Question 1hardmultiple choice
Full question →hardmultiple choiceObjective-mapped
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?
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
172.16.0.0/16
This is wrong because it is less specific than the matching /24 and /25 entries.
B
Distractor review
172.16.20.0/24
This is wrong because the /24 is less specific than the /25 that also matches.
C
Best answer
172.16.20.128/25
This is correct because .200 falls inside the 172.16.20.128/25 range.
D
Distractor review
The default route
This is wrong because multiple more specific routes already match the destination.
Common exam trap
Common exam trap: answer the scenario, not the keyword
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.
Technical deep dive
How to think about this question
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.
KKey Concepts to Remember
- Cisco routers use the longest prefix match rule to select the most specific route that matches the destination IP address.
- A route with a longer subnet mask (higher prefix length) covers fewer IP addresses and is preferred over broader routes.
- Routing tables can contain overlapping routes, but the router always forwards traffic using the route with the narrowest subnet that matches the destination.
- The subnet mask determines the size of the network and directly influences route specificity in Cisco routing decisions.
- If multiple routes match a destination, the router ignores less specific routes in favor of the one with the longest prefix.
- Default routes are only used when no more specific routes exist in the routing table for the destination IP.
- Understanding subnetting and prefix lengths is critical for interpreting routing table entries and predicting router behavior.
- Exam questions on routing often test the ability to apply longest prefix match logic to overlapping subnet routes.
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 that matches the destination IP address.
What is the correct answer to this question?
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.
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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