N10-009 Question 6
Single answerLayer 3 - NetworkA company has two IPv4 subnets connected by a router: 192.168.10.0/24 for the accounting department and 192.168.20.0/24 for the warehouse. Users in accounting can reach hosts in their own subnet and can ping the router interface at 192.168.10.1, but they cannot reach any devices in the warehouse subnet. A technician verifies that the router interface for the warehouse network is up and configured as 192.168.20.1/24. On an accounting PC, the IP configuration is 192.168.10.50/24 with a default gateway of 192.168.10.254. Which action is the BEST way to restore connectivity between the two subnets?
- A
Change the accounting PC default gateway to 192.168.10.1
- B
Change the accounting PC subnet mask to 255.255.0.0
- C
Assign the warehouse router interface an address in the 192.168.10.0/24 subnet
- D
Enable NAT on the router for traffic between the two internal subnets
Show answer and explanation
Correct answer: A
Explanation
This scenario tests practical Layer 3 troubleshooting. A host uses its subnet mask to determine whether a destination is local or remote. If the destination is remote, the host forwards the packet to its configured default gateway, which must be a router interface on the same local subnet. Here, the accounting PC is on 192.168.10.0/24, and the known router interface on that subnet is 192.168.10.1. Because the PC is configured with 192.168.10.254 instead, traffic for 192.168.20.0/24 is sent to the wrong gateway and cannot be routed correctly. Best practice for IPv4 inter-subnet communication is to configure each host with the correct local IP, subnet mask, and default gateway that points to the local router interface. This aligns with standard TCP/IP host routing behavior described in common networking references and vendor documentation such as Cisco and Microsoft TCP/IP configuration guidance.
- A. Correct.
Correct. For hosts to reach remote Layer 3 networks, their default gateway must be the IP address of the local router interface on the same subnet. The accounting PC is in 192.168.10.0/24, and the reachable router interface on that subnet is 192.168.10.1. Using 192.168.10.254 as the gateway would prevent proper forwarding unless a router interface actually existed at that address. Since the user can already ping 192.168.10.1, updating the default gateway to that address is the appropriate fix.
- B. Incorrect.
Incorrect. Changing the mask to 255.255.0.0 would make the host believe that 192.168.20.0 is part of the local network range, causing it to attempt direct Layer 2 delivery instead of sending traffic to a router. Because the warehouse hosts are actually on a different subnet and likely a different broadcast domain, this would not solve the routing problem and would create addressing issues.
- C. Incorrect.
Incorrect. Router interfaces that connect different networks must be in the correct subnet for each connected segment. The warehouse-side interface should remain in 192.168.20.0/24 so it can serve as the gateway for that subnet. Moving it into 192.168.10.0/24 would break the Layer 3 boundary and create an invalid design with overlapping interface addressing for separate segments.
- D. Incorrect.
Incorrect. NAT is not required for routing between two internal private subnets directly connected to the same router. NAT is typically used when translating between private and public addresses or between overlapping address spaces. The issue here is basic host gateway configuration, not address translation.