- A
The DHCP server has exhausted its address pool.
Why wrong: If the DHCP server were out of addresses, other users on the same VLAN would also fail to obtain leases. This is inconsistent with the scenario.
- B
The switch port is in a blocked state due to a spanning tree loop.
Why wrong: A spanning tree loop would affect all devices on that port, not just DHCP, and would likely cause intermittent connectivity. Also, the port should transition to forwarding after convergence.
- C
The user's computer has not authenticated via 802.1X and is placed in a restricted VLAN.
With 802.1X, unauthenticated devices are often placed in a restricted VLAN that may not include the DHCP server or have limited access. This explains why the computer gets an APIPA address while other devices on the same physical port (if authenticated) work normally.
- D
The network cable is faulty.
Why wrong: If the cable were faulty, the link would likely be down or have excessive errors. The computer obtained a link and attempted DHCP, so a cable fault is improbable.
Quick Answer
The answer is that the user’s computer has not authenticated via 802.1X and is placed in a restricted VLAN. When 802.1X authentication is enabled on a switch port, the port begins in an unauthorized state, blocking all traffic except EAPoL frames. If the client fails to authenticate—due to a missing supplicant, bad credentials, or certificate errors—the switch can drop the port into a restricted or guest VLAN that lacks a DHCP server. This explains the 169.254.x.x APIPA address, as the client cannot obtain a lease. On the CompTIA Network+ N10-009 exam, this scenario tests your understanding of 802.1X authentication failure and its direct consequence: isolation from the production network. A common trap is assuming a physical cabling issue or a faulty DHCP server, but the key clue is that other users on the same VLAN get IPs successfully. Remember the mnemonic “No Auth, No DHCP”—if 802.1X fails, the client lands in a VLAN with no DHCP, triggering APIPA.
N10-009 Network Troubleshooting Practice Question
This N10-009 practice question tests your understanding of network troubleshooting. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
A user's computer is unable to obtain a DHCP lease after connecting to a new switch port. The user's computer displays an IP address in the 169.254.x.x range. Other users on the same VLAN can obtain IP addresses successfully. The switch port is configured for 802.1X authentication. What is the most likely cause?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue:
"most likely"Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
The user's computer has not authenticated via 802.1X and is placed in a restricted VLAN.
When 802.1X authentication is enabled on a switch port, the port initially operates in an unauthorized state, restricting traffic to only EAPoL (802.1X) frames. If the user's computer fails to authenticate (e.g., due to missing supplicant, incorrect credentials, or certificate issues), the switch can place the port into a restricted or guest VLAN that lacks a DHCP server or is isolated from the production network. The 169.254.x.x address (APIPA) indicates the client could not reach a DHCP server, which aligns with being placed in a VLAN without DHCP services.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
The DHCP server has exhausted its address pool.
- ✗
The switch port is in a blocked state due to a spanning tree loop.
Why it's wrong here
A spanning tree loop would affect all devices on that port, not just DHCP, and would likely cause intermittent connectivity. Also, the port should transition to forwarding after convergence.
- ✓
The user's computer has not authenticated via 802.1X and is placed in a restricted VLAN.
Why this is correct
With 802.1X, unauthenticated devices are often placed in a restricted VLAN that may not include the DHCP server or have limited access. This explains why the computer gets an APIPA address while other devices on the same physical port (if authenticated) work normally.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
The network cable is faulty.
Why it's wrong here
If the cable were faulty, the link would likely be down or have excessive errors. The computer obtained a link and attempted DHCP, so a cable fault is improbable.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that a 169.254.x.x address always indicates a DHCP server problem, but the trap here is that the issue is actually an 802.1X authentication failure placing the port into a restricted VLAN without DHCP, not a global DHCP exhaustion or spanning-tree issue.
Trap categories for this question
Scenario analysis trap
If the DHCP server were out of addresses, other users on the same VLAN would also fail to obtain leases. This is inconsistent with the scenario.
Detailed technical explanation
How to think about this question
With 802.1X, the switch port transitions through states: unauthorized (only EAPoL allowed), authenticating, and authorized (full access). If authentication fails or is not attempted, the switch can apply a restricted VLAN (RFC 3580) or a guest VLAN, which often has no DHCP server or is isolated. The client's OS then auto-configures an APIPA address (169.254.0.0/16) after failing to receive a DHCP offer, as defined in RFC 3927. In real-world scenarios, this is common when a new device lacks the correct 802.1X supplicant configuration or when certificates are expired.
KKey Concepts to Remember
- Read the scenario before looking for a memorised answer.
- Find the constraint that changes the correct option.
- Eliminate answers that are true in general but not in this case.
TExam Day Tips
- Watch for words such as best, first, most likely and least administrative effort.
- Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A help-desk technician troubleshoots why a newly connected PC cannot reach shared printers on the same floor. The cable is good, the switch port is active, but the PC is in VLAN 20 and the printers are in VLAN 10. The uplink trunk only allows VLAN 10. A trunk being up does not mean every VLAN crosses it.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
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Network Troubleshooting — study guide chapter
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FAQ
Questions learners often ask
What does this N10-009 question test?
Network Troubleshooting — This question tests Network Troubleshooting — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The user's computer has not authenticated via 802.1X and is placed in a restricted VLAN. — When 802.1X authentication is enabled on a switch port, the port initially operates in an unauthorized state, restricting traffic to only EAPoL (802.1X) frames. If the user's computer fails to authenticate (e.g., due to missing supplicant, incorrect credentials, or certificate issues), the switch can place the port into a restricted or guest VLAN that lacks a DHCP server or is isolated from the production network. The 169.254.x.x address (APIPA) indicates the client could not reach a DHCP server, which aligns with being placed in a VLAN without DHCP services.
What should I do if I get this N10-009 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 11, 2026
This N10-009 practice question is part of Courseiva's free CompTIA certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the N10-009 exam.
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