The central problem
Connect independent networks.
Good network reasoning separates the mechanism from the symptom. Start with the boundary, identify the evidence, and test the explanation that accounts for the observation.
One signal rarely proves the whole path.Compare observations from the relevant layer before changing a configuration.
Build the model
Four useful signals
Local network
Connects nearby devices under one administration.
Provider
Carries traffic between customer and wider networks.
Router
Forwards packets toward another network.
Interconnection
Exchanges reachability between independent systems.
Mini investigation
Inspect the evidence
Select the observation and identify the most direct next check.
NEXT CHECKDirect evidence
This observation narrows the mechanism but does not prove every downstream layer.
Compare it with one nearby-layer check.
Knowledge check
Test your model
Answer five questions.