CCNA Network Infrastructure and Connectivity Practice Test 15 — 15 Questions

Question 1

A host address is 172.31.9.200/27. Which address is the first usable host in that subnet?

Accepted Answer

172.31.9.193. A /27 subnet has a block size of 32. In practical terms, the relevant last-octet blocks are 0-31, 32-63, 64-95, 96-127, 128-159, 160-191, 192-223, and 224-255. Because 200 falls within the 192-223 block, the network address is 172.31.9.192 and the first usable host is 172.31.9.193.

This is a host-range interpretation problem rather than just network-or-broadcast recognition.

Answer

172.31.9.192

Answer

172.31.9.223

Answer

172.31.9.201

Question 2

A host address is 10.100.12.94/26. Which address is the broadcast address for that subnet?

Accepted Answer

10.100.12.127. A /26 subnet has a block size of 64, so the fourth-octet ranges are 0–63, 64–127, 128–191, and 192–255. The host address 10.100.12.94 lies in the 64–127 range, making the broadcast address the last address in that range: 10.100.12.127. Option A (10.100.12.63) is the broadcast of the previous subnet (0–63). Option C (10.100.12.64) is the network address of the subnet containing the host. Option D (10.100.12.128) is the network address of the next subnet (128–191).

Answer

10.100.12.63

Answer

10.100.12.64

Answer

10.100.12.128

Question 3

A host address is 192.168.90.33/28. Which address is the last usable host in the subnet?

Accepted Answer

192.168.90.46. The /28 subnet mask (255.255.255.240) gives a block size of 16 addresses. The network address for 192.168.90.33 is 192.168.90.32, so the broadcast address is 192.168.90.47. The last usable host is the broadcast address minus one, which is 192.168.90.46.

Answer

192.168.90.47

Answer

192.168.90.33

Answer

192.168.90.48

Question 4

A host address is 10.77.4.141/28. Which address is the network address of the subnet?

Accepted Answer

10.77.4.128. A /28 subnet has a block size of 16. In practical terms, the last-octet blocks are 0-15, 16-31, 32-47, and so on. Because 141 falls within the 128-143 block, the network address is 10.77.4.128.

This is a clean addressing-boundary question that rewards careful block calculation rather than guesswork.

Answer

10.77.4.143

Answer

10.77.4.144

Answer

10.77.4.112

Question 5

A host address is 10.55.8.117/29. Which address is the network address of the subnet?

Accepted Answer

10.55.8.112. A /29 subnet has a block size of 8. In practical terms, the relevant last-octet blocks are 112-119 for this host. That means the network address is 10.55.8.112. Once you identify the correct block, the first address in the block is the network address.

This is a useful addressing-boundary question because it checks careful block calculation, not memorized guesses.

Answer

10.55.8.119

Answer

10.55.8.120

Answer

10.55.8.116

Question 6

A host address is 172.22.14.99/27. Which address is the broadcast address of the subnet?

Accepted Answer

172.22.14.127. A /27 subnet has a block size of 32. In practical terms, the relevant blocks are 0-31, 32-63, 64-95, 96-127, and so on. Because 99 falls within the 96-127 block, the broadcast address is the last address in that block: 172.22.14.127.

This is a classic subnet-boundary question and remains important because addressing precision appears throughout the CCNA blueprint.

Answer

172.22.14.95

Answer

172.22.14.96

Answer

172.22.14.128

Question 7

Which field in an IPv4 packet is primarily used to prevent packets from looping forever in the network?

Accepted Answer

Time to Live. Each router decrements the TTL field by one. When TTL reaches zero, the packet is discarded. That mechanism prevents indefinite looping.

Answer

Version

Answer

Header checksum

Answer

Protocol

Question 8

Exhibit: A host has address 192.168.14.77/27. Which address is its valid default gateway if the first usable address in the subnet is chosen for the router interface?

Accepted Answer

192.168.14.65. A /27 has a block size of 32. Address 192.168.14.77 falls in the 192.168.14.64/27 subnet, where the usable host range is 192.168.14.65 through 192.168.14.94. The first usable address is 192.168.14.65.

Answer

192.168.14.63

Answer

192.168.14.64

Answer

192.168.14.95

Question 9

Exhibit: A laptop has IP address 10.20.30.44/27. Which address is its directed broadcast for that subnet?

Accepted Answer

10.20.30.63. A /27 gives a block size of 32 addresses. The host 10.20.30.44 falls in the 10.20.30.32 to 10.20.30.63 subnet, so the broadcast address is 10.20.30.63.

Answer

10.20.30.31

Answer

10.20.30.32

Answer

10.20.30.64

Question 10

Exhibit: A host on VLAN 10 can ping the local SVI but not a server in VLAN 20. The Layer 3 switch has both VLAN interfaces up. What is the next item to verify first?

Accepted Answer

Whether the server in VLAN 20 has the correct default gateway. If the source host can reach its own default gateway, the local VLAN and access port are probably fine. The next practical check is whether the server in VLAN 20 has the correct IP address, mask, and default gateway configured.

Answer

Whether the switch should disable IP routing

Answer

Whether VLAN 10 should be the native VLAN

Answer

Whether the host should use a /8 mask instead of /24

Question 11

A support engineer is explaining why a host uses ARP before sending a frame on an Ethernet LAN. Which two statements are correct?

Accepted Answer

ARP resolves an IPv4 address to a MAC address. On Ethernet, the sender needs a destination MAC address. For remote destinations, that usually means ARPing for the default gateway's MAC.

Answer

ARP is used to discover the remote router's OSPF router ID

Answer

ARP replaces DNS for hostname resolution

Question 12

An engineer is reviewing transport protocols for a new application. Which two characteristics are associated with TCP rather than UDP?

Accepted Answer

Connection establishment before data transfer. TCP is connection-oriented and provides reliability with sequence numbers, acknowledgments, and retransmissions. UDP is lighter but does not guarantee delivery.

Answer

Best-effort delivery with no acknowledgments

Answer

Lower overhead because no session state is tracked

Question 13

A network technician is troubleshooting a link between two Cisco switches, SW1 and SW2, connected via a single-mode fiber optic cable. The interface on SW1 is up/up, but the interface on SW2 remains down/down. The technician has verified that the fiber cable is not damaged and that the SFP modules are correctly seated. Which configuration change should the technician make to resolve the issue?

Accepted Answer

Replace the 1000BaseSX SFP on SW2 with a 1000BaseLX SFP.. The issue is that SW1 is up/up but SW2 is down/down, indicating a unidirectional link. Since the fiber cable and SFP seating are verified, the most likely cause is an SFP wavelength mismatch. SW1 likely has a 1000BaseLX SFP (long-wavelength, single-mode), while SW2 has a 1000BaseSX SFP (short-wavelength, multimode). Single-mode fiber requires LX optics; SX optics are designed for multimode fiber and will not produce a signal that can be received correctly over single-mode fiber, causing the remote interface to remain down. Replacing the 1000BaseSX SFP on SW2 with a 1000BaseLX SFP resolves the wavelength incompatibility.

Answer

Configure the interface on SW2 to use the same speed and duplex settings as SW1.

Answer

Enable MDIX on both interfaces to allow automatic crossover detection.

Answer

Change the VLAN assignment on SW2's interface to match that of SW1.

Question 14

An engineer is deploying a new Cisco Catalyst 9300 switch in a campus wiring closet. The uplink to the distribution switch uses a 1000BASE-LX SFP module. After connecting the fiber, the interface shows 'up/up' but the engineer notices that the 'input errors' counter is incrementing rapidly, with many CRC errors, runts, and giants being reported. What is the most likely cause of these input errors?

Accepted Answer

Check the fiber distance and ensure it is within the 5 km limit for 1000BASE-LX; if over, use a single-mode fiber extender.. The 1000BASE-LX standard uses long-wavelength laser optics (1300 nm) over single-mode fiber with a maximum distance of 5 km. Exceeding this limit causes signal attenuation and dispersion, generating bit errors that corrupt the frame check sequence (FCS). Cisco IOS counts these as CRC errors. Runts (frames shorter than 64 bytes) and giants (frames longer than 1518 bytes) also appear because the damaged frames are misinterpreted. Option B correctly identifies the distance limit as the root cause and recommends verifying it or using a fiber extender. Options A and D propose incorrect media, and option C would break the link since 1000BASE-LX operates at fixed 1000/full speed.

Answer

Replace the SFP with a 1000BASE-SX module.

Answer

Configure the interface with 'speed 100' and 'duplex full' to match the SFP capabilities.

Answer

Replace the fiber patch cable with a CAT6a copper cable and use a 1000BASE-T SFP.

Question 15

A network technician connects a new Cisco switch to an existing access switch using a Category 5e copper patch cable. The link fails to come up, and the interface status shows 'err-disabled'. The technician checks the interface diagnostics and the running configuration. What action should the technician take to resolve the problem?

Accepted Answer

Configure the interface as a trunk port and disable spanning-tree BPDU guard.. The most likely reason for the err-disabled state is a spanning-tree BPDU guard violation on the access switch's port configured with PortFast. When another switch is connected, it sends BPDUs, triggering BPDU guard to error-disable the port. Configuring the interface as a trunk port and disabling BPDU guard (option B) resolves the issue by allowing BPDUs without triggering the protection. Option A is unnecessary because Category 5e supports Gigabit Ethernet. Option C is irrelevant since the problem is not distance-related. Option D is risky because auto-negotiation is preferred and manual settings can cause mismatches.

Answer

Replace the Category 5e cable with a Category 6 cable to support Gigabit Ethernet.

Answer

Replace the copper SFP with a fiber SFP to increase the distance.

Answer

Manually set the speed and duplex to 1000 Mbps and full duplex on both switches.