The answer is that the IP pool configured with overload (PAT) changes source ports, which breaks protocols like SIP that require fixed ports. When FortiGate performs IP pool overload using Port Address Translation, it remaps the original source port to a random high port to multiplex many internal IPs to a single external address. For SIP, which relies on UDP port 5060 for signaling, this port preservation issue causes the SIP server to reject or misroute traffic because it expects communication from a consistent, well-known port. On the Fortinet NSE 4 exam, this scenario tests your understanding of how NAT overload interacts with application-layer protocols; a common trap is assuming IP pools only change IP addresses. Remember the mnemonic: “SIP sticks to its port—overload will distort.”
NSE4 Firewall Policies and NAT Practice Question
This NSE4 practice question tests your understanding of firewall policies and nat. 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 FortiGate has a policy that allows traffic from 10.0.0.0/8 to any destination with NAT enabled using an IP pool 'Pool1' (203.0.113.10-203.0.113.20). The admin notices that internal servers using fixed ports (e.g., SIP) are failing. What is the likely cause?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The IP pool is configured with overload (PAT), which changes source ports
When an IP pool is configured with overload (PAT), the FortiGate translates the source IP address and also changes the source port to a random high port. For protocols like SIP that rely on fixed source ports (e.g., UDP 5060), this port remapping breaks the application because the SIP server expects traffic from a specific port. Option D correctly identifies this as the root cause.
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 policy order is incorrect
Why it's wrong here
Policy order would affect matching, not port preservation.
✗
The IP pool is configured with one-to-one NAT
Why it's wrong here
One-to-one preserves IP and does not change ports; it would work better.
✗
The IP pool uses fixed port range, which should work
Why it's wrong here
Fixed port range preserves ports; it would not cause failure.
✓
The IP pool is configured with overload (PAT), which changes source ports
Why this is correct
Overload modifies source ports; protocols like SIP need consistent ports.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates assume any IP pool will preserve source ports, but overload (PAT) mode explicitly changes them, which breaks applications that require fixed source ports like SIP, DNS, or TFTP.
Detailed technical explanation
How to think about this question
PAT (Port Address Translation) modifies the source port to a unique value from a pool of high ports (typically 1024-65535) to allow many internal hosts to share a single public IP. SIP uses UDP port 5060 as both source and destination; if the source port is changed, the SIP server may reject the packet or the SIP client may not recognize the response. This is a common issue with ALG (Application Layer Gateway) handling, but in this case, the NAT configuration itself is the culprit.
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.
Firewall Policies and NAT — This question tests Firewall Policies and NAT — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The IP pool is configured with overload (PAT), which changes source ports — When an IP pool is configured with overload (PAT), the FortiGate translates the source IP address and also changes the source port to a random high port. For protocols like SIP that rely on fixed source ports (e.g., UDP 5060), this port remapping breaks the application because the SIP server expects traffic from a specific port. Option D correctly identifies this as the root cause.
What should I do if I get this NSE4 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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