Question 1hardmultiple choice
Full question →hardmultiple choiceObjective-mapped
A router has routes to 192.168.0.0/16 and 192.168.100.0/24. Which route is used for traffic to 192.168.100.77?
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
192.168.0.0/16
This is wrong because the /16 is less specific than the matching /24.
B
Best answer
192.168.100.0/24
This is correct because it is the more specific matching prefix.
C
Distractor review
Both routes are discarded because they overlap.
This is wrong because overlapping prefixes are normal and route specificity resolves the choice.
D
Distractor review
The default route is preferred if present.
This is wrong because more specific matching routes are preferred before the default route.
Common exam trap
Common exam trap: answer the scenario, not the keyword
A frequent exam trap is to select the broader route 192.168.0.0/16 because it seems to cover more addresses, including the destination. Candidates may mistakenly think that a larger subnet mask means a better route or that overlapping routes cause the router to discard both. However, Cisco routers always prefer the most specific route based on the longest-prefix match rule. Overlapping routes are normal and do not cause discarding; instead, the router uses the route with the longest subnet mask. Misunderstanding this can lead to incorrect answers and confusion about routing behavior.
Technical deep dive
How to think about this question
Routing tables in Cisco routers use prefix matching to determine the best path for forwarding packets. Each route in the table has a network prefix and a subnet mask, which together define the range of IP addresses covered by that route. When a packet arrives, the router compares the destination IP address against all routes and selects the one with the longest matching prefix, meaning the most specific subnet mask that fits the destination address. The longest-prefix match rule means that if multiple routes cover the destination IP, the router prefers the route with the smallest subnet (largest mask). In this question, 192.168.0.0/16 covers all addresses from 192.168.0.0 to 192.168.255.255, while 192.168.100.0/24 covers only 192.168.100.0 to 192.168.100.255. Since 192.168.100.77 falls within both ranges, the router chooses the /24 route because it is more specific and thus a better match. A common exam trap is to assume that the broader route (192.168.0.0/16) is preferred because it covers a larger address space. However, Cisco routers always use the longest-prefix match rule first, regardless of route size or administrative distance unless no more specific route exists. This behavior ensures precise routing and prevents routing ambiguity in overlapping subnets, which is critical for efficient network operation and troubleshooting.
KKey Concepts to Remember
- Cisco routers use the longest-prefix match rule to select the most specific route for forwarding packets based on subnet mask length.
- A route with a smaller subnet (larger mask) is preferred over a broader route when both cover the destination IP address.
- Overlapping routes in a routing table are normal and do not cause routes to be discarded; the router chooses the best match.
- The router compares destination IP addresses against all routes and selects the one with the longest matching prefix first.
- Default routes are only used when no more specific matching route exists in the routing table.
- Routing decisions in Cisco devices prioritize prefix specificity before considering administrative distance or metrics.
- The presence of multiple routes to overlapping subnets requires understanding subnetting and prefix lengths for correct route selection.
- Misinterpreting route specificity can lead to incorrect routing decisions and exam mistakes in CCNA routing questions.
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?
Cisco routers use the longest-prefix match rule to select the most specific route for forwarding packets based on subnet mask length.
What is the correct answer to this question?
The correct answer is: 192.168.100.0/24 — The route to 192.168.100.0/24 is used because it is more specific. In plain language, even though the /16 route covers a large address range that includes the destination, the /24 route describes the destination network more precisely. Longest-prefix match therefore prefers the /24. This is a basic but critical routing-table concept. The router does not choose the broader route when a narrower one matches the same destination.
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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