence is completely different.