- A
The DHCP server does not have a scope configured for the 192.168.1.0/24 subnet (VLAN 10)
The DHCP server uses the gateway IP (giaddr) in the relayed packet to determine which scope to use. Without a matching scope, the server does not respond.
- B
The router's ip helper-address is configured on the wrong interface
Why wrong: The scenario states the helper address is on the VLAN 10 interface; if it were on the wrong interface, users would not reach the DHCP server at all, but they can ping the server from the router.
- C
The switch port connecting users is configured as a trunk instead of an access port
Why wrong: A trunk port would still allow traffic in VLAN 10, but if misconfiguration caused the user to be in the wrong VLAN, they might not be in VLAN 10. However, they could still likely reach the DHCP relay if routing works. A trunk misconfiguration is less likely to cause APIPA when the relay is working.
- D
The router's ACL is blocking DHCP offers from the server
Why wrong: If an ACL were blocking DHCP replies, the router would still receive them but drop them; however, the users would not get replies, leading to APIPA. But since the helper address is configured, the ACL would need to explicitly block UDP ports 67/68, which is less common than a missing scope.
Quick Answer
The answer is that the DHCP server lacks a scope configured for the client VLAN’s subnet, specifically 192.168.1.0/24. The ip helper-address command on the router successfully forwards DHCPDISCOVER broadcasts from VLAN 10 to the DHCP server in VLAN 20, and since users can ping the server, Layer 3 connectivity is not the issue. However, the server will only respond with an IP offer if it has a defined scope matching the requesting client’s subnet; without that scope, it silently drops the request, forcing clients to self-assign APIPA addresses (169.254.x.x). On the CompTIA Network+ N10-009 exam, this scenario tests your understanding of DHCP relay operations and the common trap of assuming connectivity alone guarantees address assignment. A frequent mistake is troubleshooting routing or the helper address itself when the real problem is a missing scope on the server. Memory tip: “No scope, no hope—the server needs a pool for the client’s subnet to play its role.”
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.
Users in VLAN 10 cannot obtain IP addresses from the DHCP server located in VLAN 20. The router interface for VLAN 10 has an ip helper-address 192.168.20.5 command configured, and users can ping the DHCP server IP (192.168.20.5) from the router. However, users receive APIPA addresses. 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 DHCP server does not have a scope configured for the 192.168.1.0/24 subnet (VLAN 10)
The ip helper-address command on the router correctly forwards DHCPDISCOVER broadcasts from VLAN 10 to the DHCP server at 192.168.20.5. Since users can ping the server from the router, Layer 3 connectivity exists. However, the DHCP server must have a scope (or address pool) for the subnet of the requesting clients (192.168.1.0/24) to offer an IP address; without it, the server ignores the request, and clients fall back to APIPA (169.254.x.x).
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 does not have a scope configured for the 192.168.1.0/24 subnet (VLAN 10)
Why this is correct
The DHCP server uses the gateway IP (giaddr) in the relayed packet to determine which scope to use. Without a matching scope, the server does not respond.
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 router's ip helper-address is configured on the wrong interface
- ✗
The switch port connecting users is configured as a trunk instead of an access port
Why it's wrong here
A trunk port would still allow traffic in VLAN 10, but if misconfiguration caused the user to be in the wrong VLAN, they might not be in VLAN 10. However, they could still likely reach the DHCP relay if routing works. A trunk misconfiguration is less likely to cause APIPA when the relay is working.
- ✗
The router's ACL is blocking DHCP offers from the server
Why it's wrong here
If an ACL were blocking DHCP replies, the router would still receive them but drop them; however, the users would not get replies, leading to APIPA. But since the helper address is configured, the ACL would need to explicitly block UDP ports 67/68, which is less common than a missing scope.
Common exam traps
Common exam trap: answer the scenario, not the keyword
CompTIA often tests the misconception that ip helper-address alone guarantees DHCP success, but the trap is that the DHCP server must have a scope matching the client's subnet (identified by the giaddr) to issue an address.
Trap categories for this question
Scenario analysis trap
The scenario states the helper address is on the VLAN 10 interface; if it were on the wrong interface, users would not reach the DHCP server at all, but they can ping the server from the router.
Detailed technical explanation
How to think about this question
DHCP relay (ip helper-address) uses UDP port 67 (server) and 68 (client). When the relay agent forwards a DISCOVER, it sets the giaddr (gateway IP address) field to the router's interface IP (192.168.1.1). The DHCP server uses this giaddr to select the correct scope; if no scope matches 192.168.1.0/24, the server silently drops the packet. In real-world scenarios, misconfigured scopes are a common cause of DHCP failures even when relay is correctly set up.
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
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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 DHCP server does not have a scope configured for the 192.168.1.0/24 subnet (VLAN 10) — The ip helper-address command on the router correctly forwards DHCPDISCOVER broadcasts from VLAN 10 to the DHCP server at 192.168.20.5. Since users can ping the server from the router, Layer 3 connectivity exists. However, the DHCP server must have a scope (or address pool) for the subnet of the requesting clients (192.168.1.0/24) to offer an IP address; without it, the server ignores the request, and clients fall back to APIPA (169.254.x.x).
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.
About these practice questions
Courseiva creates original exam-style practice questions with explanations and wrong-answer analysis. It does not publish real exam questions, exam dumps, or protected exam content. Learn why practice questions differ from exam dumps →
Same concept, more angles
1 more ways this is tested on N10-009
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. Users in VLAN 10 cannot obtain IP addresses from a DHCP server located in VLAN 20. The router has an ip helper-address configured on VLAN 10 interface pointing to the DHCP server. Users can ping the DHCP server IP from the router. However, users are receiving APIPA addresses. What is the most likely cause?
medium- A.The DHCP server is not reachable from the router
- ✓ B.The DHCP server scope does not include the VLAN 10 subnet
- C.The router's ip helper-address is configured incorrectly
- D.The switch ports are not configured for VLAN 10
Why B: The correct answer is B because the DHCP server must have a scope configured for the subnet of the requesting clients (VLAN 10) to assign IP addresses from that range. Since users receive APIPA addresses (169.254.x.x), the DHCP discovery process is failing, which typically occurs when the server receives the request via the ip helper-address but has no matching scope for VLAN 10. The router's ability to ping the server confirms Layer 3 reachability, isolating the issue to the server's scope configuration.
Keep practising
More N10-009 practice questions
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Last reviewed: Jun 30, 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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