FE Electrical & Computer Practice Question Solution included after investigation
CASE E-040 · FE

The Router That Ignored the Shorter-Looking Route

Two routes matched the destination. The router chose the more specific prefix, not the visually simpler route.

▶ INTERACTIVE CASE FILE • 5–10 MIN • CLICK TO INVESTIGATE

Inspect, commit, prove, fix, and sign off.

Case E-040 engineering failure visual
HOW THIS WORKS

Think like
a real engineer.

Follow the investigation process used in the field — in five guided steps.

1

Inspect

Review the problem, diagram, and evidence.

2

Commit

Choose your hypothesis.

3

Prove

Run calculations and test your idea.

4

Fix

Select and validate a safe correction.

5

Sign Off

See the full debrief and key takeaways.

Case Brief

SAME PRINCIPLES. HIGHER STANDARDS.
PROBLEM STATEMENT

A router has two relevant entries: 10.0.0.0/8 via Interface A and 10.1.2.0/24 via Interface B. A packet is destined for 10.1.2.55. A technician expects the /8 route because it appears first in the table. The router sends the packet through Interface B. Diagnose the route-selection rule and identify which entry wins.

Computer Networks Electrical & Computer Design Verification Root Cause Analysis Safe Correction
E-040
INVESTIGATION WORKSPACE WORK THROUGH THE CASE — YOUR CHOICES MATTER.
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DEBRIEF (LOCKED)

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Find the root cause, confirm the fix, and see how this connects to the exam.

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CASE CLOSED · ENGINEERING VERDICT

What actually failed — and what should change.

What failed

The expected route was based on table order and breadth rather than specificity.

What was assumed

The shorter prefix would win.

Corrected model

Use longest-prefix matching among all routes that contain the destination.

QUICK ANSWER

When multiple IP routes match a destination, a router selects the longest matching prefix. For destination 10.1.2.55, both 10.0.0.0/8 and 10.1.2.0/24 match, but /24 is more specific and is selected.

ENGINEERING TAKEAWAY

Route lookup uses the most specific matching network prefix: the longest prefix length among matching entries.

CASE QUESTIONS

Questions this case should settle.

What does longest-prefix match mean?

Choose the matching route with the greatest prefix length, meaning the most specific network.

Which route wins between a matching /8 and /24?

The /24 route, assuming no even more-specific match exists.

NEXT FE PRACTICE QUESTION

The 1500-Byte Packet That Did Not Arrive in 12 Microseconds

Computer Networks · 5–10 min

Continue Practice →
WHAT YOU’LL LEARN

Real failures. Lasting skills.

Each case is designed to build the judgment, analysis, and confidence you need for engineering exams — and beyond.

  • Apply core engineering concepts to real-world problems
  • Practice structured troubleshooting and analysis
  • Strengthen exam-ready thinking through realistic scenarios
COMMON QUESTIONS
Do I need prior knowledge to use these cases?

Basic subject familiarity helps, but every case is designed to teach through the investigation itself.

How long does a case take?

Most cases are designed for a focused 5–10 minute investigation.

Are the cases aligned with engineering exams?

Each case is mapped to a verified exam, subject, topic, and misconception before publication.

What happens after I complete a case?

The sealed debrief unlocks with the root cause, corrected reasoning, fix, and takeaway.