This N10-009 practice question tests your understanding of network security. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. 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 network security analyst notices that the firewall is logging traffic on the external interface that has a source IP address of 10.0.1.5, which is within the internal network range. This is most likely the result of which type of attack?
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 the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
IP spoofing
The firewall is logging traffic on its external interface with a source IP address from the internal RFC 1918 range (10.0.1.5). This indicates the source IP has been forged, because private IP addresses should never appear as source addresses on a public-facing interface. This is the classic signature of an IP spoofing attack, where the attacker modifies the source IP in the packet header to impersonate an internal host.
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.
✗
DNS poisoning
Why it's wrong here
DNS poisoning manipulates DNS resolution, not source IP addresses.
✓
IP spoofing
Why this is correct
IP spoofing involves crafting packets with a forged source IP address to impersonate an internal host.
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.
✗
ARP poisoning
Why it's wrong here
ARP poisoning manipulates ARP tables to intercept traffic on a local network.
✗
VLAN hopping
Why it's wrong here
VLAN hopping is an attack to gain access to traffic on other VLANs, not typically involving source IP spoofing.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates confuse IP spoofing with ARP poisoning, because both involve address impersonation, but ARP poisoning is a Layer 2 attack confined to the local subnet, whereas IP spoofing can originate from anywhere on the Internet and is visible on the external interface.
Detailed technical explanation
How to think about this question
IP spoofing relies on the fact that IP routing is destination-based; routers forward packets based on the destination address without validating the source address. Ingress filtering per BCP 38 (RFC 2827) is the mitigation, where edge routers drop packets with source addresses that do not belong to the expected inbound prefix. In this scenario, the firewall should have an ACL blocking RFC 1918 addresses on the external interface, which would have prevented the spoofed packet from being logged.
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.
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 Security — This question tests Network Security — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: IP spoofing — The firewall is logging traffic on its external interface with a source IP address from the internal RFC 1918 range (10.0.1.5). This indicates the source IP has been forged, because private IP addresses should never appear as source addresses on a public-facing interface. This is the classic signature of an IP spoofing attack, where the attacker modifies the source IP in the packet header to impersonate an internal host.
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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Question Discussion
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