Question 1mediummultiple choice
Full question →mediummultiple choiceObjective-mapped
A router has routes for 10.10.0.0/16, 10.10.20.0/24, and a default route. Which route is used for destination 10.10.20.55?
Answer choices
Why each option matters
Good practice is not just finding the correct option. The wrong answers often show the exact trap the exam wants you to fall into.
A
Distractor review
The 10.10.0.0/16 route
The /16 matches, but it is less specific than the /24.
B
Distractor review
The default route
The default route is used only when no more specific route matches.
C
Best answer
The 10.10.20.0/24 route
Correct. The /24 route is chosen.
D
Distractor review
The router load-balances across all matching routes
Routers do not load-balance across unrelated prefix lengths like this.
Common exam trap
Common exam trap: answer the scenario, not the keyword
A common exam trap is selecting the broader route (10.10.0.0/16) or the default route for the destination 10.10.20.55 simply because they appear first or seem more general. Candidates may mistakenly believe that a larger network range is preferred or that the default route acts as a catch-all even when more specific routes exist. This misunderstanding ignores the longest prefix match rule, which always prioritizes the most specific subnet mask. Another trap is thinking the router load-balances across all matching routes regardless of prefix length, which is incorrect since load balancing only occurs among routes with equal prefix lengths and administrative distances.
Technical deep dive
How to think about this question
In Cisco routing, the fundamental principle for route selection is the longest prefix match. This means the router examines all routes that match the destination IP address and selects the one with the most bits specified in the subnet mask. For example, a /24 prefix (255.255.255.0) specifies 24 bits of the address, making it more specific than a /16 prefix (255.255.0.0), which specifies only 16 bits. In the given scenario, the destination IP 10.10.20.55 matches both 10.10.0.0/16 and 10.10.20.0/24 routes. Since 10.10.20.0/24 covers a smaller, more specific subnet, the router chooses this route over the broader /16. The default route, which matches all addresses, is only used if no other route matches. Cisco routers do not load-balance across routes with different prefix lengths because that would violate the longest prefix match principle. Load balancing is possible only among routes with identical prefix lengths and administrative distances. This behavior ensures predictable and efficient routing. Practically, network engineers must design subnetting schemes carefully to avoid unintended route overlaps and ensure traffic flows through the intended paths. Understanding how prefix lengths affect route selection is essential for troubleshooting routing issues and optimizing network performance.
KKey Concepts to Remember
- Routers use the longest prefix match rule to select the most specific route for a given destination IP address.
- A /24 subnet mask (255.255.255.0) is more specific than a /16 subnet mask (255.255.0.0) because it matches more bits of the IP address.
- When multiple routes match a destination, the router chooses the route with the longest subnet mask, not the default route.
- Default routes (0.0.0.0/0) are only used when no other more specific route exists in the routing table.
- Routing decisions in Cisco routers prioritize prefix length before considering administrative distance or metrics.
- Overlapping routes with different prefix lengths can coexist, but the most specific route always takes precedence.
- Load balancing occurs only among routes with equal prefix length and equal administrative distance, not across different prefix lengths.
- Understanding subnetting and prefix lengths is critical for interpreting routing table entries and predicting routing behavior.
TExam Day Tips
- Watch for words such as best, first, most likely and least administrative effort.
- Review why wrong options are wrong, not only why the correct option is correct.
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More questions from this exam
Keep practising from the same exam bank, or move into a focused topic page if this question exposed a weak area.
Question 1
A router learns the same prefix from both OSPF and EIGRP. Which route is installed by default?
Question 2
A router shows this output: R1#show ip ospf neighbor Neighbor ID Pri State Dead Time Address Interface 10.1.1.2 1 FULL/DR 00:00:34 192.168.12.2 GigabitEthernet0/0 10.1.1.3 1 2WAY/DROTHER 00:00:39 192.168.12.3 GigabitEthernet0/0 Which statement is correct?
Question 3
What is the OSPF metric called?
Question 4
A non-root switch has two uplinks toward the root bridge. One path has a lower total STP cost than the other. What role will the lower-cost uplink have?
Question 5
A router interface applies this ACL inbound: 10 deny tcp any any eq 80 20 permit ip any any A user reports that web browsing to a server by IP address fails, but ping works. Which statement best explains the behavior?
Question 6
A router learns route 198.51.100.0/24 from OSPF with AD 110 and also has a static route to the same prefix configured with AD 150. Which route is installed?
FAQ
Questions learners often ask
What does this 200-301 question test?
Routers use the longest prefix match rule to select the most specific route for a given destination IP address.
What is the correct answer to this question?
The correct answer is: The 10.10.20.0/24 route — The destination matches multiple routes, but the /24 is the most specific match and wins by longest prefix match.
What should I do if I get this 200-301 question wrong?
Then try more questions from the same exam bank and focus on understanding why the wrong options are tempting.
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