CCNA IP Routing Practice Question
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?
⚠ Common exam trap
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.
Answer choices
Why each option matters
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.
Answer analysis
Option-by-option breakdown
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
Although 172.16.20.200 is within the 172.16.0.0/16 address block, this route has a prefix length of only 16 bits. The router selects the route with the longest matching prefix; since the routing table contains more specific routes (172.16.20.0/24 and 172.16.20.128/25), the /16 route is not considered for forwarding. It would only be used if no longer prefix matches existed. Its broad summary covers many subnets, but that generality disqualifies it in favor of the more exact /25 entry.
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
While 172.16.20.0/24 does encompass the destination IP address 172.16.20.200, it is not the route selected. Routers utilise the Longest Prefix Match rule, which dictates that the most specific route available in the routing table for a given destination IP address will be chosen. In this scenario, 172.16.20.128/25 is a more precise match. This option would be correct if the 172.16.20.128/25 route were absent, as it would then represent the longest prefix match.
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
The destination IP 172.16.20.200 falls within the subnet 172.16.20.128/25 (range 172.16.20.128 to 172.16.20.255). According to the longest prefix match rule, this route has a prefix length of 25, which is longer (more specific) than the other candidate routes (default /0 and 172.16.0.0/16 /16), so it is preferred. The route 172.16.20.0/24 also matches but is less specific because /24 is shorter than /25. Thus, the router forwards the packet using the /25 route.
- ✗
The default route
Why it's wrong here
A default route, typically 0.0.0.0/0, matches every destination IP address. However, it has the shortest possible prefix length (0), making it the least specific route. The longest prefix match algorithm always prefers the route with the greatest number of matching bits; here, both 172.16.20.0/24 and 172.16.20.128/25 match the destination with longer prefixes, so the default route is never selected for this packet. It serves only as a last resort when no other route matches the destination network.
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
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.
✓172.16.20.128/25Correct answer▾
Why this is correct
The destination IP 172.16.20.200 falls within the subnet 172.16.20.128/25 (range 172.16.20.128 to 172.16.20.255). According to the longest prefix match rule, this route has a prefix length of 25, which is longer (more specific) than the other candidate routes (default /0 and 172.16.0.0/16 /16), so it is preferred. The route 172.16.20.0/24 also matches but is less specific because /24 is shorter than /25. Thus, the router forwards the packet using the /25 route.
✗172.16.0.0/16Wrong answer — click to see why▾
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.
✗172.16.20.0/24Wrong answer — click to see why▾
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.
✗The default routeWrong answer — click to see why▾
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?”
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Key term
Route
A route is a path that data takes through a network from one device or network to another, determined by routing protocols and configured rules.
Key term
Router
A router is a networking device that connects different networks together and directs data traffic between them by choosing the best path for data to travel.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This 200-301 practice question is part of Courseiva's free Cisco certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 200-301 exam.