CCNA Network Services and Security Practice Question
Exhibit
R1# show ip nat translations Pro Inside global Inside local Outside local Outside global --- 203.0.113.10 192.168.1.10 --- --- --- 203.0.113.11 192.168.1.11 --- --- --- 203.0.113.12 192.168.1.12 --- --- --- 203.0.113.13 192.168.1.13 --- --- --- 203.0.113.14 192.168.1.14 --- --- --- 203.0.113.15 192.168.1.15 --- --- --- 203.0.113.16 192.168.1.16 --- --- --- 203.0.113.17 192.168.1.17 --- --- --- 203.0.113.18 192.168.1.18 --- --- --- 203.0.113.19 192.168.1.19 --- --- --- 203.0.113.20 192.168.1.20 --- --- --- 203.0.113.21 192.168.1.21 --- --- --- 203.0.113.22 192.168.1.22 --- --- --- 203.0.113.23 192.168.1.23 --- --- --- 203.0.113.24 192.168.1.24 --- --- --- 203.0.113.25 192.168.1.25 --- --- --- 203.0.113.26 192.168.1.26 --- --- --- 203.0.113.27 192.168.1.27 --- --- --- 203.0.113.28 192.168.1.28 --- --- --- 203.0.113.29 192.168.1.29 --- --- --- 203.0.113.30 192.168.1.30 --- ---
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?
⚠ Common exam trap
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.
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
✓
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.
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.
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).
- ✗
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 normal RFC 1918 addresses, and the NAT table shows them being translated to public addresses, which implies the router is successfully receiving and forwarding their outbound traffic. If these addresses were not in the same subnet as the inside interface, the router would not have a directly connected route to them and no translations would appear in the first place. Moreover, asymmetric routing—where outbound and return traffic take different paths—would be a routing topology issue, not a NAT configuration issue, and the shortage of inbound connectivity is explained by the lack of overload or static mappings.
- ✗
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?”
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Key term
NAT
NAT (Network Address Translation) is a method that allows multiple devices on a private network to share a single public IP address when accessing the internet.
Key term
Port Address Translation
A networking technique that maps multiple private IP addresses and their ports to a single public IP address using unique port numbers.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This 200-301 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 200-301 exam.