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A host uses the subnet mask 255.255.254.0. Which prefix length does this represent?

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A host uses the subnet mask 255.255.254.0. Which prefix length does this represent?

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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

/22

This is wrong because /22 would correspond to 255.255.252.0.

B

Best answer

/23

This is correct because 255.255.254.0 corresponds to 23 network bits.

C

Distractor review

/24

This is wrong because /24 would correspond to 255.255.255.0.

D

Distractor review

/25

This is wrong because /25 would correspond to 255.255.255.128.

Common exam trap

Common exam trap: answer the scenario, not the keyword

A frequent exam trap is mistaking the subnet mask 255.255.254.0 for /24 or /22. Many candidates memorize common masks like 255.255.255.0 (/24) and 255.255.252.0 (/22) but overlook that 254 in the third octet means only 7 bits are set, not 8 or 6. This leads to selecting /24 or /22 incorrectly. The trap arises because 254 looks similar to 255 but actually represents one fewer network bit, making the correct prefix /23. Misreading this causes subnetting errors and incorrect network boundary calculations in Cisco routing and addressing.

Technical deep dive

How to think about this question

Subnetting is the process of dividing a larger IP network into smaller, more manageable subnetworks by manipulating the subnet mask or prefix length. The subnet mask 255.255.254.0 is a non-standard mask that extends beyond the common /24 boundary, indicating that the network portion spans more bits than a typical Class C network. This mask means the first two octets (255.255) are fully network bits, the third octet (254) contributes 7 bits, and the last octet (0) contributes none, totaling 23 bits for the network prefix. The prefix length, also called CIDR notation, counts the number of bits set to 1 in the subnet mask. For 255.255.254.0, the binary form is 11111111.11111111.11111110.00000000, which equals /23. This means the first 23 bits identify the network, and the remaining 9 bits identify hosts within that subnet. Cisco devices use this prefix length to determine routing boundaries, broadcast domains, and address allocation. A common exam trap is confusing 255.255.254.0 with masks like 255.255.252.0 (/22) or 255.255.255.0 (/24). Candidates often default to /24 because it is a common subnet size, but the presence of 254 in the third octet means one bit fewer than 255, indicating a /23 prefix. Understanding this distinction is critical for accurate subnetting, IP address planning, and configuring Cisco routers or switches correctly in the CCNA exam context.

KKey Concepts to Remember

  • Subnet masks define the boundary between network and host portions of an IP address by setting bits to 1 for network and 0 for host.
  • The prefix length counts the total number of 1 bits in the subnet mask and is expressed in CIDR notation, such as /23.
  • A subnet mask of 255.255.254.0 corresponds to a /23 prefix because the third octet has 7 bits set to 1 (binary 11111110).
  • Cisco devices use the prefix length to determine routing boundaries, broadcast domains, and address allocation within a subnet.
  • Misinterpreting subnet masks with non-255 octets, like 254, leads to common exam mistakes by confusing them with /24 or /22 prefixes.
  • Converting subnet mask octets to binary is the most reliable method to identify the correct prefix length during subnetting tasks.
  • Subnetting with a /23 prefix doubles the host capacity compared to a /24 subnet by borrowing one bit from the host portion.
  • Understanding subnet mask to prefix length conversion is essential for configuring IP addressing and routing in Cisco CCNA environments.

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?

Subnet masks define the boundary between network and host portions of an IP address by setting bits to 1 for network and 0 for host.

What is the correct answer to this question?

The correct answer is: /23 — The mask 255.255.254.0 represents /23. In plain language, the first two octets are fully network bits, the third octet contributes 7 more network bits because 254 in binary is 11111110, and the last octet contributes none. That totals 23 network bits. This is a common prefix-conversion question because candidates often memorize the usual masks but hesitate when a non-/24 boundary appears. Recognizing that 255.255.254.0 equals /23 is important for subnetting and local-scope calculations.

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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