PCNSE Core Concepts and Architecture Practice Question
This PCNSE practice question tests your understanding of core concepts and architecture. 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 firewall is configured with a destination NAT rule to translate public IP 203.0.113.10 to internal server 10.0.0.5 on port 443. Internal users from 10.0.0.0/24 can access the server using its private IP, but cannot access using the public IP. What should be configured to allow internal users to reach the server using the public IP?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Configure a source NAT rule that translates the internal source IP to the firewall's interface IP when the destination is the public IP.
Option A is correct because when internal users send traffic to the public IP (203.0.113.10), the firewall performs destination NAT, translating the destination to 10.0.0.5. However, the return traffic from the server is sent directly to the internal user's IP (since they are on the same subnet), bypassing the firewall and causing asymmetric routing. A source NAT rule (often called NAT hairpin or NAT reflection) translates the internal source IP to the firewall's interface IP, forcing return traffic to go through the firewall and maintain session state.
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.
✓
Configure a source NAT rule that translates the internal source IP to the firewall's interface IP when the destination is the public IP.
Why this is correct
This hairpin NAT rule ensures reply traffic goes through the firewall.
Related concept
Read the scenario before looking for a memorised answer.
✗
Create a policy-based forwarding (PBF) rule to send the traffic to the server.
Add a security policy allowing traffic from internal zone to the public IP.
Why it's wrong here
Security policy is needed but the NAT problem remains; traffic will not be translated properly without a hairpin NAT rule.
✗
Add a static route on the firewall for the public IP pointing to the internal server.
Why it's wrong here
The firewall already knows the public IP from NAT; routing is not the issue.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often think a security policy or route is sufficient, but they miss the fundamental requirement for symmetric routing in stateful firewalls, where the return traffic must traverse the same firewall that performed the NAT.
Detailed technical explanation
How to think about this question
This scenario is known as NAT hairpin or NAT loopback, where internal hosts need to access an internal server via its public IP. The firewall must perform both destination NAT (to translate the public IP to the private IP) and source NAT (to translate the internal source IP to the firewall's own IP) so that return traffic is forced back through the firewall. Without source NAT, the server sees the original internal source IP and sends replies directly, causing asymmetric routing and session failure. In Palo Alto Networks firewalls, this is configured under the source NAT rule with a 'None' or 'Dynamic IP and Port' translation to the firewall's interface IP.
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 small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation applies.
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.
Core Concepts and Architecture — This question tests Core Concepts and Architecture — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Configure a source NAT rule that translates the internal source IP to the firewall's interface IP when the destination is the public IP. — Option A is correct because when internal users send traffic to the public IP (203.0.113.10), the firewall performs destination NAT, translating the destination to 10.0.0.5. However, the return traffic from the server is sent directly to the internal user's IP (since they are on the same subnet), bypassing the firewall and causing asymmetric routing. A source NAT rule (often called NAT hairpin or NAT reflection) translates the internal source IP to the firewall's interface IP, forcing return traffic to go through the firewall and maintain session state.
What should I do if I get this PCNSE 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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Question Discussion
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