Question 1hardmultiple choice
Full question →hardmultiple choiceObjective-mapped
A router has routes to 172.20.0.0/16, 172.20.10.0/24, and 172.20.10.64/26. Which route is used for traffic to 172.20.10.70?
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
172.20.0.0/16
This is wrong because it is less specific than the matching /24 and /26 routes.
B
Distractor review
172.20.10.0/24
This is wrong because it is less specific than the matching /26 route.
C
Best answer
172.20.10.64/26
This is correct because .70 falls within the 172.20.10.64/26 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 common exam trap is to select a less specific route such as 172.20.0.0/16 or 172.20.10.0/24 because they appear to cover the destination IP 172.20.10.70. Candidates may overlook that the router always prefers the most specific route, which in this case is the /26 subnet. Choosing a broader subnet ignores the longest prefix match principle and leads to incorrect routing decisions. This trap tests your understanding of how subnet masks influence route selection and the importance of prefix length in Cisco routing tables.
Technical deep dive
How to think about this question
Routing in IP networks relies on the longest prefix match rule, where the router selects the route with the most specific subnet mask that matches the destination IP address. In this scenario, the router has three routes: 172.20.0.0/16, 172.20.10.0/24, and 172.20.10.64/26. Each route covers a progressively smaller range of IP addresses, with the /26 subnet being the most specific. The IP address 172.20.10.70 falls within the 172.20.10.64/26 subnet, which ranges from 172.20.10.64 to 172.20.10.127, making it the best match for routing decisions. The router uses the longest prefix match algorithm to determine the best route. This means it compares the destination IP against all available routes and selects the one with the longest subnet mask that still includes the destination address. Even though the /16 and /24 routes also cover 172.20.10.70, the /26 route is preferred because it is more specific. This behavior ensures traffic is routed efficiently and accurately to the intended subnet, minimizing unnecessary hops or routing ambiguity. A common exam trap is to assume that broader routes like /16 or /24 are sufficient or preferred simply because they are listed first or cover more addresses. However, Cisco routers always prioritize the most specific route due to longest prefix matching. In practical networks, this behavior allows for hierarchical subnetting and route summarization, improving scalability and performance. Understanding this principle is critical for troubleshooting routing issues and designing efficient IP addressing schemes.
KKey Concepts to Remember
- A router uses the longest prefix match rule to select the most specific route that matches the destination IP address.
- Routes with longer subnet masks (higher prefix length) are preferred over broader routes when multiple matches exist.
- The 172.20.10.64/26 subnet is more specific than 172.20.10.0/24 and 172.20.0.0/16 because it has a longer prefix length.
- Traffic destined for 172.20.10.70 matches the 172.20.10.64/26 subnet because the IP falls within that subnet's address range.
- Default routes are only used when no more specific matching routes exist in the routing table.
- Cisco routers perform route lookup by comparing destination IPs against all routes and choosing the one with the longest matching prefix.
- Subnetting allows hierarchical IP address division, enabling routers to make precise forwarding decisions based on subnet specificity.
- Misunderstanding longest prefix match can lead to incorrect route selection and routing failures in complex 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.
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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?
A router uses 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: 172.20.10.64/26 — The /26 route is used because it is the most specific matching prefix. In practical terms, 172.20.10.70 falls inside the 172.20.10.64/26 range, so that route narrows the destination more precisely than the broader /24 and /16 routes. Longest-prefix match therefore selects the /26 entry. This question is designed to reinforce that specificity comes first in route lookup. Broader routes remain useful, but they lose when a more exact route matches.
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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