Question 1hardmultiple choice
Full question →hardmultiple choiceObjective-mapped
A router has routes to 10.50.0.0/16, 10.50.10.0/24, and 10.50.10.128/25. Which route is used for traffic to 10.50.10.140?
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
10.50.0.0/16
This is wrong because the /16 is less specific than the matching /24 and /25 routes.
B
Distractor review
10.50.10.0/24
This is wrong because the /24 is less specific than the matching /25.
C
Best answer
10.50.10.128/25
This is correct because .140 falls inside the 10.50.10.128/25 range.
D
Distractor review
The default route
This is wrong because more specific matching routes exist.
Common exam trap
Common exam trap: answer the scenario, not the keyword
A frequent exam trap is choosing a less specific route such as 10.50.0.0/16 or 10.50.10.0/24 because they also include the destination IP address. Candidates might overlook the importance of the subnet mask length and assume any matching route is acceptable. This mistake ignores the longest-prefix match rule, which always prefers the route with the most bits matching the destination IP. Selecting a broader route leads to incorrect routing decisions and fails to reflect Cisco’s routing behavior.
Technical deep dive
How to think about this question
Routing tables in Cisco routers use prefix matching to determine the best path for forwarding packets. Each route in the table has a network prefix and a subnet mask, which together define the range of IP addresses covered. When a packet arrives, the router compares the destination IP address against all routes and selects the one with the longest matching prefix, meaning the most specific subnet mask. This process is known as longest-prefix match and ensures traffic is routed as precisely as possible. In the given scenario, the router has three routes: 10.50.0.0/16, 10.50.10.0/24, and 10.50.10.128/25. The destination IP 10.50.10.140 falls within all three ranges, but the /25 subnet mask is the most specific because it covers fewer addresses and matches more bits of the destination IP. Therefore, the router chooses the 10.50.10.128/25 route to forward the traffic, following the longest-prefix match rule that Cisco routers strictly enforce. A common exam trap is to select a less specific route like the /16 or /24 because they also include the destination IP. However, Cisco routers always prefer the most specific route, even if broader routes exist. Practically, this behavior allows network administrators to create hierarchical and overlapping subnets, where more specific routes override general ones. Understanding this helps avoid misconfigurations and ensures efficient routing in complex networks.
KKey Concepts to Remember
- Cisco routers use longest-prefix match to select the route with the most specific subnet mask 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.
- Overlapping routes can exist in a routing table, but the router always forwards packets using the most specific matching route.
- The subnet 10.50.10.128/25 covers IP addresses from 10.50.10.128 to 10.50.10.255, making it more specific than 10.50.10.0/24 or 10.50.0.0/16.
- Routing decisions in Cisco devices do not rely on administrative distance when multiple routes to the same destination exist in the same routing protocol; prefix length is the primary factor.
- Default routes are only used when no more specific matching routes exist in the routing table.
- Understanding subnetting and binary IP matching is essential for correctly determining which route a router will use.
- Longest-prefix match ensures efficient and precise routing by directing traffic to the narrowest subnet that includes the destination IP.
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 longest-prefix match to select the route with the most specific subnet mask that matches the destination IP address.
What is the correct answer to this question?
The correct answer is: 10.50.10.128/25 — The /25 route is used because it is the most specific matching prefix. In practical terms, 10.50.10.140 belongs to the upper half of the 10.50.10.0/24 space, which is exactly what 10.50.10.128/25 describes. Even though the /24 and /16 also match, longest-prefix match prefers the narrowest route. This is a direct route-selection question. It reinforces that the router chooses the route that describes the destination most precisely.
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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