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: cisco routers use the longest prefix match rule to select the most specific route that matches the 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 the following routes in its table: 172.16.0.0/16, 172.16.20.0/24, and 172.16.20.128/25. Which route is used for traffic to 172.16.20.200?
172.16.0.0/16
Why wrong: This is wrong because it is less specific than the matching /24 and /25 entries.
172.16.20.0/24
Why wrong: This is wrong because the /24 is less specific than the /25 that also matches.
172.16.20.128/25
This is correct because .200 falls inside the 172.16.20.128/25 range.
The default route
Why wrong: This is wrong because multiple more specific routes already match the destination.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
172.16.20.128/25
The /25 route is used because it is the most specific match. In plain language, the router looks for the narrowest route that still contains the destination address. Since 172.16.20.200 falls inside 172.16.20.128/25, that route wins over the broader /24 and /16 entries. This is a direct longest-prefix-match question. It is meant to reinforce that specificity comes first in routing-table lookup. Broader routes remain useful, but they are not chosen when a more precise matching entry exists.
Key principle: Cisco routers use the longest prefix match rule to select the most specific route that matches the destination IP address.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
172.16.0.0/16
Why it's wrong here
This is wrong because it is less specific than the matching /24 and /25 entries.
When this WOULD be correct
In a different scenario where the routing table only contains the route 172.16.0.0/16 and no more specific routes, any traffic destined for 172.16.20.200 would be routed using this option, making it the correct answer.
172.16.20.0/24
Why it's wrong here
This is wrong because the /24 is less specific than the /25 that also matches.
When this WOULD be correct
In a different scenario where the routing table only includes 172.16.20.0/24 and no more specific routes, a question asking which route would be used for traffic to 172.16.20.200 would correctly identify option B as the answer, since it would be the only applicable route.
172.16.20.128/25
Why this is correct
This is correct because .200 falls inside the 172.16.20.128/25 range.
Related concept
Cisco routers use the longest prefix match rule to select the most specific route that matches the destination IP address.
The default route
Why it's wrong here
This is wrong because multiple more specific routes already match the destination.
When this WOULD be correct
If the question specified that the router had no specific routes for the 172.16.20.200 address and only a default route was configured, then the default route would be the correct answer. For example, if the routing table only contained a default route and no other specific routes.
Option-by-option analysis
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.
Why this is correct
This is correct because .200 falls inside the 172.16.20.128/25 range.
Why this is wrong here
The route 172.16.0.0/16 is too broad for the specific destination 172.16.20.200, as it encompasses a larger range of addresses and is not the most specific match available in the routing table.
★ When this WOULD be the correct answer
In a different scenario where the routing table only contains the route 172.16.0.0/16 and no more specific routes, any traffic destined for 172.16.20.200 would be routed using this option, making it the correct answer.
Why candidates choose this
Candidates may choose this option due to a misunderstanding of subnetting, thinking that a broader route would always be applicable, especially if they overlook the importance of specificity in routing decisions.
Why this is wrong here
Option B is incorrect because the route 172.16.20.0/24 encompasses the IP address 172.16.20.200, but it is not the most specific match compared to 172.16.20.128/25, which has a longer prefix and thus takes precedence in routing decisions.
★ When this WOULD be the correct answer
In a different scenario where the routing table only includes 172.16.20.0/24 and no more specific routes, a question asking which route would be used for traffic to 172.16.20.200 would correctly identify option B as the answer, since it would be the only applicable route.
Why candidates choose this
Candidates might choose this option because they recognize that 172.16.20.0/24 covers the target IP address, leading them to mistakenly assume it is the most appropriate route without considering the specificity of the other routes.
Why this is wrong here
The default route is used when no other specific route matches the destination IP address. In this case, 172.16.20.200 matches the more specific route 172.16.20.128/25, making the default route irrelevant.
★ When this WOULD be the correct answer
If the question specified that the router had no specific routes for the 172.16.20.200 address and only a default route was configured, then the default route would be the correct answer. For example, if the routing table only contained a default route and no other specific routes.
Why candidates choose this
Candidates might choose the default route because they may assume that if no specific match is found, the default route is automatically used, reflecting a common misunderstanding of route specificity in routing tables.
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
A frequent exam trap is selecting a broader subnet route such as 172.16.20.0/24 or 172.16.0.0/16 instead of the more specific 172.16.20.128/25. Candidates may mistakenly believe that any matching route is acceptable or that larger subnets are preferred. This misunderstanding ignores the fundamental longest prefix match rule used by Cisco routers, which always prioritizes the most specific route. Falling for this trap leads to incorrect routing decisions and exam errors.
Detailed technical explanation
Routing tables in Cisco routers use the longest prefix match rule to determine the best path for forwarding packets. Each route in the table has a subnet mask that defines the size of the network. The router compares the destination IP address against all routes and selects the one with the most specific subnet mask that still includes the destination address. This ensures traffic is routed as precisely as possible, optimizing network efficiency and reducing unnecessary hops. In this scenario, the router has three routes: 172.16.0.0/16, 172.16.20.0/24, and 172.16.20.128/25. The destination IP 172.16.20.200 falls within all three ranges, but the /25 subnet mask is the most specific because it covers a smaller address range. Therefore, the router selects the 172.16.20.128/25 route to forward the traffic, following the longest prefix match principle. A common exam trap is to choose a less specific route like /24 or /16 because they also match the destination IP. However, Cisco routers always prefer the route with the longest subnet mask that matches the destination. Practically, this behavior ensures that traffic is routed through the most precise path available, which can affect performance and security by avoiding broader, less controlled routes.
Key takeaway
Cisco routers use the longest prefix match rule to select the most specific route that matches the destination IP address.
Real-world example
A small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation applies.
What to study next
Review cisco routers use the longest prefix match rule to select the most specific route that matches the destination IP address., then practise related 200-301 questions on the same topic to reinforce the concept.
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FAQ
IP Routing — This question tests IP Routing — Cisco routers use the longest prefix match rule to select the most specific route that matches the destination IP address..
The correct answer is: 172.16.20.128/25 — The /25 route is used because it is the most specific match. In plain language, the router looks for the narrowest route that still contains the destination address. Since 172.16.20.200 falls inside 172.16.20.128/25, that route wins over the broader /24 and /16 entries. This is a direct longest-prefix-match question. It is meant to reinforce that specificity comes first in routing-table lookup. Broader routes remain useful, but they are not chosen when a more precise matching entry exists.
Review cisco routers use the longest prefix match rule to select the most specific route that matches the destination IP address., then practise related 200-301 questions on the same topic to reinforce the concept.
Cisco routers use the longest prefix match rule to select the most specific route that matches the destination IP address.
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Last reviewed: May 17, 2026
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