Prove a port with a loopback and distinguish it from the loopback address

short · 40 min · Objective 5.2

Task

Test a network port with a physical loopback or its nearest available substitute, and establish precisely what that result proves compared with a loopback address ping -- two tests with similar names and different scopes.

Steps

  1. Ping the loopback address and record the result in lab/loop/address.txt, then write in lab/loop/address-proves.md exactly what that proved and what it did not touch.
  2. Where a loopback plug is available, fit it and run the adapter's own diagnostic, recording the result in lab/loop/plug.txt. WHERE IT IS NOT: connect the adapter to a known-good switch port with a known-good cable and record the link state instead, noting the substitution in lab/loop/substitute.md.
  3. Write lab/loop/compare.csv with header test,layer,proves,does_not_prove comparing the address ping, the plug or substitute, and a ping to another host.
  4. Write lab/loop/tools.csv with header tool,question,what_a_pass_misses for the wire-map tester, the certifier, the reflectometer, the toner and the loopback plug.
  5. Write lab/loop/scenario.md describing a machine that passes the address ping and fails everything else, and what that narrows the fault to.

Verify

grep -c . lab/loop/address.txt
grep -c . lab/loop/address-proves.md
ls lab/loop/plug.txt lab/loop/substitute.md 2>/dev/null | wc -l
awk -F, 'NR>1 && NF>=4 {n++} END {print n" test(s) compared"}' lab/loop/compare.csv
awk -F, 'NR>1 && NF>=3 {n++} END {print n" tool(s) with a stated blind spot"}' lab/loop/tools.csv
grep -c . lab/loop/scenario.md

An address ping with its scope stated, either a plug result or a documented substitute, three tests compared, five tools each with a stated blind spot, and the scenario. A tools table with an empty blind-spot column has recorded what each tool does and missed the point of the lesson.

Notes

The address ping tests software and never reaches the adapter. Reporting 'the loopback test passed' without saying which loopback is how a failed adapter gets cleared.

This is an independent study companion for CompTIA A+ Core 1 220-1201 and is not produced by or endorsed by CompTIA.