Question 1hardmultiple choice
Full question →hardmultiple choiceObjective-mapped
A router has routes to 192.168.100.0/24 and 192.168.100.128/25. Which route is used for traffic to 192.168.100.140?
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
192.168.100.0/24
This is wrong because the /24 is less specific than the matching /25.
B
Best answer
192.168.100.128/25
This is correct because .140 falls within the 192.168.100.128/25 range.
C
Distractor review
The default route
This is wrong because a more specific route already matches.
D
Distractor review
Neither route, because the prefixes overlap
This is wrong because overlapping prefixes are normal and longest-prefix match resolves the choice.
Common exam trap
Common exam trap: answer the scenario, not the keyword
A frequent exam trap is assuming that the less specific route (192.168.100.0/24) will be used simply because it covers the entire subnet range or because it might have a better administrative distance. Candidates might also think overlapping prefixes cause routing conflicts that prevent either route from being used. However, Cisco routers resolve overlapping routes by always selecting the longest prefix match, which is the most specific subnet. Ignoring this rule leads to incorrect answers and misunderstanding of routing behavior.
Technical deep dive
How to think about this question
In IP routing, routers use the longest prefix match rule to determine the best route for forwarding packets. This means that when multiple routes match a destination IP address, the router selects the route with the most specific subnet mask, i.e., the route with the longest subnet prefix. For example, a /25 subnet mask (255.255.255.128) is more specific than a /24 mask (255.255.255.0) because it covers fewer addresses and thus provides a narrower range. When a router has overlapping routes such as 192.168.100.0/24 and 192.168.100.128/25, and it receives traffic destined for 192.168.100.140, it evaluates both routes. The IP 192.168.100.140 falls within the 192.168.100.128/25 subnet, which covers addresses from 192.168.100.128 to 192.168.100.255. Since /25 is more specific than /24, the router chooses the 192.168.100.128/25 route to forward the packet. A common exam trap is misunderstanding overlapping routes and assuming the broader route (/24) will be used because it was learned first or has a better administrative distance. However, Cisco routers always apply longest prefix match first before considering administrative distance or metrics. This behavior ensures traffic is routed as precisely as possible, avoiding routing ambiguity and improving network efficiency.
KKey Concepts to Remember
- A router uses the longest prefix match rule to select the most specific route for forwarding traffic to a destination IP address.
- When multiple routes match a destination, the router prefers the route with the longest subnet mask prefix length.
- Overlapping IP prefixes are common in routing tables and are resolved by selecting the route with the narrowest subnet range.
- Administrative distance and routing metrics are only considered after the router identifies routes with equal prefix lengths.
- A /25 subnet mask covers half the address space of a /24 subnet, making it more specific and preferred for matching addresses within its range.
- Cisco routers always forward packets based on the most specific matching route to optimize network traffic flow.
- Ignoring longest prefix match can lead to routing errors and inefficient traffic forwarding in practical network environments.
- Understanding subnetting and prefix length comparison is essential for correctly interpreting routing decisions in Cisco networks.
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?
A router uses the longest prefix match rule to select the most specific route for forwarding traffic to a destination IP address.
What is the correct answer to this question?
The correct answer is: 192.168.100.128/25 — The /25 route is used because it is the most specific matching prefix. In practical terms, 192.168.100.140 falls inside the upper half of the /24, which is exactly what the 192.168.100.128/25 route describes. Even though the /24 also matches, the router always prefers the narrower route when both are valid. This is a direct longest-prefix-match question. It reinforces that specificity is checked before broader route-source preferences matter.
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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