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300-410 Practice Question: Router R1 is configured with ip nat inside source…
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?
⚠ Common exam trap
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.
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 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.
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.
Why this is correct
ICMP is permitted, but TCP 443 is likely denied by an implicit or explicit ACL.
- ✗
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.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This 300-410 practice question is part of Courseiva's free Cisco certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 300-410 exam.