Network Defense Fundamentals

Modern network defense begins with a shared vocabulary for describing how data moves between systems, and the OSI (Open Systems Interconnection) model provides exactly that. Developed by the International Organization for Standardization, it divides network communication into seven layers — Physical, Data Link, Network, Transport, Session, Presentation, and Application — each responsible for a distinct function and each communicating only with the layers immediately above and below it. The TCP/IP model, which actually underlies the modern internet, condenses this into four layers — Link, Internet, Transport, and Application — but the two frameworks map onto each other closely enough that security professionals use OSI terminology as a diagnostic shorthand: a "Layer 3" problem points to routing and IP addressing, while a "Layer 7" problem points to the application itself. This layered thinking matters for defense because different attacks and different controls operate at different layers. A firewall filtering by IP address operates mostly at Layer 3; a web application firewall inspecting HTTP requests operates at Layer 7. Understanding which layer a given technology protects, and which layers it leaves exposed, is the starting point for building a defense with no obvious gaps.