The answer is that source port 12345 indicates the translated source port after PAT. When a FortiGate uses an IP pool with overload (Port Address Translation), it replaces the original source port from the internal host with a new, dynamically assigned port number to multiplex multiple internal connections to a single public IP. The session list output from `diagnose sys session list` always displays the post-NAT source port, not the original ephemeral port from the host at 10.0.1.10. On the Fortinet NSE 4 exam, this tests your understanding of how NAT hides internal addressing and port details; a common trap is assuming the session list shows the original client port. Remember: the session list shows the translated port—think of it as the "outside" port seen by the destination server. A useful memory tip is "Session list = post-NAT; packet capture = pre-NAT."
NSE4 Firewall Policies and NAT Practice Question
This NSE4 practice question tests your understanding of firewall policies and nat. Examine the command output carefully: the correct answer depends on what the output actually shows, not on general recall alone. 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.
An admin runs 'diagnose sys session filter saddr 10.0.1.10' and 'diagnose sys session list' to check sessions from a specific internal host. The output shows multiple sessions with destination IP 203.0.113.50 using source port 12345. The admin then checks the firewall policy and sees that the policy uses an IP pool for source NAT. What does the source port 12345 indicate?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
This is the translated source port after PAT
When using IP pool with overload (PAT), the FortiGate will perform port translation. The source port in the session list is the translated port (the port number after NAT). The original source port (from the internal host) is not shown in the session list; the output shows the post-NAT source port.
Key principle: NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
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 IP pool is configured for one-to-one NAT (no port translation)
Why it's wrong here
One-to-one NAT would preserve the original source port, not translate it.
✗
The IP pool is using port range 12345-12345
Why it's wrong here
Port ranges are typically wider; a single port number does not indicate a range.
✓
This is the translated source port after PAT
Why this is correct
In PAT, the source port is modified to a unique number to differentiate sessions sharing the same public IP.
Related concept
Static NAT maps one inside address to one outside address.
✗
The internal host's original source port is 12345
Why it's wrong here
The session list shows the post-NAT source port, not the original.
Common exam traps
Common exam trap: NAT rules depend on direction and matching traffic
NAT is not only about the public address. The inside/outside interface roles and the ACL or rule that matches traffic are just as important.
Trap categories for this question
Command / output trap
The session list shows the post-NAT source port, not the original.
Detailed technical explanation
How to think about this question
NAT questions usually test address translation, overload/PAT behaviour, static mappings and whether the right traffic is being translated. Read the interface direction and address terms carefully.
KKey Concepts to Remember
Static NAT maps one inside address to one outside address.
PAT allows many inside hosts to share one public address using ports.
Inside local and inside global describe the private and translated addresses.
NAT ACLs identify traffic for translation, not always security filtering.
TExam Day Tips
→Identify inside and outside interfaces first.
→Check whether the scenario needs static NAT, dynamic NAT or PAT.
→Do not confuse NAT matching ACLs with normal packet-filtering intent.
Key takeaway
NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
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.
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related NSE4 NAT questions on configuration and troubleshooting.
Firewall Policies and NAT — This question tests Firewall Policies and NAT — Static NAT maps one inside address to one outside address..
What is the correct answer to this question?
The correct answer is: This is the translated source port after PAT — When using IP pool with overload (PAT), the FortiGate will perform port translation. The source port in the session list is the translated port (the port number after NAT). The original source port (from the internal host) is not shown in the session list; the output shows the post-NAT source port.
What should I do if I get this NSE4 question wrong?
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related NSE4 NAT questions on configuration and troubleshooting.
What is the key concept behind this question?
Static NAT maps one inside address to one outside address.
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