- A
An ACL on the outside interface is blocking TCP port 443; configure an ACL to permit it.
ICMP is permitted, but TCP 443 is likely denied by an implicit or explicit ACL.
- B
The NAT translation is failing for TCP due to port exhaustion.
Why wrong: The debug shows a successful translation, so port exhaustion is not the issue.
- C
The internal host has a firewall blocking outbound TCP.
Why wrong: The host can ping, so it is likely not blocking all traffic.
- D
The external host is not responding to TCP SYN packets.
Why wrong: The external host shows no received packets, indicating the issue is on the path.
Question 336 of 2,152
300-410 NAT and PAT Practice Question
This 300-410 practice question tests your understanding of nat and pat. 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.
Router R1 is configured with ip nat inside source list 100 interface GigabitEthernet0/1 overload. Internal host 192.168.1.10 can ping external host 203.0.113.50, but cannot establish a TCP connection to port 443. Router R1 shows: debug ip nat: NAT: s=192.168.1.10->203.0.113.1, d=203.0.113.50 [0]. The external host shows no received packets. What is the root cause?
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
An ACL on the outside interface is blocking TCP port 443; configure an ACL to permit it.
The correct answer is A because the debug output shows a successful NAT translation (s=192.168.1.10->203.0.113.1, d=203.0.113.50), yet the external host receives no packets. This indicates that the translated packets are being dropped after leaving R1, most likely by an ACL applied to the outside interface (GigabitEthernet0/1) that blocks TCP port 443. Since ICMP (ping) succeeds but TCP/443 fails, the ACL is filtering only TCP traffic on that port, not all traffic.
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.
- ✓
An ACL on the outside interface is blocking TCP port 443; configure an ACL to permit it.
- ✗
The NAT translation is failing for TCP due to port exhaustion.
Why it's wrong here
The debug shows a successful translation, so port exhaustion is not the issue.
- ✗
The internal host has a firewall blocking outbound TCP.
Why it's wrong here
The host can ping, so it is likely not blocking all traffic.
- ✗
The external host is not responding to TCP SYN packets.
Why it's wrong here
The external host shows no received packets, indicating the issue is on the path.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the distinction between NAT translation success and packet forwarding success; candidates mistakenly assume that a successful NAT debug entry means the packet reached the destination, but the trap is that an ACL on the outside interface can drop the packet after translation, causing asymmetric behavior between ICMP and TCP.
Trap categories for this question
Command / output trap
The debug shows a successful translation, so port exhaustion is not the issue.
Detailed technical explanation
How to think about this question
When NAT is configured with 'overload' (PAT), the router translates the source IP and port to the outside interface IP and a unique port. The debug output shows the translation succeeded, but the packet is dropped after NAT processing. An ACL applied to the outside interface (e.g., 'ip access-group 101 in') can filter traffic based on Layer 4 protocols and ports; if it denies TCP/443, ICMP (protocol 1) would pass while TCP/443 fails. This is a common misconfiguration where an ACL is too restrictive for specific application traffic.
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.
Visual reference
What to study next
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FAQ
Questions learners often ask
What does this 300-410 question test?
NAT and PAT — This question tests NAT and PAT — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: An ACL on the outside interface is blocking TCP port 443; configure an ACL to permit it. — The correct answer is A because the debug output shows a successful NAT translation (s=192.168.1.10->203.0.113.1, d=203.0.113.50), yet the external host receives no packets. This indicates that the translated packets are being dropped after leaving R1, most likely by an ACL applied to the outside interface (GigabitEthernet0/1) that blocks TCP port 443. Since ICMP (ping) succeeds but TCP/443 fails, the ACL is filtering only TCP traffic on that port, not all traffic.
What should I do if I get this 300-410 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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Last reviewed: Jul 4, 2026
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