How it works
Distinct protocols handle distinct jobs: resolving names to addresses, assigning configuration, addressing and delivering packets, and choosing between reliable delivery and speed.
Match the symptom to the protocol. Reaching a service by address but not by name points somewhere quite specific and rules out a great deal.
Common error
Reliable delivery and fast delivery are a deliberate tradeoff. Neither is simply better; they suit different traffic.
Worked example
Separate naming, addressing, and transport responsibilities in a web request.
Evidence
Name: portal.example
DNS answer: 192.0.2.20
Application: HTTPS
Transport: TCP in this exampleDNS maps the requested name to an address; IP carries packets toward that address; TCP supports reliable ordered transport in this example; TLS protects the HTTPS session. DNS does not transfer the whole page. Other protocol stacks can implement secure web transport differently.
Exercises
4.3.1
A service is reachable by its documented address but not its hostname. Which service is a leading suspect, and what check supports the diagnosis?
Show solution
Name resolution is a leading suspect. Query the configured resolver for the exact hostname and compare its result with the documented address or a known working client.
4.3.2
Explain why a real-time media application might accept some loss rather than wait indefinitely for retransmission.
Show solution
Late media data may no longer be useful for timely playback. The application may favor lower delay over complete delivery, while still needing appropriate congestion handling and security.