The OSI Model

The OSI reference model describes network communication in 7 layers. It helps to isolate problems and classify protocols.

7 Application  HTTP, DNS, SMTP
6 Presentation Encoding, TLS
5 Session      Sessions (often merged)
4 Transport    TCP, UDP
3 Network      IP, Routing
2 Data Link    Ethernet, MAC, Switches
1 Physical     Cables, Radio

Practical Use

Check from the bottom up: cable/LEDs (1-2), ping IP (3), DNS (7), port test nc -zv host port (4), then browser (7).

Related: TCP/IP Basics · Subnetting Explained