The answer is that the FortiGate does not have a route to the destination IP. In the debug flow output, "forward to port2" confirms the packet is being sent to the correct egress interface based on policy, but the subsequent "no route to host" error indicates the routing table lacks a specific or default route for that destination address, so the packet cannot be forwarded beyond that interface. On the Fortinet NSE 4 Network Security Professional exam, this scenario tests your ability to distinguish between policy-based forwarding and actual routing—a common trap is assuming the issue is a firewall policy block when the debug flow clearly shows the packet is permitted but fails at the routing layer. Remember, "forward to" means the policy allowed it; "no route to host" means the routing table is incomplete. A quick memory tip: if you see "forward to" but then "no route," think "policy says yes, routing says no."
NSE4 High Availability and Diagnostics Practice Question
This NSE4 practice question tests your understanding of high availability and diagnostics. 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 administrator is troubleshooting an issue where internal users cannot access a public web server. The administrator runs 'diagnose debug flow' and sees the output shows 'forward to port2' but then 'no route to host'. 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
✓
The FortiGate does not have a route to the destination IP
Option A is correct. 'No route to host' indicates that the FortiGate does not have a route to the destination IP in the routing table. The packet is being forwarded to the correct interface (port2) but then cannot be routed further.
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 source IP is not in the routing table
Why it's wrong here
Source IP does not require routing.
✗
The interface port2 is down
Why it's wrong here
If port2 were down, the debug would show 'interface down' or packet would not be forwarded.
✓
The FortiGate does not have a route to the destination IP
Why this is correct
The debug output clearly shows 'no route to host', meaning the destination is not reachable via any route.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Static NAT maps one inside address to one outside address.
✗
The destination port is blocked by a firewall policy
Why it's wrong here
A policy block would show 'deny by policy' or similar, not 'no route to host'.
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
Similar concept trap
A policy block would show 'deny by policy' or similar, not 'no route to host'.
Command / output trap
If port2 were down, the debug would show 'interface down' or packet would not be forwarded.
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.
High Availability and Diagnostics — This question tests High Availability and Diagnostics — Static NAT maps one inside address to one outside address..
What is the correct answer to this question?
The correct answer is: The FortiGate does not have a route to the destination IP — Option A is correct. 'No route to host' indicates that the FortiGate does not have a route to the destination IP in the routing table. The packet is being forwarded to the correct interface (port2) but then cannot be routed further.
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.
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?
Static NAT maps one inside address to one outside address.
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