The answer is a subnet mask mismatch between the router’s GigabitEthernet0/0 interface and the host, specifically a /24 mask on the router versus a /25 mask on the host. This mismatch causes the router to treat 192.168.1.200 as a local address within its directly connected 192.168.1.0/24 network, so it attempts to forward packets via ARP without a gateway. However, the host’s /25 mask defines its local subnet as 192.168.1.128/25, meaning 192.168.1.200 falls within its own range, but the host will not respond to ARP requests from the router for addresses outside that /25 boundary, dropping the traffic. On the CCNA 200-301 v2 exam, this scenario tests your understanding of how subnet masks define the boundary between local and remote destinations—a common trap where both devices share the same IP but different masks. Remember the key insight: the router’s larger /24 subnet sees the host as local, but the host’s smaller /25 subnet sees the router’s source address as remote, breaking two-way communication. A quick memory tip: “Big mask sees all, small mask sees half—if they don’t match, ARP won’t laugh.”
CCNA Network Infrastructure and Connectivity Practice Question
This 200-301 practice question tests your understanding of network infrastructure and connectivity. 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 ip interface GigabitEthernet0/0
GigabitEthernet0/0 is up, line protocol is up
Internet address is 192.168.1.2/24
Broadcast address is 255.255.255.255
MTU is 1500 bytes
Helper address is not set
Outgoing access list is not set
Inbound access list is not set
Proxy ARP is enabled
Local Proxy ARP is disabled
Split horizon is enabled
ICMP redirects are always sent
ICMP unreachables are always sent
ICMP mask replies are never sent
Refer to the exhibit. An engineer is troubleshooting a connectivity issue where packets from the local network destined to host 192.168.1.200 are being dropped. The host 192.168.1.200 is configured with IP address 192.168.1.200/25, while the router’s GigabitEthernet0/0 interface is on the same physical segment. Based on the output, what is the most likely cause of the problem?
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 ip interface GigabitEthernet0/0
GigabitEthernet0/0 is up, line protocol is up
Internet address is 192.168.1.2/24
Broadcast address is 255.255.255.255
MTU is 1500 bytes
Helper address is not set
Outgoing access list is not set
Inbound access list is not set
Proxy ARP is enabled
Local Proxy ARP is disabled
Split horizon is enabled
ICMP redirects are always sent
ICMP unreachables are always sent
ICMP mask replies are never sent
A
The host 192.168.1.200 has an incorrect default gateway set, preventing it from communicating with the router.
Why wrong: Even if the default gateway were incorrect, the host would still reply to ARP requests from the router if the router’s IP were considered on-subnet. The root issue is that the host and router are in different subnets due to mismatched masks, not the gateway setting.
B
The router interface Gi0/0 is using a /24 subnet mask while the host is in a /25 subnet, making 192.168.1.200 appear as a local address to the router but off-subnet to the host.
The output shows 'Internet address is 192.168.1.2/24'. With this mask, the router treats 192.168.1.200 as directly connected and sends ARP requests for it. The host, configured with a /25 mask, sees 192.168.1.2 as outside its /25 subnet (192.168.1.128/25), so it does not process the ARP request or respond, breaking connectivity.
C
An inbound access list on GigabitEthernet0/0 is blocking ICMP traffic from host 192.168.1.200.
Why wrong: The exhibit explicitly states 'Inbound access list is not set', meaning no filtering is applied. This rules out any ACL-related blocking.
D
The GigabitEthernet0/0 interface is in a shutdown state, preventing traffic from being forwarded to the host.
Why wrong: The first line of the output says 'GigabitEthernet0/0 is up, line protocol is up', confirming the interface is active and fully operational.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The router interface Gi0/0 is using a /24 subnet mask while the host is in a /25 subnet, making 192.168.1.200 appear as a local address to the router but off-subnet to the host.
The router's GigabitEthernet0/0 interface is configured with a /24 subnet mask, which means its directly connected network is 192.168.1.0/24. The host 192.168.1.200/25 has a subnet mask of /25, so its local network is 192.168.1.128/25. When the router receives a packet destined for 192.168.1.200, it considers the address to be within its own directly connected /24 subnet and attempts to forward it locally via ARP. However, the host's /25 mask causes it to treat 192.168.1.200 as a local address, but the router's /24 mask means the router expects the host to be on the same subnet without a gateway. The mismatch prevents the host from responding to ARP requests from the router for addresses outside its /25 range, leading to dropped packets.
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 host 192.168.1.200 has an incorrect default gateway set, preventing it from communicating with the router.
Why it's wrong here
Even if the default gateway were incorrect, the host would still reply to ARP requests from the router if the router’s IP were considered on-subnet. The root issue is that the host and router are in different subnets due to mismatched masks, not the gateway setting.
✓
The router interface Gi0/0 is using a /24 subnet mask while the host is in a /25 subnet, making 192.168.1.200 appear as a local address to the router but off-subnet to the host.
Why this is correct
The output shows 'Internet address is 192.168.1.2/24'. With this mask, the router treats 192.168.1.200 as directly connected and sends ARP requests for it. The host, configured with a /25 mask, sees 192.168.1.2 as outside its /25 subnet (192.168.1.128/25), so it does not process the ARP request or respond, breaking connectivity.
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.
✗
An inbound access list on GigabitEthernet0/0 is blocking ICMP traffic from host 192.168.1.200.
Why it's wrong here
The exhibit explicitly states 'Inbound access list is not set', meaning no filtering is applied. This rules out any ACL-related blocking.
✗
The GigabitEthernet0/0 interface is in a shutdown state, preventing traffic from being forwarded to the host.
Why it's wrong here
The first line of the output says 'GigabitEthernet0/0 is up, line protocol is up', confirming the interface is active and fully operational.
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 interface Gi0/0 is using a /24 subnet mask while the host is in a /25 subnet, making 192.168.1.200 appear as a local address to the router but off-subnet to the host.Correct answer▾
Why this is correct
The output shows 'Internet address is 192.168.1.2/24'. With this mask, the router treats 192.168.1.200 as directly connected and sends ARP requests for it. The host, configured with a /25 mask, sees 192.168.1.2 as outside its /25 subnet (192.168.1.128/25), so it does not process the ARP request or respond, breaking connectivity.
✗The host 192.168.1.200 has an incorrect default gateway set, preventing it from communicating with the router.Wrong answer — click to see why▾
Why this is wrong here
Candidates often think connectivity problems are caused by gateway misconfiguration, ignoring the subnet mask mismatch that actually prevents Layer 2 communication.
✗An inbound access list on GigabitEthernet0/0 is blocking ICMP traffic from host 192.168.1.200.Wrong answer — click to see why▾
Why this is wrong here
Candidates may glance at the output and assume an access list is blocking traffic, but the output clearly shows no ACL is applied.
✗The GigabitEthernet0/0 interface is in a shutdown state, preventing traffic from being forwarded to the host.Wrong answer — click to see why▾
Why this is wrong here
A common troubleshooting mistake is to assume an interface is down, but the state is clearly shown as up in the exhibit.
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 concept that a router will ARP for any destination IP within its directly connected network, regardless of the host's subnet mask, leading candidates to overlook the subnet mask mismatch and instead blame the default gateway or interface status.
Trap categories for this question
Command / output trap
The first line of the output says 'GigabitEthernet0/0 is up, line protocol is up', confirming the interface is active and fully operational.
Detailed technical explanation
How to think about this question
When a router has a directly connected network with a /24 mask, it will ARP for any destination IP within that /24 range, assuming the host is on the same Layer 2 segment. However, if the host is configured with a /25 mask, it only considers IPs in its own subnet (e.g., 192.168.1.128/25) as local; for addresses outside that range (like the router's own IP), the host will try to send traffic to its default gateway, which may not exist or be reachable. This mismatch causes asymmetric routing and ARP failures, as the router sends ARP requests for addresses the host does not consider local, so the host never replies. In real-world scenarios, this often happens when a network administrator changes a host's subnet mask without updating the router interface, or when using DHCP with mismatched scopes.
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 network engineer segments a warehouse floor into three subnets: 20 scanners, 5 printers, and 2 management hosts. Picking the wrong mask wastes addresses or leaves too few usable hosts. Exam questions test whether you can apply CIDR notation, calculate block size, and identify the correct usable-host range for a given prefix.
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 Infrastructure and Connectivity — This question tests Network Infrastructure and Connectivity — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The router interface Gi0/0 is using a /24 subnet mask while the host is in a /25 subnet, making 192.168.1.200 appear as a local address to the router but off-subnet to the host. — The router's GigabitEthernet0/0 interface is configured with a /24 subnet mask, which means its directly connected network is 192.168.1.0/24. The host 192.168.1.200/25 has a subnet mask of /25, so its local network is 192.168.1.128/25. When the router receives a packet destined for 192.168.1.200, it considers the address to be within its own directly connected /24 subnet and attempts to forward it locally via ARP. However, the host's /25 mask causes it to treat 192.168.1.200 as a local address, but the router's /24 mask means the router expects the host to be on the same subnet without a gateway. The mismatch prevents the host from responding to ARP requests from the router for addresses outside its /25 range, leading to dropped packets.
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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