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 calls the help desk stating that they cannot access any network resources. The technician asks the user to run ipconfig and the output shows an IP address of 169.254.15.20 with a subnet mask of 255.255.0.0. Which of the following 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.
Why wrong: DNS failure would not cause an APIPA address; it would still get a valid IP from DHCP but fail name resolution.
B
The DHCP server is unreachable
When DHCP fails, Windows automatically assigns an APIPA address, indicating the device could not contact a DHCP server.
C
The default gateway is misconfigured
Why wrong: A default gateway issue would affect off-subnet communication but not the IP address assignment; the device would still receive a valid DHCP IP.
D
There is a duplicate IP address on the network
Why wrong: A duplicate IP address would cause a conflict but not result in an APIPA address; the system would still have its configured IP.
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 is unreachable
The IP address 169.254.15.20 with a subnet mask of 255.255.0.0 is an Automatic Private IP Addressing (APIPA) address, which Windows assigns when a DHCP client fails to receive a lease from a DHCP server. Since the user cannot access any network resources, the most likely cause is that the DHCP server is unreachable, preventing the client from obtaining a valid IP address, default gateway, and DNS server settings.
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 DNS server is not responding
Why it's wrong here
DNS failure would not cause an APIPA address; it would still get a valid IP from DHCP but fail name resolution.
✓
The DHCP server is unreachable
Why this is correct
When DHCP fails, Windows automatically assigns an APIPA address, indicating the device could not contact a DHCP server.
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 default gateway is misconfigured
Why it's wrong here
A default gateway issue would affect off-subnet communication but not the IP address assignment; the device would still receive a valid DHCP IP.
✗
There is a duplicate IP address on the network
Why it's wrong here
A duplicate IP address would cause a conflict but not result in an APIPA address; the system would still have its configured IP.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often confuse APIPA with a DNS failure or gateway issue, but APIPA specifically indicates the DHCP process failed, not that other network services are misconfigured.
Detailed technical explanation
How to think about this question
APIPA is defined in RFC 3927 and uses the 169.254.0.0/16 range; the client performs a gratuitous ARP probe to ensure no other host uses the randomly selected address before assigning it. In a real-world scenario, a DHCP server might be unreachable due to a misconfigured DHCP relay agent, a VLAN mismatch, or a switchport in the wrong VLAN, causing the DHCPDISCOVER broadcast to never reach the server.
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 network engineer segments a warehouse floor into three subnets: 20 scanners, 5 printers, and 2 management hosts. Picking the wrong mask wastes addresses or leaves too few usable hosts. Exam questions test whether you can apply CIDR notation, calculate block size, and identify the correct usable-host range for a given prefix.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this N10-009 question in full detail.
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 is unreachable — The IP address 169.254.15.20 with a subnet mask of 255.255.0.0 is an Automatic Private IP Addressing (APIPA) address, which Windows assigns when a DHCP client fails to receive a lease from a DHCP server. Since the user cannot access any network resources, the most likely cause is that the DHCP server is unreachable, preventing the client from obtaining a valid IP address, default gateway, and DNS server settings.
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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