What is the OSI model for?
The OSI model describes network communication in seven layers. It is a reference framework for responsibilities and interfaces, not a requirement that every application must follow exactly. For troubleshooting, it helps separate physical connectivity, addressing, transport and application behavior.
The seven layers
1 Physical: signals, cables and radio. 2 Data link: frames and local transfer, such as Ethernet. 3 Network: IP addressing and routing. 4 Transport: endpoint communication, such as TCP and UDP. 5 Session: dialogue management. 6 Presentation: data representation and related functions. 7 Application: application-oriented protocols such as DNS and HTTP.
OSI and TCP/IP
The TCP/IP model commonly groups responsibilities into network access, internet, transport and application. This mapping is a simplification. TLS does not fit neatly into one OSI layer; in practice it sits between applications and transport. A TCP connection test is not equivalent to an ICMP test at the network layer.
Troubleshooting with the model
Start with connectivity and local settings, then check routing and the required port, followed by DNS, TLS and application behavior. A working lower layer does not guarantee that a higher layer works. Our TCP ping, DNS lookup and SSL checker provide limited server-side observations, not a complete test from the visitor's device.