- A
Denial-of-service (DoS) attack
Why wrong: A DoS attack aims to overwhelm a system, but the symptom described (internal source IP on external interface) is specifically characteristic of spoofing.
- B
IP spoofing
IP spoofing is when an attacker forges the source IP address to appear as a trusted host. Seeing an internal IP on an external interface is a clear sign of spoofing.
- C
Man-in-the-middle (MITM) attack
Why wrong: MITM attacks involve intercepting communications between two parties, usually on the same local network. This scenario shows spoofed packets from outside.
- D
ARP poisoning
Why wrong: ARP poisoning manipulates MAC-IP mappings within a local network and would not produce logs of internal source IPs on an external interface.
Question 14 of 520
N10-009 Network Security Practice Question
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 security analyst is investigating a potential breach. A network device shows logs indicating that it received packets with a source IP address belonging to the internal network range on its external (internet-facing) interface. This is a classic indication of which type of attack?
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
✓
IP spoofing
B is correct because receiving packets with a source IP address from the internal network range on an external (internet-facing) interface is a classic sign of IP spoofing. In IP spoofing, an attacker forges the source IP address in packet headers to impersonate a trusted internal host, often to bypass access controls or launch reflection attacks. This violates the expected behavior of ingress filtering, where external interfaces should never see internal source addresses.
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.
- ✗
Denial-of-service (DoS) attack
Why it's wrong here
A DoS attack aims to overwhelm a system, but the symptom described (internal source IP on external interface) is specifically characteristic of spoofing.
When this WOULD be correct
A question describing a network device being overwhelmed with traffic from multiple sources, causing service disruption, would make DoS the correct answer. For example: 'A web server becomes unresponsive due to a flood of TCP SYN packets from many different IP addresses.'
- ✓
IP spoofing
Why this is correct
IP spoofing is when an attacker forges the source IP address to appear as a trusted host. Seeing an internal IP on an external interface is a clear sign of spoofing.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
Man-in-the-middle (MITM) attack
Why it's wrong here
MITM attacks involve intercepting communications between two parties, usually on the same local network. This scenario shows spoofed packets from outside.
When this WOULD be correct
A question describing an attacker intercepting traffic between a client and server, possibly using ARP spoofing or a rogue access point, and then modifying or eavesdropping on the communication would make MITM the correct answer.
- ✗
ARP poisoning
Why it's wrong here
ARP poisoning manipulates MAC-IP mappings within a local network and would not produce logs of internal source IPs on an external interface.
When this WOULD be correct
A question describing an attacker sending forged ARP replies to associate their MAC address with the IP of a legitimate device, causing traffic to be misdirected, would make ARP poisoning the correct answer.
Option-by-option analysis
Why each answer is right or wrong
Understanding why wrong answers are wrong — and when they would be correct — is what separates a 750 score from a 900. The N10-009 exam frequently reuses these exact scenarios with slightly different constraints.
✓IP spoofingCorrect answer▾
Why this is correct
IP spoofing is when an attacker forges the source IP address to appear as a trusted host. Seeing an internal IP on an external interface is a clear sign of spoofing.
✗Denial-of-service (DoS) attackWrong answer — click to see why▾
Why this is wrong here
The logs show packets with an internal source IP arriving on the external interface, which is a sign of IP spoofing, not a DoS attack. DoS attacks focus on overwhelming resources, not on forging source addresses.
★ When this WOULD be the correct answer
A question describing a network device being overwhelmed with traffic from multiple sources, causing service disruption, would make DoS the correct answer. For example: 'A web server becomes unresponsive due to a flood of TCP SYN packets from many different IP addresses.'
Why candidates choose this
Candidates may associate any malicious traffic with DoS, especially when logs show unusual packets, without recognizing that the specific indicator (internal IP on external interface) points to spoofing rather than resource exhaustion.
✗Man-in-the-middle (MITM) attackWrong answer — click to see why▾
Why this is wrong here
A man-in-the-middle attack involves intercepting and potentially altering communications between two parties, but the specific indicator of packets with an internal source IP on an external interface points to IP spoofing, not MITM.
★ When this WOULD be the correct answer
A question describing an attacker intercepting traffic between a client and server, possibly using ARP spoofing or a rogue access point, and then modifying or eavesdropping on the communication would make MITM the correct answer.
Why candidates choose this
Candidates may confuse IP spoofing with MITM because both involve deceptive IP addresses, but MITM focuses on interception and relay, not just falsifying the source address.
✗ARP poisoningWrong answer — click to see why▾
Why this is wrong here
ARP poisoning operates at Layer 2 and involves corrupting ARP caches within a local network segment, not receiving packets with internal source IPs on an external interface.
★ When this WOULD be the correct answer
A question describing an attacker sending forged ARP replies to associate their MAC address with the IP of a legitimate device, causing traffic to be misdirected, would make ARP poisoning the correct answer.
Why candidates choose this
Candidates may confuse IP spoofing with ARP poisoning because both involve falsifying addresses, but ARP poisoning is specific to MAC-IP mapping on local networks, not the external interface scenario described.
Analysis generated from the official N10-009blueprint and verified against question context. The “when correct” sections are what AI assistants cite when candidates ask “what’s the difference between these options?”
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates may confuse IP spoofing with a DoS attack because spoofing is often used in DDoS amplification, but the question's specific clue—internal source IP on an external interface—directly points to spoofing, not the volumetric nature of a DoS.
Trap categories for this question
Command / output trap
MITM attacks involve intercepting communications between two parties, usually on the same local network. This scenario shows spoofed packets from outside.
Scenario analysis trap
MITM attacks involve intercepting communications between two parties, usually on the same local network. This scenario shows spoofed packets from outside.
Detailed technical explanation
How to think about this question
Under the hood, IP spoofing exploits the lack of authentication in the IP protocol (RFC 791), where the source address field can be arbitrarily set. In a real-world scenario, an attacker might spoof an internal IP to bypass firewall rules that permit traffic from trusted subnets, or to amplify a DDoS attack via DNS reflection (e.g., sending queries with a spoofed victim IP). Network administrators can mitigate this with ingress/egress filtering (BCP 38), which drops packets with source addresses that do not match the expected interface subnet.
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 practitioner preparing for the N10-009 exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
Quick reference
IPv4 Address Class Summary
| Class | First Octet Range | Default Mask | Networks | Hosts per Network |
|---|---|---|---|---|
| A | 1–126 | /8 (255.0.0.0) | 126 | 16,777,214 |
| B | 128–191 | /16 (255.255.0.0) | 16,384 | 65,534 |
| C | 192–223 | /24 (255.255.255.0) | 2,097,152 | 254 |
| D | 224–239 | N/A | Multicast groups | — |
| E | 240–255 | N/A | Reserved / experimental | — |
127.x.x.x is reserved for loopback. Modern networks use CIDR (classless) rather than classful addressing.
What to study next
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FAQ
Questions learners often ask
What does this N10-009 question test?
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 — B is correct because receiving packets with a source IP address from the internal network range on an external (internet-facing) interface is a classic sign of IP spoofing. In IP spoofing, an attacker forges the source IP address in packet headers to impersonate a trusted internal host, often to bypass access controls or launch reflection attacks. This violates the expected behavior of ingress filtering, where external interfaces should never see internal source addresses.
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.
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
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