Chapter 2: The OSI Model and TCP/IP Model
2.1 Why do we need a reference model?
Networking involves cables, Wi-Fi signals, switches, routers, IP addresses, TCP connections, TLS encryption, HTTP requests and finally the web page. Different companies build each piece. A layered model divides this big problem into small, independent layers so that:
- each layer does one job and provides a service to the layer above it,
- vendors can build compatible products (a Cisco router works with a Dell laptop),
- troubleshooting becomes systematic ("Is it a cable problem (Layer 1) or a DNS problem (Layer 7)?").
The OSI (Open Systems Interconnection) model was published by ISO. It has 7 layers. The real internet uses the simpler TCP/IP model, but OSI is the standard vocabulary — engineers say "Layer 4 load balancer" or "Layer 7 firewall" every day.
Mnemonic
Top to bottom (7 → 1): All People Seem To Need Data Processing — Application, Presentation, Session, Transport, Network, Data Link, Physical. Bottom to top (1 → 7): Please Do Not Throw Sausage Pizza Away.