N10-009 Networking Concepts Practice Question
A network technician is configuring a small office network with two subnets: 10.0.1.0/24 and 10.0.2.0/24. Each subnet has its own switch, and both switches are connected to a router with interfaces 10.0.1.1 and 10.0.2.1. Hosts on subnet A can ping the router's interface in their subnet but cannot ping hosts on subnet B. Which of the following is the most likely cause?
⚠ Common exam trap
The N10-009 exam often tests the distinction between 'routing protocol' and 'IP routing' — candidates mistakenly think a routing protocol is required for directly connected subnets, when in fact the 'ip routing' global command is the fundamental enabler of any Layer 3 forwarding.
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
✓
IP routing is disabled on the router.
The hosts on subnet A can ping their default gateway (10.0.1.1) but cannot reach hosts on subnet B, which indicates that the router is not forwarding packets between the two directly connected subnets. This behavior is characteristic of a router with IP routing disabled, as the router will not perform inter-VLAN or inter-subnet forwarding unless the 'ip routing' command is enabled globally. Without IP routing, the router acts as a host and will only respond to traffic destined for its own interfaces, dropping any packets that require forwarding to another subnet.
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 router is not configured with a routing protocol.
Why it's wrong here
Routing protocols, such as OSPF or EIGRP, are used by routers to dynamically learn about remote networks that are not directly connected to their interfaces. For directly connected networks, a router automatically adds these routes to its routing table upon interface configuration and activation, requiring no additional routing protocol. Since the hosts can successfully ping the router's local interface, the router already knows about its directly connected subnet, making a routing protocol irrelevant for this specific communication failure.
When this WOULD be correct
In a scenario where the router does not have directly connected interfaces to both subnets (e.g., subnets are behind other routers), and the router needs to dynamically learn routes to reach them, then a routing protocol would be necessary for inter-subnet communication.
- ✓
IP routing is disabled on the router.
Why this is correct
If IP routing is disabled, the router functions merely as a multi-port switch, preventing it from performing its fundamental Layer 3 role of forwarding packets between different IP subnets. While hosts can reach the router's interface on their local subnet, the router will not process and forward traffic destined for other subnets, effectively isolating them. This directly explains why communication fails beyond the local segment.
- ✗
The hosts in subnet A have the wrong default gateway.
Why it's wrong here
The default gateway is the IP address of the router interface on the local subnet, serving as the exit point for traffic destined outside that subnet. The ability of hosts in subnet A to successfully ping the router's interface (e.g., 10.0.1.1) conclusively demonstrates that their default gateway is correctly configured. If the default gateway were incorrect, these hosts would be unable to establish any Layer 3 communication with the router, even within their own subnet.
When this WOULD be correct
In a scenario where hosts on subnet A cannot ping the router's interface (10.0.1.1) but can ping other hosts on the same subnet, the most likely cause would be an incorrect default gateway configured on the hosts.
- ✗
The switch in subnet A is blocking ICMP traffic.
Why it's wrong here
Switches operate primarily at Layer 2 of the OSI model, forwarding frames based on MAC addresses, and do not inherently block Layer 3 ICMP traffic. Blocking ICMP would require a Layer 3 switch with an Access Control List (ACL) configured, which is not a default behavior and typically beyond the scope of a basic small office network setup. Furthermore, if hosts can ping the router interface, basic Layer 2 connectivity and ICMP are already functional within the local subnet.
When this WOULD be correct
In a scenario where hosts on the same subnet cannot ping each other or the local gateway, and the switch is configured with ACLs or port security that blocks ICMP, then the switch blocking ICMP would be the likely cause.
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.
✓IP routing is disabled on the router.Correct answer▾
Why this is correct
If IP routing is disabled, the router functions merely as a multi-port switch, preventing it from performing its fundamental Layer 3 role of forwarding packets between different IP subnets. While hosts can reach the router's interface on their local subnet, the router will not process and forward traffic destined for other subnets, effectively isolating them. This directly explains why communication fails beyond the local segment.
✗The router is not configured with a routing protocol.Wrong answer — click to see why▾
Why this is wrong here
The router is directly connected to both subnets (10.0.1.0/24 and 10.0.2.0/24), so it automatically has routes to these networks. A routing protocol is only needed to learn routes to non-directly connected networks, which is not the case here.
★ When this WOULD be the correct answer
In a scenario where the router does not have directly connected interfaces to both subnets (e.g., subnets are behind other routers), and the router needs to dynamically learn routes to reach them, then a routing protocol would be necessary for inter-subnet communication.
Why candidates choose this
Candidates often assume that any routing between subnets requires a routing protocol, not realizing that directly connected routes are automatically added and sufficient for communication between directly attached networks.
✗The hosts in subnet A have the wrong default gateway.Wrong answer — click to see why▾
Why this is wrong here
The hosts can ping the router's interface in their subnet, indicating the default gateway is correctly configured. The issue is inter-subnet routing, not the gateway address.
★ When this WOULD be the correct answer
In a scenario where hosts on subnet A cannot ping the router's interface (10.0.1.1) but can ping other hosts on the same subnet, the most likely cause would be an incorrect default gateway configured on the hosts.
Why candidates choose this
Candidates often assume that connectivity issues between subnets stem from incorrect gateway settings, overlooking that the router's routing capability (IP routing enabled) is required for forwarding between subnets.
✗The switch in subnet A is blocking ICMP traffic.Wrong answer — click to see why▾
Why this is wrong here
The switch in subnet A is a Layer 2 device and does not block ICMP traffic between subnets; inter-subnet communication requires a router, and the issue is that the router is not forwarding packets because IP routing is disabled.
★ When this WOULD be the correct answer
In a scenario where hosts on the same subnet cannot ping each other or the local gateway, and the switch is configured with ACLs or port security that blocks ICMP, then the switch blocking ICMP would be the likely cause.
Why candidates choose this
Candidates may think switches can filter traffic between subnets or that ICMP blocking is a common issue, but switches operate at Layer 2 and do not route between subnets.
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?”
Visual reference
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Related to this question
Learn chapter
Interior vs Exterior Routing Protocols
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
Interface
An interface is a point of connection or interaction between two systems, devices, or software components that allows them to exchange information or signals.
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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.