CCNA Network Services and Security Practice Question
This 200-301 practice question tests your understanding of network services and security. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. A key principle to apply: nAT overload uses an access control list (ACL) to identify which inside local IP addresses should be translated to the outside global IP address.. 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
access-list 1 permit 10.10.20.0 0.0.0.255
ip nat inside source list 1 interface GigabitEthernet0/0 overload
interface GigabitEthernet0/0
ip address 198.51.100.2 255.255.255.252
ip nat outside
interface GigabitEthernet0/1
ip address 10.10.10.1 255.255.255.0
ip nat inside
Users are in 10.10.10.0/24.
A branch router is configured for NAT overload. The inside interface Gi0/0 is correctly marked ip nat inside, and the outside interface Gi0/1 is ip nat outside. The NAT statement uses access-list 1 permit 10.1.1.0 0.0.0.255 with ip nat inside source list 1 interface Gi0/1 overload. Inside hosts are in the 192.168.1.0/24 subnet and still reach the ISP with their private addresses. What is the most likely reason?
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.
access-list 1 permit 10.10.20.0 0.0.0.255
ip nat inside source list 1 interface GigabitEthernet0/0 overload
interface GigabitEthernet0/0
ip address 198.51.100.2 255.255.255.252
ip nat outside
interface GigabitEthernet0/1
ip address 10.10.10.1 255.255.255.0
ip nat inside
Users are in 10.10.10.0/24.
A
The ACL used for NAT does not match the inside client subnet.
The overload statement references ACL 1, but ACL 1 permits 10.10.20.0/24 instead of 10.10.10.0/24.
B
GigabitEthernet0/0 should be configured as ip nat inside.
Why wrong: The ISP-facing interface is correctly marked outside.
C
PAT cannot use an interface address as the translated source.
Why wrong: That is exactly how NAT overload commonly works.
D
The router must run OSPF before NAT overload can function.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The ACL used for NAT does not match the inside client subnet.
The ACL matches the wrong inside subnet. NAT overload will only translate traffic that matches the source list or route map tied to the NAT statement. The interfaces are marked inside and outside correctly, so the bad match criteria is the most likely failure point.
Key principle: NAT overload uses an access control list (ACL) to identify which inside local IP addresses should be translated to the outside global IP address.
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 ACL used for NAT does not match the inside client subnet.
Why this is correct
The overload statement references ACL 1, but ACL 1 permits 10.10.20.0/24 instead of 10.10.10.0/24.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
NAT overload uses an access control list (ACL) to identify which inside local IP addresses should be translated to the outside global IP address.
✗
GigabitEthernet0/0 should be configured as ip nat inside.
Why it's wrong here
The ISP-facing interface is correctly marked outside.
When this WOULD be correct
In a different scenario where a question asks about a router not performing NAT due to misconfigured interface roles, 'GigabitEthernet0/0 should be configured as ip nat inside' would be correct if the interface was incorrectly set as 'ip nat outside' instead of 'inside'.
✗
PAT cannot use an interface address as the translated source.
In a scenario where a question asks about limitations of PAT in a configuration that explicitly prohibits using an interface address for translation, this option would be correct. For example, if the question specifies a device that only supports static NAT without overload, this option would apply.
✗
The router must run OSPF before NAT overload can function.
In a different scenario where a question states that a router is configured for NAT overload but is experiencing issues, and it specifies that OSPF must be running for proper routing of internal traffic, then this option would be correct. This would imply that without OSPF, the NAT translations cannot be properly routed.
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 ACL used for NAT does not match the inside client subnet.Correct answer▾
Why this is correct
The overload statement references ACL 1, but ACL 1 permits 10.10.20.0/24 instead of 10.10.10.0/24.
✗GigabitEthernet0/0 should be configured as ip nat inside.Wrong answer — click to see why▾
Why this is wrong here
The ISP-facing interface (GigabitEthernet0/0) is correctly configured as 'ip nat outside'. The issue is not with the outside interface designation; the inside interface (GigabitEthernet0/1) is already marked as 'ip nat inside'. Changing the outside interface to 'ip nat inside' would be incorrect and would break NAT functionality.
★ When this WOULD be the correct answer
In a different scenario where a question asks about a router not performing NAT due to misconfigured interface roles, 'GigabitEthernet0/0 should be configured as ip nat inside' would be correct if the interface was incorrectly set as 'ip nat outside' instead of 'inside'.
Why candidates choose this
Students often confuse the 'ip nat inside' and 'ip nat outside' commands, thinking that both interfaces must be marked as 'inside' for NAT to work. They may also assume that the interface facing the ISP should be 'inside' because it is the source of the translated traffic, but in reality, the inside interface is the one connected to the private network.
✗PAT cannot use an interface address as the translated source.Wrong answer — click to see why▾
Why this is wrong here
PAT (NAT overload) commonly uses the interface's IP address as the translated source address. This is a standard and correct configuration. The statement that PAT cannot use an interface address is factually incorrect.
★ When this WOULD be the correct answer
In a scenario where a question asks about limitations of PAT in a configuration that explicitly prohibits using an interface address for translation, this option would be correct. For example, if the question specifies a device that only supports static NAT without overload, this option would apply.
Why candidates choose this
A student might confuse PAT with static NAT, where a specific global IP is often used, or they might think that using an interface address would cause issues with overlapping addresses. However, PAT is designed to use a single public IP (often the interface address) for many private hosts.
✗The router must run OSPF before NAT overload can function.Wrong answer — click to see why▾
Why this is wrong here
NAT overload does not require any routing protocol such as OSPF to function. NAT operates independently of the routing protocol; it only requires IP connectivity and proper interface designations. OSPF is irrelevant to the NAT process.
★ When this WOULD be the correct answer
In a different scenario where a question states that a router is configured for NAT overload but is experiencing issues, and it specifies that OSPF must be running for proper routing of internal traffic, then this option would be correct. This would imply that without OSPF, the NAT translations cannot be properly routed.
Why candidates choose this
Students might think that because OSPF is used for routing in many enterprise networks, it must be running for NAT to work. They may also confuse the need for routing to reach the ISP with the need for a dynamic routing protocol, but static routes or default routes are sufficient for NAT to function.
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
A frequent exam trap is assuming that NAT overload requires routing protocols such as OSPF to function or that the outside interface cannot be used as the source address for translation. Some candidates also mistakenly believe that misconfigured interface NAT designations cause the problem when the real issue is the ACL mismatch. The trap lies in overlooking the ACL's role in defining which inside addresses are translated. If the ACL does not include the correct inside subnet, NAT will not translate those packets, causing inside hosts to leak private IPs to the ISP. This subtle misconfiguration is often missed under exam pressure.
Detailed technical explanation
How to think about this question
Network Address Translation (NAT) overload, also known as Port Address Translation (PAT), allows multiple inside hosts to share a single public IP address by translating their private source IP addresses and port numbers to the router's outside interface IP. The translation is controlled by an access control list (ACL) that specifies which inside local IP addresses are eligible for translation. The router uses the ACL to match packets originating from the inside network and applies NAT to those packets as they exit the outside interface.
The key to successful NAT overload is that the ACL must exactly match the inside subnet of the hosts whose traffic needs translation. If the ACL permits a different subnet than the actual inside hosts, the router will not translate those packets because they do not meet the ACL criteria. The interfaces must also be correctly designated with ip nat inside and ip nat outside commands to define the NAT direction. The router then translates the source IP addresses of matching packets to the outside interface's IP address, allowing multiple hosts to share one public IP.
A common exam trap is misinterpreting the cause of NAT failure. Candidates might incorrectly assume that the interface configuration or routing protocols like OSPF affect NAT operation. However, NAT translation depends solely on the ACL match and interface NAT designations. If the ACL references the wrong subnet, inside hosts will send packets with private IPs that are not translated, causing connectivity issues to the ISP. Understanding this behavior helps avoid misdiagnosing NAT problems and ensures correct NAT overload implementation in Cisco environments.
KKey Concepts to Remember
NAT overload uses an access control list (ACL) to identify which inside local IP addresses should be translated to the outside global IP address.
The ACL used in a NAT overload configuration must precisely match the inside subnet of the hosts to ensure their traffic is translated correctly.
Cisco routers require interfaces to be explicitly designated as ip nat inside or ip nat outside to determine the direction of NAT translation.
NAT overload (PAT) commonly uses the IP address of the outside interface as the translated source address for multiple inside hosts.
Routing protocols like OSPF are unrelated to NAT operation and do not affect NAT overload functionality.
If the ACL does not match the inside subnet, the router will not translate the source IP addresses, causing inside hosts to appear with private IPs externally.
The NAT translation process depends on matching traffic against the ACL or route map specified in the ip nat inside source command.
Misconfiguration of the ACL is a frequent cause of NAT overload failure, especially when the permitted subnet does not correspond to the actual inside hosts.
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
NAT overload uses an access control list (ACL) to identify which inside local IP addresses should be translated to the outside global IP address.
Real-world example
How this comes up in practice
A network engineer at a university connects two campus buildings via a fibre link. Both routers run OSPF, but no adjacency forms — even though both routers can ping each other. The engineer finds one router is in area 0 and the other in area 1. OSPF adjacency requires matching area numbers, hello/dead timers, and network type. IP reachability alone is not enough.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 200-301 question in full detail.
Review nAT overload uses an access control list (ACL) to identify which inside local IP addresses should be translated to the outside global IP address., then practise related 200-301 questions on the same topic to reinforce the concept.
Network Services and Security — This question tests Network Services and Security — NAT overload uses an access control list (ACL) to identify which inside local IP addresses should be translated to the outside global IP address..
What is the correct answer to this question?
The correct answer is: The ACL used for NAT does not match the inside client subnet. — The ACL matches the wrong inside subnet. NAT overload will only translate traffic that matches the source list or route map tied to the NAT statement. The interfaces are marked inside and outside correctly, so the bad match criteria is the most likely failure point.
What should I do if I get this 200-301 question wrong?
Review nAT overload uses an access control list (ACL) to identify which inside local IP addresses should be translated to the outside global IP address., then practise related 200-301 questions on the same topic to reinforce the concept.
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?
NAT overload uses an access control list (ACL) to identify which inside local IP addresses should be translated to the outside global IP address.
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