N10-009 Question 312
Single answerRoute selection: Routing table, Default routesA network administrator is troubleshooting why a branch office file server at 10.20.30.50 cannot reach a cloud backup target at 198.51.100.25. The server has the following IPv4 routing table:\n\n- 10.20.30.0/24 via 0.0.0.0\n- 10.20.0.0/16 via 10.20.30.1\n- 172.16.0.0/12 via 10.20.30.2\n- 0.0.0.0/0 via 10.20.30.254\n\nThe local subnet gateway 10.20.30.1 routes traffic to internal corporate networks only. The firewall at 10.20.30.254 provides Internet access. Which route will the server use to send traffic to 198.51.100.25?
- A
10.20.30.0/24 via 0.0.0.0
- B
10.20.0.0/16 via 10.20.30.1
- C
172.16.0.0/12 via 10.20.30.2
- D
0.0.0.0/0 via 10.20.30.254
Show answer and explanation
Correct answer: D
Explanation
IP route selection uses longest prefix match. A device compares the destination address against entries in its routing table and chooses the most specific matching route. If no specific network route matches, the device uses the default route, represented as 0.0.0.0/0 in IPv4. In this scenario, 198.51.100.25 does not belong to 10.20.30.0/24, 10.20.0.0/16, or 172.16.0.0/12, so the server sends the packet to the default gateway at 10.20.30.254. This aligns with standard TCP/IP routing behavior documented by major vendors and networking references, including Cisco and Microsoft host routing documentation, which describe route selection based primarily on destination match specificity before considering other factors such as metrics.
- A. Incorrect.
Incorrect. The 10.20.30.0/24 route is the directly connected local subnet route. It is only used for destinations within 10.20.30.0 through 10.20.30.255. Because 198.51.100.25 is outside that subnet, this route does not match. A common mistake is assuming the connected route is preferred for all traffic simply because it has no next hop, but route selection first depends on whether the destination matches the prefix.
- B. Incorrect.
Incorrect. The 10.20.0.0/16 route covers internal addresses from 10.20.0.0 to 10.20.255.255. The destination 198.51.100.25 is not in that private address range, so this route is not eligible. Someone might pick this option if they confuse the source network with the destination network, but routing decisions are made based on the destination IP address.
- C. Incorrect.
Incorrect. The 172.16.0.0/12 route applies only to destinations in the 172.16.0.0 through 172.31.255.255 range. Since 198.51.100.25 is not within that range, this route will not be used. This distractor targets the misconception that any internal route might be tried before a default route even when the destination does not match.
- D. Correct.
Correct. The default route 0.0.0.0/0 matches any destination that does not have a more specific entry in the routing table. Since 198.51.100.25 does not match the 10.20.30.0/24, 10.20.0.0/16, or 172.16.0.0/12 routes, the server forwards the traffic to 10.20.30.254. This reflects standard longest-prefix-match behavior used by hosts and routers.