SSCP Network and Communications Security Practice Question
This SSCP practice question tests your understanding of network and communications security. 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 helpdesk ticket reports that users can browse internal web servers but cannot access external websites. The IT team checks firewall logs and sees dropped packets with the DF flag set. 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 the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
A firewall is blocking ICMP 'fragmentation needed' messages, preventing Path MTU Discovery.
The DF (Don't Fragment) flag being set in dropped packets indicates that packets are too large to traverse a network path without fragmentation, but the firewall is blocking the ICMP 'fragmentation needed' (Type 3, Code 4) messages. Without these ICMP messages, Path MTU Discovery (PMTUD) fails, causing the sending host to believe the packet was delivered when it was actually dropped. This prevents external websites from loading while internal servers (likely on the same MTU-friendly network) remain accessible.
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 firewall is blocking outbound TCP port 80 and 443.
Why it's wrong here
If internal web servers work, outbound ports are likely allowed.
✓
A firewall is blocking ICMP 'fragmentation needed' messages, preventing Path MTU Discovery.
Why this is correct
This is a classic problem where DF packets are dropped because the firewall blocks ICMP type 3 code 4 messages.
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 DNS server is not resolving external domain names.
Why it's wrong here
DNS failure would prevent any name resolution, but internal browsing works.
✗
The switch port connecting the firewall is set to the wrong VLAN.
Why it's wrong here
This would affect all traffic, not just external.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often assume the DF flag indicates a firewall rule explicitly blocking fragmented packets, rather than recognizing it as a symptom of missing ICMP feedback in the PMTUD process.
Detailed technical explanation
How to think about this question
Path MTU Discovery works by sending packets with the DF flag set; if a router along the path cannot forward the packet without fragmentation, it sends an ICMP Type 3 Code 4 message back to the source with the next-hop MTU. Many firewalls are misconfigured to block all ICMP, including these essential messages, causing TCP connections to stall or fail for hosts behind a smaller MTU link (e.g., PPPoE with MTU 1492). RFC 1191 defines PMTUD, and RFC 2923 documents the problems caused by ICMP filtering.
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 security analyst at a medium-sized enterprise encounters this scenario during an investigation or architecture review. The correct answer reflects best practice for the specific threat or control described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Security exam questions test whether you can match controls to threats in context — not just recall definitions.
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.
Network and Communications Security — This question tests Network and Communications Security — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: A firewall is blocking ICMP 'fragmentation needed' messages, preventing Path MTU Discovery. — The DF (Don't Fragment) flag being set in dropped packets indicates that packets are too large to traverse a network path without fragmentation, but the firewall is blocking the ICMP 'fragmentation needed' (Type 3, Code 4) messages. Without these ICMP messages, Path MTU Discovery (PMTUD) fails, causing the sending host to believe the packet was delivered when it was actually dropped. This prevents external websites from loading while internal servers (likely on the same MTU-friendly network) remain accessible.
What should I do if I get this SSCP 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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