15-minute session: read for 3 minutes, trace for 3, experiment for 7, and complete the check for 2. Spend longer whenever you need.
DNS resolves a name; transport establishes communication; TLS authenticates and encrypts a channel; HTTP describes requests and responses. Failure at one layer should not be mislabeled as another. The University DNS incident is a useful example: a healthy web server could not help a client that still cached a failed name lookup. Draw the layers and choose a test for each.
Practice and evidence: Predict the example or draw its state transitions. Change one assumption, test the boundary, and record the actual result. Explain what this small model does not establish about a production system.
Keep three notes: your prediction, the actual result, and why they match or differ. The completion check grades understanding; the practice task builds skill.