OSI model
also: OSI Reference Model, ISO/IEC 7498
A seven-layer conceptual framework that standardizes how computer networks communicate, with each layer handling specific functions from physical transmission to application-level interaction.
The OSI (Open Systems Interconnection) model divides network communication into seven layers, each building on the one below it. This separation of concerns helps network engineers understand where problems occur and how different protocols interact.
The layers from bottom to top are: Physical (cables, voltages), Data Link (MAC addresses, switches), Network (IP routing), Transport (TCP/UDP), Session (connection management), Presentation (encryption, compression), and Application (HTTP, SSH, DNS).
For example, when you run ssh user@server, SSH operates at layer 7, TCP operates at layer 4, IP routing happens at layer 3, and physical cables at layer 1—all working together seamlessly.