The answer is dynamic NAT without overload (PAT), which blocks inbound connections because it lacks the port-mapping mechanism needed to route external traffic back to a specific internal host. When dynamic NAT assigns a unique public IP from a pool to each inside host, the router creates a one-to-one translation for outbound sessions, but without a static mapping or explicit port forwarding, it has no way to determine which internal host should receive an unsolicited inbound packet. On the CCNA 200-301 v2 exam, this scenario tests your understanding of how NAT types handle session directionality—a common trap is confusing dynamic NAT with PAT, assuming that any NAT allows inbound traffic. Remember that dynamic NAT without overload is essentially a temporary one-to-one mapping; it supports outbound-initiated traffic only. A helpful memory tip: “No overload, no inbound road—dynamic NAT is outbound-only mode.”
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.
A network administrator has configured dynamic NAT on a Cisco router to allow internal hosts to access the Internet. Internal hosts can ping external servers, but external hosts cannot initiate connections to any internal host. The administrator checks the NAT translations. What is the most likely cause of this behavior?
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.
The NAT pool is exhausted because it contains only 21 addresses, and more than 21 internal hosts are trying to access the Internet simultaneously.
Why wrong: While pool exhaustion is possible, the output shows many translations already, and the symptom is that external hosts cannot initiate connections—a problem that would persist even with a larger pool if overload is not used.
B
The router is configured for dynamic NAT without overload (PAT), so it assigns one public IP per inside host and does not allow inbound connections without a static mapping.
The absence of protocol/port in the translations indicates one-to-one dynamic NAT without overload. This explains why internal hosts can initiate outbound traffic (they get a public IP) but external hosts cannot reach internal hosts (no return path).
C
The inside local addresses are not in the same subnet as the inside interface, causing asymmetric routing.
Why wrong: The inside local addresses (192.168.1.x) are typical private IPs and are assumed to be correctly configured on the inside interface. The translations show them mapping to public IPs, so routing is likely symmetric.
D
The outside global addresses are not routable on the Internet, so external hosts cannot send return traffic.
Why wrong: The public IPs shown (203.0.113.x) are from the TEST-NET-3 range, which is not globally routable. However, this is a common documentation range used in examples; in a real network, these would be replaced with routable IPs. The question assumes they are routable for the scenario.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The router is configured for dynamic NAT without overload (PAT), so it assigns one public IP per inside host and does not allow inbound connections without a static mapping.
Option B is correct because dynamic NAT without overload maps each inside host to a unique public IP from the pool, but it does not allow external hosts to initiate connections because there is no static mapping or port forwarding to direct inbound traffic. The described behavior—internal hosts reaching the Internet while external hosts cannot initiate connections—is typical of dynamic NAT without PAT. With PAT (overload), many internal hosts share a single public IP and inbound connections would still require explicit port forwarding; without overload, the router simply does not know how to translate incoming requests back to the correct inside host.
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 NAT pool is exhausted because it contains only 21 addresses, and more than 21 internal hosts are trying to access the Internet simultaneously.
Why it's wrong here
While pool exhaustion is possible, the output shows many translations already, and the symptom is that external hosts cannot initiate connections—a problem that would persist even with a larger pool if overload is not used.
✓
The router is configured for dynamic NAT without overload (PAT), so it assigns one public IP per inside host and does not allow inbound connections without a static mapping.
Why this is correct
The absence of protocol/port in the translations indicates one-to-one dynamic NAT without overload. This explains why internal hosts can initiate outbound traffic (they get a public IP) but external hosts cannot reach internal hosts (no return path).
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
The inside local addresses are not in the same subnet as the inside interface, causing asymmetric routing.
Why it's wrong here
The inside local addresses (192.168.1.x) are typical private IPs and are assumed to be correctly configured on the inside interface. The translations show them mapping to public IPs, so routing is likely symmetric.
✗
The outside global addresses are not routable on the Internet, so external hosts cannot send return traffic.
Why it's wrong here
The public IPs shown (203.0.113.x) are from the TEST-NET-3 range, which is not globally routable. However, this is a common documentation range used in examples; in a real network, these would be replaced with routable IPs. The question assumes they are routable for the scenario.
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 router is configured for dynamic NAT without overload (PAT), so it assigns one public IP per inside host and does not allow inbound connections without a static mapping.Correct answer▾
Why this is correct
The absence of protocol/port in the translations indicates one-to-one dynamic NAT without overload. This explains why internal hosts can initiate outbound traffic (they get a public IP) but external hosts cannot reach internal hosts (no return path).
✗The NAT pool is exhausted because it contains only 21 addresses, and more than 21 internal hosts are trying to access the Internet simultaneously.Wrong answer — click to see why▾
Why this is wrong here
The symptom described is that external hosts cannot initiate connections to internal hosts. Even if the NAT pool were exhausted, internal hosts would still be able to initiate outbound connections (using PAT if configured), and external hosts would still be unable to initiate inbound connections unless static NAT or port forwarding is configured. Pool exhaustion would cause outbound failures, not specifically inbound failures.
Why candidates choose this
Students might think that if the pool is exhausted, no new translations can be created, which could prevent inbound connections. However, inbound connections require a pre-existing translation or static mapping, not just an available pool address.
✗The inside local addresses are not in the same subnet as the inside interface, causing asymmetric routing.Wrong answer — click to see why▾
Why this is wrong here
The inside local addresses (192.168.1.x) are private IPs that are typically configured on the inside interface. The NAT translations show them mapping to public IPs, indicating that the router is correctly performing NAT. Asymmetric routing would cause connectivity issues in both directions, not just inbound, and is not indicated by the given symptoms.
Why candidates choose this
Students might confuse subnet mismatch with NAT behavior, thinking that if the inside local addresses are not in the same subnet as the inside interface, routing could be affected. However, NAT is designed to translate between different address spaces, so this is not a typical cause of the described problem.
✗The outside global addresses are not routable on the Internet, so external hosts cannot send return traffic.Wrong answer — click to see why▾
Why this is wrong here
The question states that internal hosts can ping external servers, which means return traffic is reaching the internal hosts. If the outside global addresses were not routable, the ping would fail because the external server would not be able to send a reply. Therefore, the addresses must be routable in this scenario.
Why candidates choose this
Students might recognize that 203.0.113.x is from the TEST-NET-3 documentation range and assume it is not routable. However, in exam scenarios, these addresses are used as placeholders for routable public IPs. The key point is that the symptom is about inbound connections, not routability.
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
Candidates often confuse dynamic NAT with Port Address Translation (PAT). The trap is assuming that any dynamic NAT configuration automatically allows inbound connections, but without overload, external hosts cannot reach internal hosts unless static NAT or port forwarding is configured.
Trap categories for this question
Command / output trap
While pool exhaustion is possible, the output shows many translations already, and the symptom is that external hosts cannot initiate connections—a problem that would persist even with a larger pool if overload is not used.
Scenario analysis trap
The public IPs shown (203.0.113.x) are from the TEST-NET-3 range, which is not globally routable. However, this is a common documentation range used in examples; in a real network, these would be replaced with routable IPs. The question assumes they are routable for the scenario.
Detailed technical explanation
How to think about this question
Dynamic NAT without overload (PAT) uses a one-to-one mapping of inside local to inside global addresses from a pool, and the router does not track port numbers for translation. When the pool is exhausted, no new outbound translations are created, but existing translations still allow outbound traffic. In contrast, PAT (overload) uses port numbers to multiplex multiple internal hosts to a single public IP, which can allow inbound connections only if static port forwarding rules are configured. The key difference is that PAT modifies the source port, enabling many-to-one mapping, while dynamic NAT without PAT does not.
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 router is configured for dynamic NAT without overload (PAT), so it assigns one public IP per inside host and does not allow inbound connections without a static mapping. — Option B is correct because dynamic NAT without overload maps each inside host to a unique public IP from the pool, but it does not allow external hosts to initiate connections because there is no static mapping or port forwarding to direct inbound traffic. The described behavior—internal hosts reaching the Internet while external hosts cannot initiate connections—is typical of dynamic NAT without PAT. With PAT (overload), many internal hosts share a single public IP and inbound connections would still require explicit port forwarding; without overload, the router simply does not know how to translate incoming requests back to the correct inside host.
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". 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.
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