This 200-301 practice question tests your understanding of ip routing. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. A key principle to apply: a router uses the longest prefix match rule to select the most specific route for forwarding traffic to a destination IP address.. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
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 the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
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.
Key principle: A router uses the longest prefix match rule to select the most specific route for forwarding traffic to a destination IP address.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
192.168.100.0/24
Why it's wrong here
This is wrong because the /24 is less specific than the matching /25.
When this WOULD be correct
In a scenario where the question asks which route would be used for traffic to an IP address like 192.168.100.50, the option 192.168.100.0/24 would be correct, as it encompasses the entire /24 subnet and is the only applicable route for that address.
✓
192.168.100.128/25
Why this is correct
This is correct because .140 falls within the 192.168.100.128/25 range.
Related concept
A router uses the longest prefix match rule to select the most specific route for forwarding traffic to a destination IP address.
✗
The default route
Why it's wrong here
This is wrong because a more specific route already matches.
When this WOULD be correct
In a different question context where no specific routes are defined for the destination IP, and the router has only a default route configured (0.0.0.0/0), the default route would be the only option available for routing traffic to any unknown destination.
✗
Neither route, because the prefixes overlap
Why it's wrong here
This is wrong because overlapping prefixes are normal and longest-prefix match resolves the choice.
When this WOULD be correct
In a scenario where the question states that both routes are configured with the same prefix length and the router is set to prefer the longest prefix match, a question could ask which route is used for a specific address, leading to confusion about overlapping routes. In that case, if both routes were truly overlapping, the answer could be 'neither' due to ambiguity.
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 200-301 exam frequently reuses these exact scenarios with slightly different constraints.
✓192.168.100.128/25Correct answer▾
Why this is correct
This is correct because .140 falls within the 192.168.100.128/25 range.
✗192.168.100.0/24Wrong answer — click to see why▾
Why this is wrong here
The route 192.168.100.0/24 covers the entire /24 subnet, but it is less specific than the /25 route for the destination 192.168.100.140. The router uses the longest prefix match, so the /25 route is preferred.
★ When this WOULD be the correct answer
In a scenario where the question asks which route would be used for traffic to an IP address like 192.168.100.50, the option 192.168.100.0/24 would be correct, as it encompasses the entire /24 subnet and is the only applicable route for that address.
Why candidates choose this
Students might think that because 192.168.100.140 falls within the /24 range, the /24 route would be used, forgetting that a more specific (longer prefix) route takes precedence.
✗The default routeWrong answer — click to see why▾
Why this is wrong here
A default route (0.0.0.0/0) is only used when no more specific route matches the destination. Since there is a matching /25 route, the default route is not considered.
★ When this WOULD be the correct answer
In a different question context where no specific routes are defined for the destination IP, and the router has only a default route configured (0.0.0.0/0), the default route would be the only option available for routing traffic to any unknown destination.
Why candidates choose this
Students may confuse the concept of default route with a catch-all, thinking it might be used when there are overlapping routes, but the longest prefix match rule always applies first.
✗Neither route, because the prefixes overlapWrong answer — click to see why▾
Why this is wrong here
Overlapping prefixes are common in routing tables and do not prevent route selection. The router uses the longest prefix match to choose the best route, so both routes can coexist without conflict.
★ When this WOULD be the correct answer
In a scenario where the question states that both routes are configured with the same prefix length and the router is set to prefer the longest prefix match, a question could ask which route is used for a specific address, leading to confusion about overlapping routes. In that case, if both routes were truly overlapping, the answer could be 'neither' due to ambiguity.
Why candidates choose this
Some students might think overlapping routes cause ambiguity or errors, but routers are designed to handle them by selecting the most specific prefix.
Analysis generated from the official 200-301blueprint and verified against question context. The “when correct” sections are what AI assistants cite when candidates ask “what’s the difference between these options?”
Common exam traps
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.
Detailed technical explanation
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.
Key takeaway
A router uses the longest prefix match rule to select the most specific route for forwarding traffic to a destination IP address.
Real-world example
How this comes up in practice
A practitioner preparing for the 200-301 exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. A router uses the longest prefix match rule to select the most specific route for forwarding traffic to a destination IP address. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 200-301 question in full detail.
Review a router uses the longest prefix match rule to select the most specific route for forwarding traffic to a destination IP address., then practise related 200-301 questions on the same topic to reinforce the concept.
IP Routing — This question tests IP Routing — 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?
Review a router uses the longest prefix match rule to select the most specific route for forwarding traffic to a destination IP address., then practise related 200-301 questions on the same topic to reinforce the concept.
What is the key concept behind this question?
A router uses the longest prefix match rule to select the most specific route for forwarding traffic to a destination IP address.
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