Courseiva
← 200-301
Full test
hardmultiple choiceObjective-mapped

What prefix length corresponds to the subnet mask 255.255.255.248?

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 /28 would correspond to 255.255.255.240.

B

Best answer

/29

This is correct because 255.255.255.248 equals 29 network bits.

C

Distractor review

/30

This is wrong because /30 would correspond to 255.255.255.252.

D

Distractor review

/27

This is wrong because /27 would correspond to 255.255.255.224.

Common exam trap

Common exam trap: answer the scenario, not the keyword

A frequent exam trap is mistaking the subnet mask 255.255.255.248 for 255.255.255.240 or 255.255.255.252, which correspond to /28 and /30 prefixes respectively. Candidates often confuse these because the decimal values are close and the subnet sizes are similar. This mistake leads to incorrect subnetting calculations, affecting the number of usable host addresses and network segmentation. The trap exploits the candidate’s tendency to rely on memorized decimal values without verifying the binary bit count, causing errors in prefix length identification.

Technical deep dive

How to think about this question

Subnetting is a fundamental concept in IP networking that divides a larger network into smaller, manageable subnetworks. The subnet mask determines which portion of an IP address refers to the network and which part refers to hosts. The prefix length, also known as CIDR notation, expresses the subnet mask as the number of bits set to 1 in the mask. For example, a subnet mask of 255.255.255.248 means the first 29 bits are network bits, leaving 3 bits for host addresses. To determine the prefix length from a subnet mask, convert each octet to binary and count the consecutive 1 bits from left to right. In 255.255.255.248, the first three octets (255.255.255) each have 8 bits set to 1, totaling 24 bits. The last octet, 248, in binary is 11111000, which adds 5 more bits set to 1. Adding these gives a prefix length of /29. This prefix length is critical in Cisco networking for configuring interfaces, routing protocols, and access control lists that rely on subnet boundaries. A common exam trap is confusing similar subnet masks with close prefix lengths, such as /28 (255.255.255.240) or /30 (255.255.255.252). These masks differ by the number of host bits and available addresses. Misidentifying the prefix length can lead to incorrect subnetting, causing routing or addressing errors in Cisco devices. Understanding the binary representation and practicing conversions helps avoid this mistake and ensures accurate subnet configuration in CCNA scenarios.

KKey Concepts to Remember

  • A subnet mask defines which bits of an IP address represent the network and which represent hosts by setting network bits to 1 and host bits to 0.
  • The prefix length or CIDR notation indicates the total number of network bits in the subnet mask, simplifying subnet representation in Cisco configurations.
  • Converting the subnet mask to binary and counting the consecutive 1 bits from left to right determines the prefix length accurately.
  • The subnet mask 255.255.255.248 corresponds to a /29 prefix because it has 29 bits set to 1, combining 24 bits from the first three octets and 5 bits from the last octet.
  • Incorrectly identifying subnet masks by decimal values without binary verification leads to common exam mistakes in subnetting questions.
  • Cisco devices use prefix lengths in routing, access control lists, and interface configurations to define network boundaries precisely.
  • Subnetting with a /29 prefix provides 8 IP addresses per subnet, including network and broadcast addresses, which is important for network design.
  • Understanding subnet mask to prefix length conversion helps avoid misconfiguration and ensures proper IP address allocation in Cisco 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.

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.

CCNA OSPF practice questions

Practise OSPF neighbours, router IDs, metrics, areas and routing-table interpretation.

CCNA VLAN practice questions

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?

A subnet mask defines which bits of an IP address represent the network and which represent hosts by setting network bits to 1 and host bits to 0.

What is the correct answer to this question?

The correct answer is: /29 — The mask 255.255.255.248 corresponds to /29. In practical terms, the first three octets provide 24 network bits, and the value 248 in the last octet is 11111000 in binary, which contributes 5 more network bits. That gives a total prefix length of 29. This is a standard dotted-decimal to prefix conversion question. It matters because subnetting often requires you to move comfortably between both forms.

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.

Discussion

Loading comments…

Sign in to join the discussion.