Lesson 5: Local Network Fault Isolation

Core path: 42 minutes

"The internet is down" is a symptom report, not a diagnosis.

Today you prove how far the endpoint can actually communicate.

Case A: no link

Evidence:

• Ethernet interface reports disconnected;
• no link indicators;
• known-good switch port available.

Start at cable, connector, NIC, and switch-port boundaries. DHCP is downstream of a usable link.

Case B: APIPA

Evidence:

IPv4: 169.254.22.19
Gateway: none

The interface has an IPv4 self-assigned address but did not receive the expected site configuration.

Strong next question: can the client reach the DHCP path/service?

Weak response: manually type a random public DNS server and hope.

Case C: local works, remote fails

Evidence:

• local printer responds;
• another same-subnet endpoint responds;
• configured gateway does not respond.

The surviving local path makes a total NIC/switch failure less likely. Focus near the gateway boundary.

Case D: duplicate address symptoms

Two endpoints were manually assigned the same IPv4 address. Connectivity becomes inconsistent.

The fix is not to reboot until luck changes. Correct the address plan and verify uniqueness.

Service record

Choose two cases and document:

symptom:
physical/link evidence:
address evidence:
local-path evidence:
gateway evidence:
strongest theory:
controlled next test:
expected result:
verification:

Week 7 defense

Explain this chain using your SOHO topology:

media/link
→ NIC
→ switch/AP
→ valid local IP configuration
→ local destination
→ gateway
→ remote network

If you cannot point to the last proven-good boundary, you are still guessing.