N10-009 Network Implementation Practice Question
A router receives a packet destined for 10.0.0.15. It has the following routes in the routing table: 10.0.0.0/8 via 192.168.1.1, 10.0.0.0/16 via 192.168.2.1, 0.0.0.0/0 via 192.168.3.1. Which route will be used?
⚠ Common exam trap
Many exam-takers assume administrative distance or metric determines the route selection, but the longest prefix match always takes precedence over these metrics when multiple routes match the destination.
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
✓
10.0.0.0/16 via 192.168.2.1
The router will use the route 10.0.0.0/16 via 192.168.2.1 because it has the longest prefix match (16 bits) for the destination 10.0.0.15. The /16 route is more specific than the /8 route and the default route, so it is preferred regardless of administrative distance or metric.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Default route (0.0.0.0/0)
Why it's wrong here
The default route, represented as 0.0.0.0/0, serves as a last resort for a router when no more specific route matches the destination IP address. It acts as a catch-all for traffic that doesn't fit any other known network. Since there are more precise routes available in the routing table that specifically match the 10.0.0.15 destination, such as 10.0.0.0/16, the default route will not be utilized for this packet.
When this WOULD be correct
If the destination IP did not match any other route in the routing table (e.g., 10.1.0.15 with only 10.0.0.0/8 and 10.0.0.0/16 present, but not matching), the default route would be used to forward the packet.
- ✓
10.0.0.0/16 via 192.168.2.1
Why this is correct
Routers prioritize routes based on the longest prefix match principle. The destination IP address 10.0.0.15 falls squarely within the 10.0.0.0/16 network because the first 16 bits of the destination IP match the network address. Since this route offers a more specific match (16 bits) compared to other available routes, it will be selected by the router to forward the packet towards the next hop, 192.168.2.1.
- ✗
10.0.0.0/8 via 192.168.1.1
Why it's wrong here
While the destination IP address 10.0.0.15 certainly falls within the broad 10.0.0.0/8 network, this route is significantly less specific than the 10.0.0.0/16 route. Routers strictly adhere to the "longest prefix match" rule, meaning they always select the route with the most matching bits in the network address. Consequently, the /8 route is overlooked in favor of the more precise /16 entry.
When this WOULD be correct
This option would be correct if the routing table did not contain the 10.0.0.0/16 route, or if the destination address was outside the 10.0.0.0/16 range (e.g., 10.1.0.15) but still within 10.0.0.0/8. In such cases, the longest prefix match would be 10.0.0.0/8.
- ✗
None; the packet is dropped
Why it's wrong here
A packet is only dropped if a router cannot find any matching route in its routing table for the destination IP address, including a default route. In this scenario, multiple routes, such as 10.0.0.0/16, 10.0.0.0/8, and the default route, are available that encompass the 10.0.0.15 destination. Therefore, the packet will be successfully forwarded, not dropped.
When this WOULD be correct
If the routing table had no matching routes (e.g., only routes for different subnets) and no default route, then the packet would be dropped. For example, a router with only 192.168.0.0/16 and no default route receiving a packet for 10.0.0.1 would drop it.
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.
✓10.0.0.0/16 via 192.168.2.1Correct answer▾
Why this is correct
Routers prioritize routes based on the longest prefix match principle. The destination IP address 10.0.0.15 falls squarely within the 10.0.0.0/16 network because the first 16 bits of the destination IP match the network address. Since this route offers a more specific match (16 bits) compared to other available routes, it will be selected by the router to forward the packet towards the next hop, 192.168.2.1.
✗Default route (0.0.0.0/0)Wrong answer — click to see why▾
Why this is wrong here
The destination 10.0.0.15 matches the 10.0.0.0/16 route with a longer prefix length (16) than the default route (0), so the router will use the more specific route, not the default.
★ When this WOULD be the correct answer
If the destination IP did not match any other route in the routing table (e.g., 10.1.0.15 with only 10.0.0.0/8 and 10.0.0.0/16 present, but not matching), the default route would be used to forward the packet.
Why candidates choose this
Candidates may mistakenly think that the default route is always used as a catch-all, forgetting that more specific routes take precedence over the default route.
✗10.0.0.0/8 via 192.168.1.1Wrong answer — click to see why▾
Why this is wrong here
The route 10.0.0.0/8 has a longer prefix length than the default route but a shorter prefix length than 10.0.0.0/16. Since the destination 10.0.0.15 falls within both 10.0.0.0/8 and 10.0.0.0/16, the router selects the most specific match, which is 10.0.0.0/16.
★ When this WOULD be the correct answer
This option would be correct if the routing table did not contain the 10.0.0.0/16 route, or if the destination address was outside the 10.0.0.0/16 range (e.g., 10.1.0.15) but still within 10.0.0.0/8. In such cases, the longest prefix match would be 10.0.0.0/8.
Why candidates choose this
Candidates may mistakenly think that a shorter prefix (larger network) is preferred, or they may overlook the existence of the more specific /16 route, assuming the /8 route is the best match.
✗None; the packet is droppedWrong answer — click to see why▾
Why this is wrong here
The router has a default route (0.0.0.0/0) and two specific routes that match the destination 10.0.0.15. Since the longest prefix match rule applies, the route with the most specific prefix (10.0.0.0/16) is used, so the packet is not dropped.
★ When this WOULD be the correct answer
If the routing table had no matching routes (e.g., only routes for different subnets) and no default route, then the packet would be dropped. For example, a router with only 192.168.0.0/16 and no default route receiving a packet for 10.0.0.1 would drop it.
Why candidates choose this
Candidates may mistakenly think that if no exact match exists, the router drops the packet, forgetting that a default route can be used as a catch-all.
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?”
Go deeper
Related to this question
Learn chapter
Inter-VLAN Routing Methods
Key term
Metric
A metric is a quantifiable measurement used to assess the performance, health, or status of IT systems, networks, or applications.
Key term
Router
A router is a networking device that connects different networks together and directs data traffic between them by choosing the best path for data to travel.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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