Question 1hardmultiple choice
Full question →hardmultiple choiceObjective-mapped
A subnet must support at least 126 usable IPv4 host addresses. Which prefix is the smallest that meets the requirement?
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
/26
This is wrong because /26 provides only 62 usable host addresses.
B
Best answer
/25
This is correct because /25 provides 126 usable host addresses.
C
Distractor review
/24
This is wrong because /24 works but is larger than necessary.
D
Distractor review
/27
This is wrong because /27 provides only 30 usable hosts.
Common exam trap
Common exam trap: answer the scenario, not the keyword
A common exam trap is selecting a /24 subnet because it is a well-known default subnet size with 254 usable hosts, which exceeds the requirement but wastes IP address space. Candidates might also incorrectly choose a /26 or /27 prefix, assuming they provide enough hosts, but these subnets only support 62 and 30 usable hosts respectively, which is insufficient. This trap arises from confusing total addresses with usable hosts or neglecting to subtract the network and broadcast addresses. Understanding the difference between total and usable hosts is essential to avoid this mistake.
Technical deep dive
How to think about this question
Subnetting is a fundamental concept in IPv4 networking that divides a larger network into smaller, manageable segments called subnets. Each subnet has a network prefix that determines the number of available IP addresses. The prefix length, expressed as a slash followed by a number (e.g., /25), indicates how many bits are fixed for the network portion, with the remaining bits available for host addressing. Usable host addresses exclude the network and broadcast addresses, which are reserved for subnet identification and broadcast traffic respectively. To determine the smallest subnet prefix that supports at least 126 usable hosts, you calculate the number of host bits needed. A /25 prefix leaves 7 bits for hosts (32 - 25 = 7), providing 2^7 = 128 total addresses. Subtracting 2 for network and broadcast addresses yields 126 usable hosts, exactly meeting the requirement. Prefixes longer than /25 (like /26 or /27) provide fewer usable hosts and thus do not meet the minimum requirement, while shorter prefixes (like /24) provide more addresses but are unnecessarily large. A common exam trap is selecting a /24 prefix because it is a familiar subnet size with 254 usable hosts, which exceeds the requirement but wastes address space. Conversely, choosing a /26 or /27 prefix is tempting due to their common usage but results in insufficient host capacity. In practical Cisco networking, efficient subnetting conserves IP address space and optimizes routing tables, making the /25 subnet the precise and efficient choice for exactly 126 hosts.
KKey Concepts to Remember
- Subnetting divides an IPv4 network into smaller subnets by extending the network prefix to allocate bits for host addressing.
- A /25 prefix allocates 7 bits for host addresses, providing 128 total IP addresses, of which 126 are usable after excluding network and broadcast addresses.
- Usable host addresses exclude the network address (all host bits zero) and the broadcast address (all host bits one) in each subnet.
- Choosing the smallest subnet prefix that meets host requirements optimizes IP address utilization and reduces routing table size.
- A /26 prefix provides only 62 usable hosts, which is insufficient for a subnet requiring at least 126 hosts.
- A /24 prefix provides 254 usable hosts, which meets the requirement but wastes address space compared to a /25 subnet.
- Cisco devices use subnet masks to determine network boundaries and route traffic appropriately within and between subnets.
- Efficient subnetting is critical in Cisco networking to conserve IPv4 address space and maintain scalable network designs.
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.
Related practice questions
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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?
Subnetting divides an IPv4 network into smaller subnets by extending the network prefix to allocate bits for host addressing.
What is the correct answer to this question?
The correct answer is: /25 — A /25 is the smallest valid answer. In practical terms, a /25 provides 128 total addresses. After subtracting the network and broadcast addresses, 126 usable hosts remain. A /26 would be too small because it supports only 62 usable hosts. This is a typical minimum-prefix question. The goal is to choose the smallest subnet that satisfies the host requirement without wasting more address space than necessary.
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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