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Digital · Guided lesson

Distinguish resolution from connection

About 29 minutes · 7 reading + 22 practice and checks
Time includes reading, practice, and questions. Extra videos or further study are optional.
See this course’s project requirements

A localhost-only client/server diagnostic packet distinguishing name resolution, connection, timeout and HTTP response, with controlled failures and no probing of external systems.

Keep your working files in your own notebook or software. Save a private description, checklist, and work link in My Proof.

What to include

  • Produce a loopback-only ownership and diagnostic-layer scope card.
  • Retain actual localhost resolution and distinctly labeled injected-failure evidence.
  • Demonstrate and classify a real owned-loopback refusal with cleanup evidence.
  • Verify and classify real local status, media type, and parsed-body cases.
  • Retain a controlled connected-silent-read timeline and cleanup record.
  • Deliver a layered, repeatable diagnostic packet with actual/simulated cases and exact scope limits.

Review and revise

Use these criteria to check your work and choose what to improve.

  • The implementation or declared simulation follows the supplied contract and retains inspectable original inputs.
  • Normal, boundary and rejected cases are recorded with expected versus observed results.
  • The changed case has a correct explanation and does not rely on hidden state.
  • The project includes source, outputs, instructions and honest tool, privacy and verification limits.
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Before you begin

The idea in everyday life

local_diagnostic.py and test-local-network.py create only owned loopback servers on temporary ports. Six cases distinguish actual localhost resolution, injected resolver failure, real HTTP 200/404, real refused local connection, and a real local read timeout. No external host is contacted.

The idea

Name resolution translates a host name into address information. A resolved address does not prove a listener exists or an HTTP request will succeed.

Follow along

Distinguish resolution from connection — original worked practice

Name resolution translates a host name into address information. A resolved address does not prove a listener exists or an HTTP request will succeed.

local_diagnostic.py and test-local-network.py create only owned loopback servers on temporary ports. Six cases distinguish actual localhost resolution, injected resolver failure, real HTTP 200/404, real refused local connection, and a real local read timeout. No external host is contacted.

  1. 1. Step 1

    Run the resolve assertion and inspect that localhost includes 127.0.0.1 under the IPv4 filter.

    You should see: The local name-to-address result is recorded.

    Name resolution translates a host name into address information. A resolved address does not prove a listener exists or an HTTP request will succeed.

  2. 2. Step 2

    Read the injected failed_resolver raising socket.gaierror.

    You should see: The failure is controlled simulation with no external DNS request.

    Name resolution translates a host name into address information. A resolved address does not prove a listener exists or an HTTP request will succeed.

  3. 3. Step 3

    Write observed system resolution separately from injected failure behavior.

    You should see: Actual and simulated evidence remain distinct.

    Name resolution translates a host name into address information. A resolved address does not prove a listener exists or an HTTP request will succeed.

  4. 4. Step 4

    Record why the next TCP stage is still required after resolution.

    You should see: A resolution-versus-connection comparison.

    Name resolution translates a host name into address information. A resolved address does not prove a listener exists or an HTTP request will succeed.

Now change one thing

localhost resolves but the selected local port has no listener. Has successful connection been established?

Your work

What to finish

Retain actual localhost resolution and distinctly labeled injected-failure evidence.

Common mistake

Watch for this

Claiming an injected resolver exception diagnosed a real network outage.

How can I tell I understand it?

Demonstrate the rule: Name resolution translates a host name into address information. A resolved address does not prove a listener exists or an HTTP request will succeed. Use the supplied variant to distinguish a changed result from a broken implementation.

Try 3 questions

Quick knowledge check

Answer all 3 questions. Get at least 2 right and the lesson completes automatically.

Not yet passed

You’ll get an explanation for each answer. Signed in? Your results save to your account.

1What does localhost resolving to 127.0.0.1 establish?
2How must the supplied gaierror case be labeled?
3Why record simulated and actual cases separately?
0 of 3 answeredApply the lesson to a short question, then review the explanation.
2
Finish this lesson

Pass the check. We’ll save the progress.

Answer all 3 questions above and get at least 2 right. You can review the explanations and try again.

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Sources and further reading

Explore the references behind this lesson.

docs.python.org — socket.html
Source & access notes · checked 2026-10-01

Relevant documented module/API behavior inspected during expansion authoring; actual execution evidence is recorded separately. No grant eligibility is established.

Access: Official page retrieved with the web reader on 2026-10-01. Documentation reading does not certify execution in every native tool or browser.

Reuse: Original local practice files, fictional data, worked examples and questions. Concepts paraphrased and linked; source exercises are not copied.

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