Given a scenario, use the appropriate tool or protocol to solve networking issues.

Objective 5.5 sits in Network Troubleshooting, which carries 24% of the Network+ exam. The questions below are original, written from the official objective title above, and each explanation cites the CompTIA page it rests on.

Objective title verbatim from the official objectives. CompTIA exam page ↗

A worked example

Shown solved, with the whole explanation open: this is what every question here carries.

5-5Network TroubleshootingModerate

A technician probes a closed UDP port on a server that responds to ping, and an unreachable message comes back at once. Which device generated it?

The sender's own stackA local stack surfaces errors for its own send failures and cannot know a remote port is closed until something tells it.

The first router in the pathThe first router would answer only if the destination network were unroutable from there, which a working ping rules out.

The destination host itselfCorrect · your answerCorrect. The destination's own IP module reports the inactive port.

The last router before itThe last router can report an unreachable host, a delivery failure on the final link rather than a closed service.

Correct.

Checked against datatracker.ietf.org, August 2026

Concept

Only the device that owns an address can know which of its own services are listening. A forwarding device can report that it failed to deliver, but it cannot speak to whether a process on another machine is running.

Why C

When the IP module on the destination cannot hand a datagram to the named protocol or port because nothing is active there, that host is the device that returns the unreachable message.

Source

Code 0 = net unreachable; 1 = host unreachable; 2 = protocol unreachable; 3 = port unreachable; 4 = fragmentation needed and DF set; 5 = source route failed.... Codes 0, 1, 4, and 5 may be received from a gateway. Codes 2 and 3 may be received from a host.

IETF RFC 792: Internet Control Message Protocol, checked August 2026

#icmp#port unreachable#troubleshooting tools

Now you: objective 5.5 questions

No account needed. The explanation opens when you answer.

Sample question 1 of 3

5-5Network TroubleshootingHard

A capture holds destination unreachable messages carrying codes 2 and 3. Which device must have produced them?

Sample question 2 of 3

5-5Network TroubleshootingModerate

A host receives part of a fragmented datagram, waits past its limit for the rest, and discards what it holds. Which message may it send?

Sample question 3 of 3

5-5Network TroubleshootingHard

A flow collector shows records arriving from a router but cannot resolve any fields. The router was reloaded an hour earlier and nothing else changed. What happened?

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Read the sources

These are the official pages the questions above cite. Reading them is studying the objective from the primary source, which is what the explanations point you toward anyway.

A worked example

Now you: objective 5.5 questions

Full Network+ question bank coming

Read the sources

More objectives in Network Troubleshooting