Question 1hardmultiple choice
Full question →hardmultiple choiceObjective-mapped
A subnet must support 30 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
/28
This is wrong because a /28 provides only 14 usable host addresses.
B
Best answer
/27
This is correct because a /27 provides 30 usable host addresses.
C
Distractor review
/26
This is wrong because /26 works but is larger than necessary.
D
Distractor review
/25
This is wrong because /25 is much larger than required.
Common exam trap
Common exam trap: answer the scenario, not the keyword
A frequent exam trap is choosing a /28 prefix because it appears to be the closest to supporting 30 hosts. However, a /28 subnet only provides 16 total addresses, of which 14 are usable for hosts after excluding the network and broadcast addresses. This mistake arises from confusing total addresses with usable hosts or failing to subtract the reserved addresses. Selecting a /28 leads to insufficient host capacity, causing network issues or exam failure. Always remember that usable hosts equal total addresses minus two, which is critical when calculating subnet sizes for CCNA questions.
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 subnetworks. Each subnet is identified by a network prefix, which determines the number of available host addresses. The prefix length, expressed as a slash followed by a number (e.g., /27), indicates how many bits are used for the network portion of the address, with the remaining bits available for host addressing. To determine the smallest subnet prefix that supports a specific number of usable hosts, you must calculate the total number of addresses needed, including network and broadcast addresses, which are not assignable to hosts. For 30 usable hosts, you need at least 32 total addresses (30 hosts + 1 network + 1 broadcast). A /27 prefix provides exactly 32 addresses (2^(32-27) = 32), making it the smallest subnet that meets the requirement. Prefixes longer than /27 (like /28) provide fewer addresses, while shorter prefixes (like /26 or /25) provide more than necessary. A common exam trap is confusing total addresses with usable host addresses. Candidates often select a /28 because it seems close to the required number, but it only provides 14 usable hosts, which is insufficient. Understanding the difference between total and usable addresses and correctly applying subnetting formulas is crucial for Cisco CCNA exam success and practical network design.
KKey Concepts to Remember
- Subnetting divides an IPv4 network into smaller segments by using a prefix length that determines the number of host bits available.
- The prefix length in CIDR notation indicates how many bits are used for the network portion, with the remainder used for host addressing.
- Usable host addresses equal total addresses in the subnet minus two reserved addresses: the network and broadcast addresses.
- A /27 subnet mask provides 32 total addresses, which yields 30 usable host addresses, making it the smallest subnet for 30 hosts.
- Choosing a subnet mask that is too long, such as /28, results in insufficient usable host addresses for the required number of hosts.
- Selecting a subnet mask that is too short, such as /26 or /25, provides more addresses than necessary, leading to inefficient IP address utilization.
- Understanding the difference between total addresses and usable hosts is critical for accurate subnetting and avoiding exam traps.
- Subnetting calculations are essential for designing efficient networks and are frequently tested on the CCNA 200-301 exam.
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
Related 200-301 practice-question pages
Use these pages to review the topic behind this question. This is how one missed question becomes focused revision.
CCNA subnetting practice questions
Practise IPv4 subnetting, CIDR, masks, host ranges and subnet selection.
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Practise OSPF neighbours, router IDs, metrics, areas and routing-table interpretation.
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Practise VLANs, access ports, trunks, allowed VLANs and switching scenarios.
CCNA STP practice questions
Practise spanning tree, root bridge election, port roles and STP troubleshooting.
CCNA EtherChannel practice questions
Practise LACP, PAgP, port-channel behaviour and bundle requirements.
CCNA ACL practice questions
Practise standard and extended ACLs, permit/deny logic and traffic filtering.
CCNA NAT practice questions
Practise static NAT, dynamic NAT, PAT and inside/outside address translation.
CCNA DHCP practice questions
Practise DHCP scopes, relay, leases and troubleshooting.
CCNA show ip route practice questions
Practise routing-table output, longest-prefix match, AD and route selection.
CCNA show interfaces trunk practice questions
Practise trunk verification and VLAN forwarding across switches.
CCNA wireless security practice questions
Practise WLAN security, authentication and wireless architecture concepts.
CCNA IPv6 practice questions
Practise IPv6 addressing, routes, neighbour discovery and common IPv6 exam traps.
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 segments by using a prefix length that determines the number of host bits available.
What is the correct answer to this question?
The correct answer is: /27 — A /27 is the smallest valid prefix. In plain language, a /27 provides 32 total addresses, and after subtracting the network and broadcast addresses, 30 usable host addresses remain. A /28 would be too small because it provides only 14 usable hosts. This is a classic minimum-prefix question because it checks whether you can work backward from a host requirement and choose the smallest subnet that fits without over-allocating more 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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