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. A key principle to apply: nAT overload allows multiple inside hosts to share a single outside IP address by translating source ports for simultaneous connections.. 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
interface GigabitEthernet0/0
ip address 192.168.10.1 255.255.255.0
ip nat outside
interface GigabitEthernet0/1
ip address 203.0.113.2 255.255.255.252
ip nat inside
access-list 1 permit 192.168.10.0 0.0.0.255
ip nat inside source list 1 interface GigabitEthernet0/1 overload
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?
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.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The NAT inside and outside interface roles are reversed.
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.
Key principle: NAT overload allows multiple inside hosts to share a single outside IP address by translating source ports for simultaneous connections.
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 inside and outside interface roles are reversed.
Why this is correct
That is the key problem shown.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
NAT overload allows multiple inside hosts to share a single outside IP address by translating source ports for simultaneous connections.
✗
The ACL must deny RFC1918 traffic before NAT can work.
Why it's wrong here
The ACL is being used to match inside addresses for translation, not for packet filtering.
When this WOULD be correct
In a different scenario where a question states that NAT is configured but only private IP addresses are being routed to the internet, and the ACL is explicitly set to deny RFC1918 addresses, this option would be correct. It would imply that the ACL is blocking necessary traffic for NAT to operate.
✗
PAT cannot use an interface address for overload.
Why it's wrong here
Using the outside interface address for PAT is standard.
When this WOULD be correct
In a scenario where a question specifies that PAT is configured incorrectly and explicitly states that it cannot use an interface address for overload, this option would be correct. For example, if the question describes a setup where multiple internal hosts are trying to access the internet but are configured to use the router's interface address directly without proper overload settings.
✗
The inside subnet must be configured as /24 on both interfaces.
Why it's wrong here
The WAN interface should not use the inside subnet.
When this WOULD be correct
In a different scenario where a question specifies that both the inside and outside interfaces must have the same subnet mask for proper routing and NAT to function, option D would be correct. For example, if the question states that mismatched subnet masks are causing routing issues, then this option would be valid.
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 NAT inside and outside interface roles are reversed.Correct answer▾
Why this is correct
That is the key problem shown.
✗The ACL must deny RFC1918 traffic before NAT can work.Wrong answer — click to see why▾
Why this is wrong here
This option is incorrect because ACLs do not need to deny RFC1918 traffic for NAT to function properly; NAT can handle private IP addresses without additional ACL configurations. The issue here is more likely related to NAT interface roles.
★ When this WOULD be the correct answer
In a different scenario where a question states that NAT is configured but only private IP addresses are being routed to the internet, and the ACL is explicitly set to deny RFC1918 addresses, this option would be correct. It would imply that the ACL is blocking necessary traffic for NAT to operate.
Why candidates choose this
Candidates may find this option tempting because they might recall that ACLs can impact NAT operations, leading them to mistakenly believe that denying private addresses is a necessary step for NAT to function properly.
✗PAT cannot use an interface address for overload.Wrong answer — click to see why▾
Why this is wrong here
This option is incorrect because PAT (Port Address Translation) can indeed use an interface address for overload, as it is designed to allow multiple hosts to share a single public IP address by differentiating traffic based on port numbers.
★ When this WOULD be the correct answer
In a scenario where a question specifies that PAT is configured incorrectly and explicitly states that it cannot use an interface address for overload, this option would be correct. For example, if the question describes a setup where multiple internal hosts are trying to access the internet but are configured to use the router's interface address directly without proper overload settings.
Why candidates choose this
Candidates may choose this option due to a misunderstanding of PAT functionality, thinking that it cannot use the interface address, especially if they confuse it with static NAT configurations that require specific mappings.
✗The inside subnet must be configured as /24 on both interfaces.Wrong answer — click to see why▾
Why this is wrong here
This option is wrong because the subnet mask configuration does not directly affect NAT functionality; NAT can operate with different subnet masks as long as the routing is correctly set up. The issue here is related to the NAT interface roles, not the subnet mask.
★ When this WOULD be the correct answer
In a different scenario where a question specifies that both the inside and outside interfaces must have the same subnet mask for proper routing and NAT to function, option D would be correct. For example, if the question states that mismatched subnet masks are causing routing issues, then this option would be valid.
Why candidates choose this
Candidates may choose this option because they associate NAT functionality with subnet configurations, believing that consistent subnet masks are essential for NAT operations, leading to confusion about the specific requirements for NAT to function correctly.
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 reversing the NAT inside and outside interface roles. Candidates may see that hosts can initiate traffic to the internet and mistakenly assume NAT is correctly configured. However, if the router’s interfaces are misassigned, return traffic from the internet will not be translated back to the inside hosts, causing connectivity failures. This trap exploits the partial functionality of NAT where outbound packets appear to succeed but inbound packets fail, leading to confusion during troubleshooting and exam scenarios.
Detailed technical explanation
How to think about this question
Network Address Translation (NAT) is a fundamental technology in Cisco routing that allows multiple devices on a private inside network to access external networks like the internet using a single public IP address. NAT overload, also known as Port Address Translation (PAT), enables many inside hosts to share one outside IP address by tracking port numbers. For NAT to function correctly, the router interfaces must be explicitly designated as 'inside' or 'outside' using the commands 'ip nat inside' and 'ip nat outside'. This classification informs the router which traffic should be translated from private to public addresses and vice versa.
The correct configuration requires the LAN-facing interface to be marked as 'ip nat inside' and the WAN-facing interface as 'ip nat outside'. This setup ensures that packets originating from the inside network are translated when they exit the router towards the internet, and return traffic from the internet is properly translated back to the inside hosts. If these roles are reversed, the router will not perform NAT translations correctly, causing return traffic to fail, even though outbound traffic appears to work.
A common exam trap is confusing the NAT inside and outside interface roles, especially since the router can still forward packets without proper NAT translation. This misconfiguration leads to asymmetric traffic flow where outbound packets are translated, but inbound packets are dropped or not translated back. Practically, this results in inside hosts being unable to receive responses from the internet, causing connectivity failures. Understanding and verifying interface roles is critical in troubleshooting NAT issues in Cisco environments.
KKey Concepts to Remember
NAT overload allows multiple inside hosts to share a single outside IP address by translating source ports for simultaneous connections.
Cisco routers require explicit configuration of interfaces as 'ip nat inside' or 'ip nat outside' to determine the direction of NAT translation.
The LAN-facing interface must be configured as 'ip nat inside' to identify private IP addresses for translation.
The WAN-facing interface must be configured as 'ip nat outside' to represent the public IP address used for NAT translations.
If the NAT inside and outside interface roles are reversed, return traffic from the internet will fail because translations are not applied correctly.
Access Control Lists (ACLs) used in NAT configurations match inside local addresses but do not filter traffic or deny RFC1918 addresses.
Using the outside interface IP address for PAT overload is standard and supported in Cisco NAT implementations.
The inside subnet must be correctly configured only on the LAN interface; the WAN interface uses a different subnet and should not share the inside subnet.
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 allows multiple inside hosts to share a single outside IP address by translating source ports for simultaneous connections.
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.
Review nAT overload allows multiple inside hosts to share a single outside IP address by translating source ports for simultaneous connections., 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 allows multiple inside hosts to share a single outside IP address by translating source ports for simultaneous connections..
What is the correct answer to this question?
The correct answer is: The NAT inside and outside interface roles are reversed. — 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.
What should I do if I get this 200-301 question wrong?
Review nAT overload allows multiple inside hosts to share a single outside IP address by translating source ports for simultaneous connections., 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 allows multiple inside hosts to share a single outside IP address by translating source ports for simultaneous connections.
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