The answer is that the wildcard mask in access list 20 is incorrect because it matches only the network address rather than the entire subnet. For NAT to translate traffic from the 192.168.20.0/24 subnet, the ACL must match all host addresses within that range, which requires a wildcard mask of 0.0.0.255. The current mask of 0.0.0.0 tells the router to match only the exact IP 192.168.20.0—the network ID itself—so no host traffic from that subnet is eligible for PAT overload, breaking Internet connectivity. On the CCNA 200-301 v2 exam, this scenario tests your understanding of how NAT ACL wildcard mask subnet match logic works, often appearing as a trick where a zeroed-out mask seems valid but actually blocks all host traffic. A common trap is confusing the ACL’s permit statement with the wildcard mask’s scope; remember that a wildcard mask of 0.0.0.0 is a host match, not a subnet match. Memory tip: for a /24, think “three octets of zeros, one octet of 255” (0.0.0.255) to let the last octet vary.
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 running-config | section access-list|ip nat|interface GigabitEthernet0/1|interface GigabitEthernet0/2|interface GigabitEthernet0/0
access-list 10 permit 192.168.10.0 0.0.0.255
access-list 20 permit 192.168.20.0 0.0.0.0
ip nat pool NAT-POOL 200.1.1.1 200.1.1.2 netmask 255.255.255.252
ip nat inside source list 10 pool NAT-POOL overload
ip nat inside source list 20 pool NAT-POOL overload
interface GigabitEthernet0/0
ip address 200.1.1.1 255.255.255.252
ip nat outside
interface GigabitEthernet0/1
ip address 192.168.10.1 255.255.255.0
ip nat inside
interface GigabitEthernet0/2
ip address 192.168.20.1 255.255.255.0
ip nat inside
A network administrator is troubleshooting an issue where hosts in the 192.168.20.0/24 subnet cannot reach the Internet, while hosts in 192.168.10.0/24 can. The router is configured for PAT overload using a dynamic pool on the outside interface. The administrator collects the configuration shown in the exhibit. What is the most likely cause of the connectivity problem for the 192.168.20.0/24 subnet?
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.
R1#show running-config | section access-list|ip nat|interface GigabitEthernet0/1|interface GigabitEthernet0/2|interface GigabitEthernet0/0
access-list 10 permit 192.168.10.0 0.0.0.255
access-list 20 permit 192.168.20.0 0.0.0.0
ip nat pool NAT-POOL 200.1.1.1 200.1.1.2 netmask 255.255.255.252
ip nat inside source list 10 pool NAT-POOL overload
ip nat inside source list 20 pool NAT-POOL overload
interface GigabitEthernet0/0
ip address 200.1.1.1 255.255.255.252
ip nat outside
interface GigabitEthernet0/1
ip address 192.168.10.1 255.255.255.0
ip nat inside
interface GigabitEthernet0/2
ip address 192.168.20.1 255.255.255.0
ip nat inside
A
The wildcard mask in access list 20 is incorrect; it matches only the network address.
Access list 20 uses mask 0.0.0.0, which matches only the single address 192.168.20.0. To encompass the entire 192.168.20.0/24 subnet, the mask must be 0.0.0.255.
B
The NAT pool does not have enough IP addresses to support both subnets.
Why wrong: Overload (PAT) allows many inside local addresses to share a single global address by multiplexing ports, so two addresses in the pool are more than sufficient.
C
Interface GigabitEthernet0/2 is missing the ip nat inside command.
Why wrong: The exhibit clearly shows the command 'ip nat inside' applied to GigabitEthernet0/2.
D
Access list 10 is incorrectly applied to the NAT pool, causing a conflict.
Why wrong: Both access lists are correctly mapped to the same pool with 'overload', and there is no configuration conflict; list 10 works fine for the 192.168.10.0/24 subnet.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The wildcard mask in access list 20 is incorrect; it matches only the network address.
The issue is that access list 20, used to define which internal addresses are eligible for NAT, has a wildcard mask of 0.0.0.0. This wildcard mask matches only the exact address 192.168.20.0, not the entire 192.168.20.0/24 subnet. For a /24 subnet, the correct wildcard mask should be 0.0.0.255, which would match all addresses from 192.168.20.1 to 192.168.20.254. Because the ACL matches only the network address (192.168.20.0), no host traffic from that subnet is translated, breaking Internet connectivity.
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 wildcard mask in access list 20 is incorrect; it matches only the network address.
Why this is correct
Access list 20 uses mask 0.0.0.0, which matches only the single address 192.168.20.0. To encompass the entire 192.168.20.0/24 subnet, the mask must be 0.0.0.255.
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 NAT pool does not have enough IP addresses to support both subnets.
Why it's wrong here
Overload (PAT) allows many inside local addresses to share a single global address by multiplexing ports, so two addresses in the pool are more than sufficient.
✗
Interface GigabitEthernet0/2 is missing the ip nat inside command.
Why it's wrong here
The exhibit clearly shows the command 'ip nat inside' applied to GigabitEthernet0/2.
✗
Access list 10 is incorrectly applied to the NAT pool, causing a conflict.
Why it's wrong here
Both access lists are correctly mapped to the same pool with 'overload', and there is no configuration conflict; list 10 works fine for the 192.168.10.0/24 subnet.
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 wildcard mask in access list 20 is incorrect; it matches only the network address.Correct answer▾
Why this is correct
Access list 20 uses mask 0.0.0.0, which matches only the single address 192.168.20.0. To encompass the entire 192.168.20.0/24 subnet, the mask must be 0.0.0.255.
✗The NAT pool does not have enough IP addresses to support both subnets.Wrong answer — click to see why▾
Why this is wrong here
NAT pool size is not a limiting factor with PAT overload; a single address can serve thousands of hosts.
✗Interface GigabitEthernet0/2 is missing the ip nat inside command.Wrong answer — click to see why▾
Why this is wrong here
The interface is correctly configured for NAT inside.
✗Access list 10 is incorrectly applied to the NAT pool, causing a conflict.Wrong answer — click to see why▾
Why this is wrong here
Applying multiple access lists to the same pool is allowed and does not create a conflict.
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 distinction between matching the network address versus matching the host range in ACLs used for NAT, where candidates incorrectly assume that using the network address with a wildcard mask of 0.0.0.0 will match all hosts in the subnet.
Trap categories for this question
Command / output trap
The exhibit clearly shows the command 'ip nat inside' applied to GigabitEthernet0/2.
Detailed technical explanation
How to think about this question
In Cisco NAT configuration, the access list used with the ip nat inside source list command determines which source IP addresses are translated. The wildcard mask in the ACL must match the range of host addresses, not the network address itself. For example, ACL 20 with permit 192.168.20.0 0.0.0.255 would correctly match all hosts in the subnet, while permit 192.168.20.0 0.0.0.0 matches only the single address 192.168.20.0, which is never used as a source IP by hosts. This is a common misconfiguration when administrators mistakenly use the network address with a host wildcard mask.
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 wildcard mask in access list 20 is incorrect; it matches only the network address. — The issue is that access list 20, used to define which internal addresses are eligible for NAT, has a wildcard mask of 0.0.0.0. This wildcard mask matches only the exact address 192.168.20.0, not the entire 192.168.20.0/24 subnet. For a /24 subnet, the correct wildcard mask should be 0.0.0.255, which would match all addresses from 192.168.20.1 to 192.168.20.254. Because the ACL matches only the network address (192.168.20.0), no host traffic from that subnet is translated, breaking Internet connectivity.
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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