The answer is that the outside interface GigabitEthernet0/0 is missing the 'ip nat outside' command. This is correct because for dynamic NAT to function bidirectionally, the router must identify which interface faces the external network; without the 'ip nat outside' command on that interface, the router will translate outbound packets from the inside hosts but will not perform reverse translation on return traffic, causing replies to be forwarded with the original private source IPs that are unroutable across the internet. On the CCNA 200-301 v2 exam, this scenario tests your understanding of the mandatory inside/outside interface pairing in NAT configuration—a common trap is assuming that only the inside interface needs the command, or that NAT will automatically apply to all interfaces. Remember the memory tip: "NAT needs both doors—inside for the source, outside for the return."
CCNA Network Services and Security Practice Question
This 200-301 practice question tests your understanding of network services and security. 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.
Exhibit
R1# show ip nat translations
Pro Inside global Inside local Outside local Outside global
--- 203.0.113.10 192.168.1.10 198.51.100.1 198.51.100.1
--- 203.0.113.11 192.168.1.20 198.51.100.2 198.51.100.2
R1# show ip nat statistics
Total active translations: 2 (0 static, 2 dynamic; 2 extended)
Pool translations: 2
Outside interfaces: GigabitEthernet0/0
Inside interfaces: GigabitEthernet0/1
Hits: 5 Misses: 0
CEF Translated packets: 5, CEF Punted packets: 0
Expired translations: 0
Dynamic mappings:
-- Inside Source
[Id] ip nat pool POOL 203.0.113.10 203.0.113.20 netmask 255.255.255.0
access-list NAT permit 192.168.1.0 0.0.0.255
Refcount: 2
A network engineer notices that internal hosts (192.168.1.0/24) can reach external servers on the internet, but replies from external servers never reach the internal hosts. The router R1 is configured with dynamic NAT to translate the internal subnet to a pool of public IPs (203.0.113.10-203.0.113.20). The engineer runs 'show ip nat translations' and sees only a few stale translations. What is the most likely cause of the issue?
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.
Clue: "never"
Why it matters: Absolute qualifier. True only if the statement has zero exceptions — be cautious of options that seem obvious but break down in edge cases.
R1# show ip nat translations
Pro Inside global Inside local Outside local Outside global
--- 203.0.113.10 192.168.1.10 198.51.100.1 198.51.100.1
--- 203.0.113.11 192.168.1.20 198.51.100.2 198.51.100.2
R1# show ip nat statistics
Total active translations: 2 (0 static, 2 dynamic; 2 extended)
Pool translations: 2
Outside interfaces: GigabitEthernet0/0
Inside interfaces: GigabitEthernet0/1
Hits: 5 Misses: 0
CEF Translated packets: 5, CEF Punted packets: 0
Expired translations: 0
Dynamic mappings:
-- Inside Source
[Id] ip nat pool POOL 203.0.113.10 203.0.113.20 netmask 255.255.255.0
access-list NAT permit 192.168.1.0 0.0.0.255
Refcount: 2
A
The access list 'NAT' is incorrect; it should permit only specific hosts, not the entire subnet.
Why wrong: The access list permits the internal subnet, which is correct for dynamic NAT. The issue is not with the access list.
B
The outside interface (GigabitEthernet0/0) is missing the 'ip nat outside' command.
Without 'ip nat outside' on the outside interface, the router does not translate return packets from the outside to the inside. Adding this command enables NAT to work bidirectionally.
C
The NAT configuration lacks the 'overload' keyword, so the pool is exhausted quickly.
Why wrong: While 'overload' is important for PAT, the immediate symptom of no reply traffic is not due to pool exhaustion but missing outside interface configuration.
D
The NAT pool 'POOL' has too few addresses; it should be expanded to a /24 subnet.
Why wrong: The pool has 11 addresses, which is sufficient for the current number of active translations (2). Expanding the pool would not fix the missing outside interface command.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The outside interface (GigabitEthernet0/0) is missing the 'ip nat outside' command.
The correct answer is B because the 'ip nat outside' command must be applied to the interface facing the external network (GigabitEthernet0/0) for the router to translate return traffic. Without it, the router does not perform NAT on packets arriving on that interface, so replies from external servers are forwarded without translation back to the inside local IPs, which are not routable on the internet. The stale translations indicate that outbound translations were created but never used for return traffic, confirming the missing outside interface command.
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 access list 'NAT' is incorrect; it should permit only specific hosts, not the entire subnet.
Why it's wrong here
The access list permits the internal subnet, which is correct for dynamic NAT. The issue is not with the access list.
✓
The outside interface (GigabitEthernet0/0) is missing the 'ip nat outside' command.
Why this is correct
Without 'ip nat outside' on the outside interface, the router does not translate return packets from the outside to the inside. Adding this command enables NAT to work bidirectionally.
Clue confirmation
The clue words "most likely", "never" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
The NAT configuration lacks the 'overload' keyword, so the pool is exhausted quickly.
Why it's wrong here
While 'overload' is important for PAT, the immediate symptom of no reply traffic is not due to pool exhaustion but missing outside interface configuration.
✗
The NAT pool 'POOL' has too few addresses; it should be expanded to a /24 subnet.
Why it's wrong here
The pool has 11 addresses, which is sufficient for the current number of active translations (2). Expanding the pool would not fix the missing outside interface command.
Option-by-option analysis
Why each answer is right or wrong
Understanding why wrong answers are wrong — and when they would be correct — is what separates a 750 score from a 900. The 200-301 exam frequently reuses these exact scenarios with slightly different constraints.
✓The outside interface (GigabitEthernet0/0) is missing the 'ip nat outside' command.Correct answer▾
Why this is correct
Without 'ip nat outside' on the outside interface, the router does not translate return packets from the outside to the inside. Adding this command enables NAT to work bidirectionally.
✗The access list 'NAT' is incorrect; it should permit only specific hosts, not the entire subnet.Wrong answer — click to see why▾
Why this is wrong here
The access list correctly matches the internal subnet, so this is not the root cause.
✗The NAT configuration lacks the 'overload' keyword, so the pool is exhausted quickly.Wrong answer — click to see why▾
Why this is wrong here
Even without overload, dynamic NAT should work for the first 11 hosts; the problem is that return packets are not being translated.
✗The NAT pool 'POOL' has too few addresses; it should be expanded to a /24 subnet.Wrong answer — click to see why▾
Why this is wrong here
The pool size is not the issue; the router is not translating return traffic due to missing 'ip nat outside'.
Analysis generated from the official 200-301blueprint and verified against question context. The “when correct” sections are what AI assistants cite when candidates ask “what’s the difference between these options?”
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the requirement that both 'ip nat inside' and 'ip nat outside' must be configured on the respective interfaces for NAT to work bidirectionally, and the trap here is that candidates assume only the inside interface needs the command or confuse the symptom with pool exhaustion or ACL issues.
Trap categories for this question
Command / output trap
The pool has 11 addresses, which is sufficient for the current number of active translations (2). Expanding the pool would not fix the missing outside interface command.
Detailed technical explanation
How to think about this question
Dynamic NAT requires both 'ip nat inside' on the internal interface and 'ip nat outside' on the external interface to create a bidirectional translation state. When the outside interface lacks the 'ip nat outside' command, the router builds a translation entry for outbound packets but does not inspect inbound packets for translation, causing return traffic to be dropped or forwarded untranslated. In real-world scenarios, this misconfiguration often leads to asymmetric routing issues where internal hosts can initiate connections but cannot receive responses, mimicking a firewall or routing problem.
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.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 200-301 question in full detail.
Network Services and Security — This question tests Network Services and Security — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The outside interface (GigabitEthernet0/0) is missing the 'ip nat outside' command. — The correct answer is B because the 'ip nat outside' command must be applied to the interface facing the external network (GigabitEthernet0/0) for the router to translate return traffic. Without it, the router does not perform NAT on packets arriving on that interface, so replies from external servers are forwarded without translation back to the inside local IPs, which are not routable on the internet. The stale translations indicate that outbound translations were created but never used for return traffic, confirming the missing outside interface command.
What should I do if I get this 200-301 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Are there clue words in this question I should notice?
Yes — watch for: "most likely", "never". 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?
Read the scenario before looking for a memorised answer.
About these practice questions
Courseiva creates original exam-style practice questions with explanations and wrong-answer analysis. It does not publish real exam questions, exam dumps, or protected exam content. Learn why practice questions differ from exam dumps →
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. Hosts on the inside network can reach the internet, but return traffic is failing after a new router was installed. The router's configuration shows that the LAN-facing interface has been configured with 'ip nat outside' and the WAN-facing interface with 'ip nat inside'. What configuration mistake is the most likely cause?
hard
✓ A.The NAT inside and outside interface roles are reversed.
B.The ACL must deny RFC1918 traffic before NAT can work.
C.PAT cannot use an interface address for overload.
D.The inside subnet must be configured as /24 on both interfaces.
Why A: NAT overload requires the LAN-facing interface to be marked as ip nat inside and the WAN-facing interface as ip nat outside. The exhibit shows those roles reversed, so translations will not occur correctly. The ACL itself is fine for matching the inside subnet.
Last reviewed: Jun 11, 2026
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