OSI Model

The OSI (Open Systems Interconnection) model is a conceptual framework that describes how data travels across a network in seven layers, each with its own job. It helps people understand networking and troubleshoot problems layer by layer.

The layers, from bottom to top, are: 1 Physical (cables and signals), 2 Data Link (MAC addresses, switches), 3 Network (IP addresses, routing), 4 Transport (TCP/UDP), 5 Session, 6 Presentation (encoding, encryption) and 7 Application (HTTP, DNS, email). The internet itself follows the simpler four-layer TCP/IP model.

7 Application   HTTP, DNS, SMTP
6 Presentation  encoding, encryption (TLS)
5 Session       sessions between applications
4 Transport     TCP, UDP
3 Network       IP, routers
2 Data Link     Ethernet, MAC addresses, switches
1 Physical      cables, Wi-Fi radio, signals