Quick Check: Message Paths and Diagnostic Evidence

20.1 What This Check Covers

This check reviews only the ideas you have already used in Devices, Packets, and Layers and Lab: Find the First Broken Boundary.

You should be ready to:

• separate local-link evidence from routed-packet evidence;
• climb through link, address, next hop, naming, transport/security, and service meaning in order;
• explain what a successful or failed observation supports;
• stop at the first failed boundary and state what remains unproved.

No IPv6 design, subnet arithmetic, topology selection, RSSI threshold memorisation, NAT, or protocol-port recall is required here. Those topics have dedicated chapters later in Core Networking.

20.2 Check the Message Path

20.3 Check the Evidence Order

20.4 Find the First Failed Boundary

20.5 Review and Continue

The strongest answers follow the same discipline as the lab:

1. name the observation;
2. map it to the smallest correct boundary;
3. state what earlier boundaries it supports;
4. state what remains unproved;
5. choose the next bounded test only when needed.

If any answer felt uncertain, return to the six-boundary evidence ladder and complete one diagnostic packet before repeating the check.

Next, continue to MAC Protocols: Sharing the Channel. That chapter builds on the local-link job you have already separated from routed packets and service meaning.