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Network Troubleshooting

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CompTIA A+ 220-1201 & 220-1202

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1 Introduction & Exam Overview

About the CompTIA A+ Certification Exam Domains & Study Strategy

2 Core 1 Domain 1: Mobile Devices (13%)

Laptop Hardware & Components Laptop Features & Display Types Mobile Device Connectivity & Accessories Mobile Operating System Features Mobile Device Synchronization & Data Transfer

3 Core 1 Domain 2: Networking (23%)

TCP/IP Fundamentals & the OSI Model Common Ports & Protocols Wireless Networking Standards Network Hardware & Devices Network Configuration & Internet Connections Network Types & Topologies

4 Core 1 Domain 3: Hardware (25%)

Cables & Connectors RAM Types & Storage Devices Motherboards, CPUs & Power Supplies Printers & Multifunction Devices Peripheral Devices & Expansion Cards Network Cable Tools & Testing

5 Core 1 Domain 4: Virtualization & Cloud Computing (11%)

Virtualization Concepts Cloud Computing Models & Services Client-Side Virtualization in Practice

6 Core 1 Domain 5: Hardware & Network Troubleshooting (28%)

CompTIA Troubleshooting Methodology Hardware Troubleshooting Network Troubleshooting Printer Troubleshooting Display & Mobile Device Troubleshooting

7 Core 2 Domain 1: Operating Systems (28%)

Windows Editions & Features Windows Administrative Tools & Utilities Windows Command Line & PowerShell macOS & Linux Basics Windows Networking & File Sharing Windows Installation & Upgrade Methods

8 Core 2 Domain 2: Security (28%)

Physical & Logical Security Measures Malware Types & Threat Mitigation Wireless & Network Security Encryption & Data Protection

9 Core 2 Domain 3: Software Troubleshooting (23%)

Windows OS Troubleshooting Mobile OS & Application Troubleshooting Browser & Application Troubleshooting

10 Core 2 Domain 4: Operational Procedures (21%)

Documentation & Change Management Backup & Recovery Methods Safety & Environmental Procedures Scripting & Remote Access Technologies Privacy, Licensing & Organizational Policies

Network Troubleshooting

Key Takeaways Explain This Topic AI

A 169.254.x.x address (APIPA) means the client failed to reach a DHCP server — troubleshoot the link, switch port, and DHCP scope, not DNS.

If ping by IP works but name resolution fails, the fault is DNS — verify DNS server settings and run ipconfig /flushdns.

Core CLI tools: ping, tracert/traceroute, ipconfig/ifconfig (and ip addr), nslookup, netstat, pathping, and on the exam, the loopback test ping 127.0.0.1.

Intermittent connectivity usually starts at the physical layer: damaged cable, loose RJ45, EMI, or a failing NIC — verify the link light first.

On 2.4 GHz Wi-Fi use non-overlapping channels 1, 6, and 11; microwaves, cordless phones, and Bluetooth all interfere on that band.

Last updated: October 2026

  • Command-Line Diagnostics (Objective 5.5)
  • ping — basic reachability

ping 127.0.0.1 tests the local TCP/IP stack (loopback); failure means a corrupt stack or driver.

ping <gateway> confirms the local LAN path; ping 8.8.8.8 confirms Internet routing by IP.

Request timed out = no reply (host down or ICMP blocked); Destination host unreachable = no route, a local-network problem.

tracert / traceroute

Lists each hop (router) to a destination with round-trip times, so you can see where the path breaks. Repeated * * * at one hop flags that router as the failure point or an ICMP-blocking device. traceroute is the Linux/macOS spelling.

ipconfig / ifconfig / ip addr

ipconfig shows IP, subnet mask, default gateway.

ipconfig /all adds the MAC address, DHCP server, DNS servers, and lease times .

ipconfig /release then /renew requests a fresh DHCP lease; /flushdns clears the resolver cache.

Linux/macOS use ifconfig or the modern ip addr .

nslookup, netstat, pathping

nslookup example.com resolves a name; nslookup example.com 8.8.8.8 forces a specific DNS server to isolate DNS faults.

netstat -an lists connections/listening ports; netstat -b (Windows) names the owning process — useful for spotting malware beacons.

pathping combines route discovery with per-hop packet-loss statistics.

The APIPA / Gateway / DNS Decision Tree

  • Observation Diagnosis Action
  • IP is 169.254.x.x (APIPA) DHCP unreachable Check cable/link light, switch port, DHCP scope/relay
  • Valid IP, cannot ping gateway Local LAN/cabling fault Reseat cable, test port, check VLAN
  • Can ping gateway and 8.8.8.8 , but not by name DNS Fix DNS server, ipconfig /flushdns
  • Two devices drop offline together IP conflict Use DHCP or reassign static IPs
  • Loopback (127.0.0.1) fails Stack/driver corrupt Reinstall NIC driver, reset stack

This tree is the heart of Domain 5 networking questions: an APIPA address is never a DNS problem , and name-only failures are never cabling problems.

Wired Problem Patterns

  • Issue Likely Cause Fix
  • No link light Dead port, bad cable, NIC disabled Swap cable, try another port, enable NIC
  • Intermittent drops Loose RJ45, EMI, failing NIC Reseat/replace cable, route away from EMI
  • Slow throughput Duplex mismatch, congestion Set both ends to auto-negotiate; apply QoS
  • Works near, fails far Cable run > 100 m Add a switch/repeater within spec
  • Wireless-Specific Troubleshooting
  • Issue Steps
  • Can't see SSID Confirm SSID broadcast on; check band — many IoT devices are 2.4 GHz only
  • Connected, no Internet Verify AP uplink, DNS, and any captive portal
  • Slow Wi-Fi Check RSSI, move to a clear channel, reduce distance
  • Frequent disconnects Update Wi-Fi driver and AP firmware; cut interference
  • Wrong-password error Re-enter key; match security type (WPA2/WPA3)
  • 2.4 GHz Channel Planning

The 2.4 GHz band has 11 usable U.S. channels but only three non-overlapping ones: 1, 6, and 11 . Neighboring APs should be assigned to different ones of these three to avoid co-channel interference. The 5 GHz band offers many non-overlapping channels and far less congestion but shorter range. Common 2.4 GHz interferers: microwave ovens, cordless phones, Bluetooth, baby monitors, and overlapping neighbor Wi-Fi . Physical attenuators include thick walls, metal, and water.

A Structured Diagnostic Walkthrough

When a user reports "no Internet," resist random reboots and walk the path outward. First confirm the physical layer : is the Ethernet cable seated and the link light lit, or is Wi-Fi enabled and associated? Next read the IP configuration with ipconfig /all . A 169.254.x.x address means DHCP never answered — chase the cable, switch port, or DHCP scope. A valid private address (for example 192.168.1.50) means addressing is fine; move up the stack. Then ping the default gateway : success proves the local LAN works, failure points to cabling or the switch. Then ping 8.8.8.8 to test Internet routing by IP.

Finally resolve a name with nslookup or by pinging a hostname; if the IP ping succeeds but the name fails, you have proven DNS is the only broken layer and can fix the DNS server entry and run ipconfig /flushdns . This bottom-up sequence — physical, addressing, gateway, Internet, DNS — turns a vague complaint into a precise fault location and mirrors how Domain-5 scenario questions are written.

Reading the Symptoms Examiners Use

Intermittent drops on a wired PC almost always start at the physical layer : a kinked cable, a loose RJ45 boot, or a run exceeding the 100-meter copper Ethernet limit. Check the link light before software.

An IP conflict ("another device on the network has the same IP") knocks two hosts offline together; resolve it by moving both to DHCP or assigning unique static addresses.

Slow throughput with lots of errors suggests a duplex/speed mismatch — set both ends to auto-negotiate rather than forcing 100/full on one side only.

High latency that climbs at one specific hop in tracert / pathping localizes congestion or a failing router on the path; the hops before it are healthy.

Connected but no Internet on Wi-Fi can be a captive portal waiting for a login, an AP whose own uplink is down, or wrong DNS pushed by the AP.

Knowing which tool proves which layer is the real skill: ping 127.0.0.1 validates the local TCP/IP stack, ping <gateway> validates the LAN, ping 8.8.8.8 validates routing, and nslookup validates name resolution — each isolates one variable, exactly as the troubleshooting methodology demands.