N10-009 Network Troubleshooting Practice Question
A technician is troubleshooting a connectivity issue. A client can successfully ping its default gateway but cannot ping a server located on a different subnet. The router's routing table shows a valid route to the server's network. What should the technician check NEXT?
⚠ Common exam trap
CompTIA often tests the misconception that a valid route in the routing table guarantees end-to-end connectivity, but ACLs can override routing decisions by filtering traffic at the interface level.
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 ACL applied to the router interface
Since the client can ping its default gateway but not a server on a different subnet, Layer 3 routing is working (the router has a valid route). The most likely cause is an ACL on the router interface that is blocking traffic to the server's network. ACLs filter traffic based on source/destination IP, protocol, or port, and can permit or deny packets even when a valid route exists.
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 applied to the router interface
Why this is correct
An Access Control List (ACL) applied to a router interface inspects packets *after* the routing table determines the next hop. If the client can ping its default gateway, routing to the local subnet is functional. However, a specific ACL rule could be blocking traffic destined for the server's IP address or port, while still permitting ICMP to the gateway or other general traffic, leading to selective connectivity issues. This scenario perfectly explains why some destinations are reachable and others are not, despite a valid route.
- ✗
The DNS resolution for the server
Why it's wrong here
Domain Name System (DNS) resolution is responsible for translating human-readable hostnames into IP addresses. The problem statement indicates the client is attempting to ping the server directly by its IP address, bypassing the need for any DNS lookup. Therefore, even if there were issues with DNS servers or their configuration, it would not impact connectivity when using an explicit IP address, making this an irrelevant factor.
When this WOULD be correct
A client can ping the server by IP address but not by hostname. In this scenario, DNS resolution is the likely cause, and checking DNS settings would be the next step.
- ✗
The ARP cache on the client
Why it's wrong here
The Address Resolution Protocol (ARP) is used to resolve IP addresses to MAC addresses within the local broadcast domain. Since the client can successfully ping its default gateway, its ARP mechanism is functioning correctly for local subnet communication. Issues with the ARP cache would prevent any local communication, including reaching the gateway, making it an unlikely cause for selective connectivity problems to a remote server.
When this WOULD be correct
A technician would check the ARP cache on the client when the client cannot ping its default gateway or any other host on the same subnet, suggesting a missing or incorrect MAC address resolution for the gateway.
- ✗
The duplex settings on the client NIC
Why it's wrong here
A duplex mismatch between the client's Network Interface Card (NIC) and the connected switch port would cause severe performance degradation, high collision rates, and excessive retransmissions. This type of issue would broadly impact all network communication, including the ability to successfully ping the local default gateway. Since the client can reach its gateway, a duplex mismatch is highly improbable as the root cause for selective connectivity to a remote server.
When this WOULD be correct
A technician would check duplex settings when a client experiences slow or intermittent connectivity, especially after hardware changes, and link errors are observed on the NIC or switch interface.
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 N10-009 exam frequently reuses these exact scenarios with slightly different constraints.
✓The ACL applied to the router interfaceCorrect answer▾
Why this is correct
An Access Control List (ACL) applied to a router interface inspects packets *after* the routing table determines the next hop. If the client can ping its default gateway, routing to the local subnet is functional. However, a specific ACL rule could be blocking traffic destined for the server's IP address or port, while still permitting ICMP to the gateway or other general traffic, leading to selective connectivity issues. This scenario perfectly explains why some destinations are reachable and others are not, despite a valid route.
✗The DNS resolution for the serverWrong answer — click to see why▾
Why this is wrong here
The client can ping the default gateway, indicating Layer 3 connectivity to the local subnet, and the router has a valid route to the server's network. DNS resolution is irrelevant because the issue is with IP connectivity, not name resolution.
★ When this WOULD be the correct answer
A client can ping the server by IP address but not by hostname. In this scenario, DNS resolution is the likely cause, and checking DNS settings would be the next step.
Why candidates choose this
Candidates may confuse connectivity issues with name resolution problems, assuming that ping failures are often due to DNS when the real issue is at the network layer.
✗The ARP cache on the clientWrong answer — click to see why▾
Why this is wrong here
The client can ping its default gateway, indicating Layer 3 connectivity to the local subnet. Since the router has a valid route to the server's network, the issue is likely at the router's outbound interface, such as an ACL blocking traffic, not the client's ARP cache.
★ When this WOULD be the correct answer
A technician would check the ARP cache on the client when the client cannot ping its default gateway or any other host on the same subnet, suggesting a missing or incorrect MAC address resolution for the gateway.
Why candidates choose this
Candidates may think ARP is needed for cross-subnet communication, but ARP only resolves MAC addresses for hosts on the same subnet; the client already has the gateway's MAC to send packets off-subnet.
✗The duplex settings on the client NICWrong answer — click to see why▾
Why this is wrong here
The client can ping its default gateway successfully, indicating Layer 2 connectivity is functional. Duplex mismatch typically causes packet loss or poor performance, not a complete inability to reach a different subnet when the gateway is reachable.
★ When this WOULD be the correct answer
A technician would check duplex settings when a client experiences slow or intermittent connectivity, especially after hardware changes, and link errors are observed on the NIC or switch interface.
Why candidates choose this
Candidates may confuse general connectivity issues with physical layer problems, assuming duplex mismatch can cause total loss of connectivity beyond the local subnet.
Analysis generated from the official N10-009blueprint 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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Related to this question
Learn chapter
Troubleshooting Methodology
Key term
Subnet
A subnet is a logical subdivision of an IP network, created by partitioning a larger network address space using subnet masks.
Key term
Routing table
A routing table is a data set stored in a router or host that contains information about network paths and is used to determine where to forward data packets.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This N10-009 practice question is part of Courseiva's free CompTIA 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 N10-009 exam.