N10-009 Network Troubleshooting — 40 Questions

Question 1

A user reports intermittent connectivity on a laptop that moves between floors. The signal strength fluctuates. Which tool would best help identify signal interference and dead zones?

Accepted Answer

Spectrum analyzer. A spectrum analyzer is the correct tool because it visualizes radio frequency (RF) energy across the 2.4 GHz and 5 GHz bands, allowing you to identify sources of interference (e.g., cordless phones, microwave ovens) and locate dead zones where signal strength drops below usable thresholds. Unlike other tools, it directly measures the RF environment rather than relying on logical-layer data.

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Cable tester

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Multimeter

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Protocol analyzer

Question 2

A user reports that they cannot access the internet, but they can access local resources on the same subnet. The network administrator pings the default gateway and gets a response. Which tool should be used next to trace the path to an external website?

Accepted Answer

traceroute. B is correct because traceroute (tracert on Windows) is the appropriate tool to identify where packets are being dropped or delayed along the path from the local host to an external website. Since the user can access local resources and the default gateway responds to pings, the issue likely lies beyond the gateway, and traceroute will reveal the hop where connectivity fails.

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netstat

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nslookup

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arp

Question 3

After replacing a faulty switch, several users in the same VLAN report that they cannot communicate with the server that is on a different subnet. The switch is connected to the router via a trunk port. Which command should the administrator run on the router to verify that the VLAN is allowed on the trunk?

Accepted Answer

show interfaces trunk. The `show interfaces trunk` command displays which VLANs are allowed on each trunk port. Since the switch is connected to the router via a trunk, this command verifies whether the VLAN of the affected users is permitted on that trunk. If the VLAN is missing from the allowed list, traffic to the server on a different subnet will be dropped at the router interface.

Answer

show vlan

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show mac address-table

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show ip route

Question 4

A user reports that they cannot access an internal web server at http://intranet.company.local but can access other internet websites. The technician runs ping intranet.company.local and receives replies successfully. Which tool should the technician use next to isolate the issue?

Accepted Answer

nslookup. The technician can reach the server by IP (ping succeeds) but not by hostname (http://intranet.company.local), which points to a name resolution failure. nslookup is the correct next step because it queries DNS to verify whether the hostname resolves to the correct IP address, isolating the issue to DNS rather than connectivity or server availability.

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tracert

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netstat

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ipconfig

Question 5

A user calls the help desk stating that they cannot access any network resources. The technician asks the user to run ipconfig and the output shows an IP address of 169.254.15.20 with a subnet mask of 255.255.0.0. Which of the following is the most likely cause?

Accepted Answer

The DHCP server is unreachable. The IP address 169.254.15.20 with a subnet mask of 255.255.0.0 is an Automatic Private IP Addressing (APIPA) address, which Windows assigns when a DHCP client fails to receive a lease from a DHCP server. Since the user cannot access any network resources, the most likely cause is that the DHCP server is unreachable, preventing the client from obtaining a valid IP address, default gateway, and DNS server settings.

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The DNS server is not responding

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The default gateway is misconfigured

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There is a duplicate IP address on the network

Question 6

A network technician is troubleshooting intermittent internet access for a single user. The user’s workstation can ping the default gateway consistently, but web pages fail to load intermittently. Which of the following should the technician check NEXT?

Accepted Answer

A) DNS server configuration. The user can ping the default gateway consistently, indicating Layer 3 connectivity to the local network is intact. However, intermittent web page failures suggest a name resolution issue, as DNS translates domain names to IP addresses. If the DNS server is misconfigured, unreachable, or returning stale records, the browser will fail to load pages even though basic IP connectivity works. Checking DNS server configuration is the logical next step because it directly addresses the symptom of name resolution failures.

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B) DHCP lease time

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C) Switch port speed and duplex settings

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D) Firewall rules blocking ICMP

Question 7

A user at a branch office reports that they cannot connect to a file server at the main office. The branch office connects to the main office via a site-to-site VPN, and the VPN tunnel shows as active. The user can ping the main office's default gateway but not the file server. Which of the following should the technician check NEXT?

Accepted Answer

A) Check the routing tables on the branch router. The VPN tunnel is active and the user can ping the main office's default gateway, which confirms that Layer 3 connectivity exists across the tunnel. However, the file server is unreachable, indicating that the traffic destined for the file server's subnet is not being forwarded correctly. The next logical step is to check the routing tables on the branch router to ensure there is a specific route (or a default route) pointing to the VPN tunnel interface for the file server's network. Without this route, the branch router may be dropping the packets or sending them out the wrong interface.

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B) Check the DNS resolution for the file server

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C) Check the VPN encryption settings

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D) Check the file server's antivirus software

Question 8

A network administrator is troubleshooting communication between two switches connected via a trunk port. The trunk link is up/up, but devices in VLAN 20 cannot communicate across the trunk. The administrator has verified that both switches have VLAN 20 created and that the access ports are configured correctly. Which command should the administrator run on each switch to verify the trunk's allowed VLAN list?

Accepted Answer

show interfaces trunk. The 'show interfaces trunk' command displays the trunking status, including the allowed VLAN list on the trunk port. Since the trunk link is up/up but VLAN 20 traffic fails, the most likely cause is that VLAN 20 is not included in the allowed VLAN list on one or both switches. This command directly shows which VLANs are permitted, pruned, or active on the trunk.

Answer

show vlan brief

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show running-config interface

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show mac address-table

Question 9

A technician is troubleshooting an issue where a wireless client can associate with an access point but cannot obtain an IP address via DHCP. The technician checks the DHCP server and sees no lease requests from the client's MAC address. Which of the following is the most likely cause?

Accepted Answer

The AP is not configured with a DHCP relay. The client can associate with the AP but cannot obtain an IP address, and the DHCP server shows no lease requests from the client's MAC. This indicates that DHCP discovery broadcasts are not reaching the DHCP server, which is common when the client and server are on different subnets and the AP (or a Layer 3 device) is not configured with a DHCP relay (ip helper-address). Without a relay, broadcast DHCP messages are dropped at the router, so the server never sees the request.

Answer

The client's wireless adapter is faulty

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The client's SSID is incorrect

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The AP's radio is operating on the wrong channel

Question 10

After replacing a faulty network cable, a user reports that they can access local resources but not the internet. The technician verifies that the user's IP address is 192.168.1.100 with a subnet mask of 255.255.255.0 and a default gateway of 192.168.1.1. The technician can ping the default gateway successfully. Which of the following should the technician check NEXT?

Accepted Answer

A) DNS configuration. The user can access local resources and successfully ping the default gateway, which confirms that Layer 2 and Layer 3 connectivity to the local network is working. The inability to access the internet while local access works points to a name resolution failure, as the browser relies on DNS to translate domain names to IP addresses. Checking DNS configuration is the logical next step because a misconfigured or missing DNS server would prevent internet access even when IP connectivity is intact.

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B) DHCP server

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C) Router's firewall ACLs

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D) Switch port VLAN assignment

Question 11

A user reports that they can access a website by its IP address but not by its domain name. Which of the following is most likely the issue?

Accepted Answer

A) DNS resolution failure. Accessing a website by IP address but not by domain name indicates that the client can reach the web server over the network, but the name-to-IP translation process is failing. This is a classic symptom of a DNS resolution failure, where the client cannot query or receive a valid A or AAAA record for the domain from its configured DNS server.

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B) Incorrect default gateway

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C) Duplicate IP address

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D) Corrupted TCP/IP stack

Question 12

A user reports that they can access internal resources such as file shares and printers by name, but they cannot access any external websites. The technician checks the IP configuration and finds the workstation has a valid IP address, subnet mask, default gateway, and DNS server addresses. The technician can successfully ping the default gateway and an external IP address like 8.8.8.8. Which of the following should the technician check NEXT?

Accepted Answer

Verify that the DNS server can resolve external domain names.. The user can access internal resources by name and can ping an external IP address (8.8.8.8), which confirms that IP routing, the default gateway, and basic network connectivity are working. The inability to access external websites by name, despite having DNS server addresses configured, points directly to a DNS resolution failure for external domains. Therefore, the next logical step is to verify that the configured DNS server can resolve external domain names, such as by using `nslookup` or `dig` to query a public domain like google.com.

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Perform a traceroute to the external website to identify the point of failure.

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Check the Windows Firewall on the workstation to ensure it is not blocking outbound HTTP traffic.

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Renew the DHCP lease on the workstation.

Question 13

A user reports that they can browse to a website by typing its IP address (e.g., 93.184.216.34) but cannot access it by typing the domain name (e.g., www.example.com). The user's workstation receives IP configuration via DHCP. Which of the following is the most likely cause?

Accepted Answer

The DNS server address is incorrect or unreachable.. The user can reach the website by IP address but not by domain name, which isolates the issue to name resolution. DNS translates domain names to IP addresses; if the DNS server address provided by DHCP is incorrect or unreachable, the workstation cannot resolve www.example.com to 93.184.216.34. This is the most likely cause because all other connectivity (default gateway, web server) is confirmed working by the successful IP-based access.

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The default gateway is misconfigured.

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The web server's SSL certificate is expired.

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The workstation's hosts file has an incorrect entry.

Question 14

A user reports that they cannot access any network resources. The technician checks the IP configuration on the workstation and sees an IP address of 169.254.10.55 with a subnet mask of 255.255.0.0. Which of the following should the technician check NEXT?

Accepted Answer

Verify that the DHCP server is available and reachable. The IP address 169.254.10.55 with a subnet mask of 255.255.0.0 is an Automatic Private IP Addressing (APIPA) address, which Windows assigns when a DHCP server is unreachable. The next logical step is to verify that the DHCP server is available and reachable, as this directly addresses the root cause of the failed DHCP lease acquisition.

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Configure a static IP address on the workstation

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Check the DNS server configuration on the workstation

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Replace the network cable

Question 15

Users in a department report that the network is extremely slow. A technician checks the access switch and notices that a single port shows a high number of CRC errors and runts. The link LED is solid green. Which of the following is the most likely cause of the issue?

Accepted Answer

Duplex mismatch between the switch port and the connected device. A duplex mismatch occurs when one device operates at full duplex and the other at half duplex. The full-duplex side does not perform Carrier Sense Multiple Access with Collision Detection (CSMA/CD), so it transmits without checking for collisions. The half-duplex side detects collisions, causing late collisions that manifest as CRC errors and runts on the switch port. The solid green link LED indicates Layer 1 connectivity is intact, ruling out a physical cable fault.

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Faulty network cable

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Broadcast storm

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VLAN misconfiguration

Question 16

A user reports that they cannot connect to a file server on the same subnet. The technician checks the IP configuration and sees an IP address of 169.254.5.10. What is the most likely cause?

Accepted Answer

The DHCP server is unreachable. The IP address 169.254.5.10 is an Automatic Private IP Addressing (APIPA) address from the 169.254.0.0/16 range, which Windows assigns when a DHCP client fails to obtain a lease from a DHCP server. Since the user is on the same subnet as the file server, a missing default gateway or DNS server would not prevent local connectivity, but the inability to reach the DHCP server indicates a broader network issue that also prevents the client from obtaining a valid IP address.

Answer

The default gateway is misconfigured

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The DNS server is down

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The file server is offline

Question 17

A user reports intermittent connectivity issues. The technician notices that the link lights on both the PC and the switch are solid, but the user experiences periodic drops. The technician runs a cable test and finds that one pair of wires is open. Which standard is the cable likely violating?

Accepted Answer

TIA/EIA-568. The TIA/EIA-568 standard specifies the wiring pinouts and performance requirements for twisted-pair cabling, including that all four pairs must be properly terminated and continuous. An open pair violates this standard because it breaks the required electrical continuity, leading to signal degradation and intermittent connectivity. The solid link lights indicate basic electrical connectivity on some pairs, but the open pair causes periodic drops when the network attempts to use that pair for data transmission.

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IEEE 802.3

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ISO 9001

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RFC 1918

Question 18

A technician is troubleshooting a user's inability to reach a specific website. The user can reach other websites without issue. The technician runs nslookup on the user's workstation and receives the correct IP address for the website. However, a ping to that IP address fails. Which of the following is the most likely cause?

Accepted Answer

The website server is blocking ICMP requests.. Since nslookup returns the correct IP address, DNS resolution is working. The ping fails despite a valid IP, which indicates that the destination server is reachable at the network layer but is not responding to ICMP echo requests. Many web servers are configured to block ICMP to reduce attack surface or prevent reconnaissance, which is a common security practice.

Answer

The default gateway is misconfigured.

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The DNS server is not resolving the domain name.

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The subnet mask is incorrect.

Question 19

A technician is troubleshooting a loss of network connectivity for a single workstation. The workstation has a valid IP address but cannot ping its default gateway. The link lights on both the workstation and the switch are solid. Which of the following should the technician check NEXT?

Accepted Answer

The switch port configuration, such as VLAN assignment. The workstation has a valid IP address and solid link lights, indicating Layer 1 (physical) and Layer 3 (IP configuration) are functional. The inability to ping the default gateway points to a Layer 2 issue, such as the switch port being in the wrong VLAN or having a misconfigured access/trunk setting. Checking the switch port configuration is the logical next step because VLAN mismatches prevent frames from reaching the gateway's subnet.

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The cable integrity with a tester

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The DNS server settings

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The workstation's ARP cache

Question 20

A user's workstation shows an IP address of 169.254.15.7 with a subnet mask of 255.255.0.0. The user cannot access any network resources. Which of the following is the most likely cause?

Accepted Answer

The DHCP server is unreachable. The IP address 169.254.15.7 with a /16 subnet mask is an Automatic Private IP Addressing (APIPA) address, assigned by Windows when a DHCP client fails to obtain a lease. This indicates the workstation cannot reach a DHCP server, so the most likely cause is that the DHCP server is unreachable.

Answer

The DNS server is not responding

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The subnet mask is incorrect

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The default gateway is down