CCNA Practice Questions: Subnetting, OSPF and STP Scenarios

Introduction

Passing the Cisco Certified Network Associate (CCNA) exam requires more than just memorizing facts; you need to apply concepts in realistic scenarios. This set of CCNA practice questions focuses on three core topics: subnetting, OSPF, and Spanning Tree Protocol (STP). Each question mirrors the style and difficulty of the actual exam. After each question, we break down the correct answer and explain why the other options are incorrect. Use these to identify your weak spots and focus your study efforts.

Question 1: Subnetting

You are given the network 192.168.10.0/24 and need to create subnets that each support at least 50 hosts. What is the new subnet mask? This connects to what we discuss in CCNA Difficulty: The Labs and Subnetting Questions That Separate Pass From Fail.

• A) /25
• B) /26
• C) /27
• D) /28

Correct Answer: B) /26

Explanation: To support at least 50 hosts, you need 6 host bits because 2^6 - 2 = 62 usable hosts. A /26 mask leaves 6 host bits (32 - 26 = 6). A /25 leaves 7 host bits (126 usable), which is more than enough but not the most efficient if you need to maximize subnets. /27 leaves only 5 host bits (30 usable), and /28 leaves 4 host bits (14 usable), both insufficient. The best answer is /26, which provides exactly 62 usable addresses per subnet.

If these subnetting calculations still take you extra time, a free CCNA study plan can set aside regular practice sessions across the full exam topics.

Question 2: Subnetting

Which subnet does host 172.16.50.100/20 belong to?

• A) 172.16.32.0
• B) 172.16.48.0
• C) 172.16.64.0
• D) 172.16.50.0

Correct Answer: B) 172.16.48.0

Explanation: A /20 prefix means the subnet mask is 255.255.240.0. The network bits are in the third octet. The block size is 256 - 240 = 16. The third octet of the IP is 50. The multiples of 16 that are less than or equal to 50 are 0, 16, 32, 48. The next multiple is 64, so the network address is 172.16.48.0. Option A (172.16.32.0) is the network for the range 172.16.32.0 to 172.16.47.255, which does not include .50. Option C is the next network after 48. Option D is incorrect because /20 does not use a /24 boundary; the network address is not the same as the host portion.

Question 3: OSPF

Which OSPF state indicates that a router has received a Hello packet from a neighbor but has not yet established bidirectional communication?

• A) Down
• B) Init
• C) Two-Way
• D) ExStart

Correct Answer: B) Init

Explanation: In OSPF neighbor state progression, the Init state means that a Hello packet has been received from a neighbor, but the neighbor's Router ID is not yet in the local router's Hello packet. The Two-Way state indicates that both routers have seen each other's Router IDs in Hello packets, confirming bidirectional communication. Down means no Hello received, and ExStart is the first state in the database exchange process. Therefore, the correct answer is Init.

Question 4: OSPF

What is the default administrative distance of OSPF?

• A) 90
• B) 100
• C) 110
• D) 120

Correct Answer: C) 110 We cover this in depth in CCNA Passing Score: How Cisco Scores the 200-301.

Explanation: The default administrative distance for OSPF is 110. EIGRP has a default AD of 90, RIP is 120, and static routes have an AD of 1. OSPF's AD of 110 is lower than RIP's 120, making OSPF more preferred. Option A is EIGRP, Option B is not a standard default AD, and Option D is RIP.

Question 5: OSPF

Which command is used to configure OSPF on a Cisco router and enter OSPF router configuration mode?

• A) router ospf 1
• B) enable ospf
• C) network ospf
• D) ip ospf

Correct Answer: A) router ospf 1

Explanation: The command router ospf [process-id] enters OSPF router configuration mode. The process ID is locally significant and can be any number. The other options are not valid Cisco IOS commands for entering OSPF configuration.

Question 6: STP

Which STP port role is assigned to a port that provides the best path to the root bridge?

• A) Root port
• B) Designated port
• C) Blocking port
• D) Alternate port

Correct Answer: A) Root port

Explanation: In STP, each non-root switch selects one port as its root port, which is the port with the lowest cost to reach the root bridge. A designated port is the port on a segment that is responsible for forwarding traffic to the root; it is the best port on that segment. Blocking ports are those that are not root or designated and do not forward traffic. An alternate port is a blocking port that provides a backup path to the root. The root port is the one with the best path to the root, so A is correct.

Question 7: STP

What is the default STP bridge priority for a Cisco switch?

• A) 32768
• B) 4096
• C) 65536
• D) 0

Correct Answer: A) 32768

Explanation: The default bridge priority in classic STP (IEEE 802.1D) is 32768. This value is used to determine the root bridge; the lowest priority becomes root. Option B (4096) is the increment for the priority value, not the default. Option C is not a valid default, and Option D is not the default. Our post on CompTIA Network+ vs CCNA: Which Networking Cert Gets You Hired Faster? covers this further.

Question 8: STP

Which STP state allows a port to learn MAC addresses but not forward frames?

• A) Listening
• B) Learning
• C) Forwarding
• D) Blocking

Correct Answer: B) Learning

Explanation: In STP, a port transitions through states: Blocking, Listening, Learning, and Forwarding. In the Learning state, the port receives frames and builds the MAC address table, but it does not forward frames. Listening state only listens to BPDUs, not learning MAC addresses. Forwarding sends frames, and Blocking does not learn or forward. So B is correct.

Question 9: Subnetting and Routing

You have a router with an interface configured as 192.168.1.129/26. How many usable host addresses are available on this subnet?

• A) 62
• B) 30
• C) 126
• D) 14

Correct Answer: A) 62

Explanation: A /26 prefix length gives 6 host bits, so the number of usable hosts is 2^6 - 2 = 62. The network address and broadcast address are not usable. Option B (30) would be for /27, Option C (126) for /25, and Option D (14) for /28.

Question 10: OSPF and STP Combined

In a network running OSPF and STP, which protocol is responsible for preventing Layer 2 loops?

• A) OSPF
• B) STP
• C) Both
• D) Neither

Correct Answer: B) STP

Explanation: STP (Spanning Tree Protocol) is a Layer 2 protocol that prevents loops in Ethernet networks by blocking redundant paths. OSPF is a Layer 3 routing protocol that determines the best path for IP packets and does not deal with Layer 2 loops. Therefore, STP is the correct answer.

Conclusion

These CCNA practice questions cover essential topics that frequently appear on the exam. By understanding not only the correct answers but also the reasoning behind the incorrect options, you deepen your conceptual knowledge. Keep practicing with a variety of scenarios to build speed and accuracy.

If you want to see which weak topics remain after answering a few questions, use a free CCNA readiness check.

Round out your revision with Cisco Certified Network Associate (CCNA) practice questions — they are the missing piece most candidates skip.