The correct answer is that the payroll server’s default gateway lacks a route back to the accounting subnet, causing asymmetric routing where the missing return path drops response packets. This occurs because the firewall permits the outbound traffic from 10.10.20.0/24 to 10.10.10.50, but the server’s default gateway—often a router with no knowledge of the accounting subnet—cannot forward the return traffic, resulting in one-way communication. On the CISSP exam, this scenario tests your understanding of routing fundamentals and stateful firewall behavior, often appearing in network security domain questions about traffic flow and access control. A common trap is assuming the firewall rule alone guarantees connectivity; in reality, you must verify that every device along the path has a route back to the source. Memory tip: think of it as a “round-trip ticket”—outbound permission is useless without a return route.
CISSP Communication and Network Security Practice Question
This CISSP practice question tests your understanding of communication and network 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 network administrator notices that users in the accounting department can access the internet but are unable to access the internal payroll server (10.10.10.50). The firewall rule allows traffic from the accounting subnet (10.10.20.0/24) to the payroll server. What is the most likely issue?
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
✓
The payroll server's default gateway does not have a route back to 10.10.20.0/24.
The most likely issue is that the payroll server's default gateway does not have a route back to the accounting subnet (10.10.20.0/24). Even if the firewall permits outbound traffic from the accounting subnet to the payroll server, the return traffic from the server must be routed back through the firewall or a router that knows how to reach 10.10.20.0/24. Without a return route, the server's response packets are dropped, causing a one-way communication failure.
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 is not resolving the payroll server's IP address.
Why it's wrong here
They are using IP directly.
✓
The payroll server's default gateway does not have a route back to 10.10.20.0/24.
Why this is correct
Without a return route, packets from the server cannot reach the accounting subnet.
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 firewall rule is applied to the outbound interface only.
Why it's wrong here
The rule should be bidirectional or stateful.
✗
The accounting subnet is blocked by an implicit deny rule.
Why it's wrong here
The explicit allow rule should override implicit deny.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often focus on firewall rule direction (inbound vs. outbound) or DNS, overlooking the fundamental requirement for symmetric routing and the fact that the server's default gateway must know how to reach the source subnet.
Detailed technical explanation
How to think about this question
This scenario illustrates asymmetric routing, where the forward path (accounting to payroll) succeeds but the reverse path fails due to missing routing information. In a stateful firewall, the firewall tracks connection state and expects return traffic to match the session; however, if the return traffic bypasses the firewall (e.g., via a different default gateway), the firewall may drop it. The correct solution is to ensure the payroll server's default gateway (or the router serving it) has a static route or dynamic routing entry for 10.10.20.0/24 pointing back through the firewall.
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 SOC analyst notices unusual lateral movement in the network at 2 AM. The IR playbook dictates: identify and contain (isolate the affected machine), then eradicate (remove the malware), then recover (restore from backup), then document. Skipping containment before eradication risks the attacker regaining access. Questions like this test the sequence and rationale of incident response phases.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this CISSP question in full detail.
Communication and Network Security — This question tests Communication and Network Security — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The payroll server's default gateway does not have a route back to 10.10.20.0/24. — The most likely issue is that the payroll server's default gateway does not have a route back to the accounting subnet (10.10.20.0/24). Even if the firewall permits outbound traffic from the accounting subnet to the payroll server, the return traffic from the server must be routed back through the firewall or a router that knows how to reach 10.10.20.0/24. Without a return route, the server's response packets are dropped, causing a one-way communication failure.
What should I do if I get this CISSP 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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