How it works
Layered models describe how a network is organized so that a problem can be located rather than merely noticed. The OSI model and the TCP/IP model divide the same territory at different granularity.
Use layers to structure diagnosis. Establishing that the lower layers are healthy narrows the search enormously before you look at any application.
Common error
The models are tools for reasoning, not a list to recite. An answer that names a layer without using it has not used the model.
Worked example
Use layers to locate a fault without confusing the two common models.
Evidence
Cable link: up
IP configuration: valid
Gateway: reachable
Web name resolution: failsThe link and local IP observations support working lower-layer connectivity for the tested path. Name resolution is an application-layer service in these models, so inspect DNS configuration and queries next. The OSI model has seven layers; TCP/IP commonly groups them into four.
Exercises
4.2.1
Match Ethernet framing, IP forwarding, TCP reliability, and DNS name lookup to OSI layer names.
Show solution
Ethernet framing: data link. IP forwarding: network. TCP reliability: transport. DNS name lookup: application. The mapping organizes diagnosis rather than proving a whole layer is fault-free.
4.2.2
Why does a successful gateway ping not prove that HTTPS and DNS both work?
Show solution
It tests one form of IP reachability to one destination. Application services can fail independently through service configuration, name records, transport policy, or server problems.