Cheatsheet: Network Troubleshooting

1. Network Troubleshooting Methodology

1.1 Seven-Step Troubleshooting Process

2. Common Network Issues and Symptoms

2.1 Connectivity Problems

2.2 Performance Issues

• Slow network speeds: Bandwidth saturation, QoS misconfiguration, duplex mismatch, cable degradation
• High latency: Network congestion, routing issues, excessive hops, device processing delays
• Jitter: Inconsistent packet delivery timing affecting VoIP/video
• Packet loss: Network congestion, faulty hardware, cable issues, buffer overflow

3. Network Hardware Troubleshooting

3.1 Cable Issues

3.2 Network Interface Card (NIC)

• No link light: Check cable, driver, NIC hardware, switch port
• Amber/orange link light: Connection issue, duplex mismatch, speed mismatch
• Green link light: Good connection established
• Driver issues: Update, rollback, or reinstall NIC drivers
• Speed/duplex mismatch: Configure both ends to match (auto-negotiate or manual)

3.3 Switch and Router Issues

• Port failure: Test with different port, check for disabled ports
• VLAN misconfiguration: Verify port VLAN assignments match network topology
• Loop detection: Spanning Tree Protocol blocking ports; check for redundant connections
• Router configuration: Verify routing tables, default gateway settings, NAT configuration

3.4 Wireless Access Point (WAP) Issues

• Weak signal: Distance from AP, physical obstructions, interference, antenna issues
• Channel interference: Overlapping channels (use 1, 6, 11 for 2.4GHz)
• Incorrect SSID/security settings: Verify WPA2/WPA3 configuration, pre-shared key
• Limited range: Adjust power settings, add access points, use repeaters/extenders

4. TCP/IP Configuration Issues

4.1 IP Address Problems

4.2 DNS Issues

• Cannot resolve names: Ping IP works but hostname fails; check DNS server addresses
• Incorrect DNS server: Verify primary and secondary DNS settings
• DNS cache corruption: Clear DNS cache with ipconfig /flushdns
• DNS server failure: Test alternate DNS servers (8.8.8.8, 1.1.1.1)

4.3 DHCP Issues

• DHCP server unavailable: Verify DHCP service running, check network connectivity
• Scope exhaustion: All addresses assigned; expand DHCP scope or reduce lease time
• Rogue DHCP server: Unauthorized DHCP providing incorrect settings; locate and disable
• Lease renewal failure: Release and renew: ipconfig /release then ipconfig /renew

5. Command-Line Troubleshooting Tools

5.1 Windows Commands

5.2 Ping Testing Sequence

1. Ping 127.0.0.1 (loopback) - tests TCP/IP stack
2. Ping local IP address - tests NIC
3. Ping default gateway - tests local network connectivity
4. Ping remote IP address - tests internet connectivity
5. Ping remote hostname - tests DNS resolution

5.3 Linux/macOS Commands

• ifconfig/ip: Display network configuration (Linux)
• dig: DNS lookup utility, more detailed than nslookup
• traceroute: Equivalent to Windows tracert
• netstat: Similar function to Windows version

6. Physical Layer Troubleshooting

6.1 Link Lights and Indicators

6.2 Cable Testing Tools

• Cable tester: Tests continuity and correct pinout; identifies open/short circuits
• Tone generator and probe: Locate specific cable among many; trace cable runs
• Loopback plug: Tests NIC or port functionality by creating local loop
• Multimeter: Test voltage, resistance, continuity
• Cable certifier: Advanced testing for compliance with Cat5e/6/6a standards
• Time-domain reflectometer (TDR): Locate cable breaks and measure distance to fault

6.3 Environmental Factors

• Temperature: Overheating causes performance degradation and failures
• Humidity: Excess moisture causes corrosion and shorts
• EMI sources: Motors, generators, fluorescent lights, microwave ovens
• RFI sources: Radio transmitters, wireless devices on same frequency
• Physical damage: Crimped cables, broken connectors, worn jacks

7. Firewall and Security Issues

7.1 Firewall Troubleshooting

• Blocked ports: Application cannot communicate; verify firewall rules allow required ports
• Firewall disabled: Security risk; enable Windows Defender Firewall or third-party firewall
• Exception rules: Add programs or ports to allowed list
• Profile settings: Different rules for domain, private, and public networks
• Testing: Temporarily disable firewall to isolate issue (re-enable immediately)

7.2 Antivirus/Security Software

• Network scanning interference: AV software blocking network traffic; configure exceptions
• VPN conflicts: Security software blocking VPN connections; adjust settings
• Performance impact: Real-time scanning consuming bandwidth; schedule scans during off-hours

7.3 Common Port Numbers

8. Wireless Network Troubleshooting

8.1 Wireless Issues

8.2 Wireless Standards and Speeds

8.3 Channel Selection

• 2.4GHz non-overlapping channels: 1, 6, 11 (use these to avoid interference)
• 5GHz channels: More available, less interference, shorter range
• Channel width: 20MHz (longer range), 40MHz (faster speed), 80/160MHz (fastest, 5GHz only)
• Auto channel selection: AP selects best channel; may need manual override

8.4 Wireless Security

• WEP: Deprecated, insecure, do not use
• WPA: Minimum acceptable security
• WPA2: Current standard, uses AES encryption
• WPA3: Latest standard, enhanced security
• MAC filtering: Allow/deny specific devices by MAC address
• SSID broadcast: Disable to hide network (security through obscurity)

9. Internet Connectivity Issues

9.1 ISP and Modem Problems

• No internet access: Check modem lights, power cycle modem and router, verify ISP service status
• Modem light indicators: Power (solid), Receive/Send (solid), Online (solid), Activity (blinking)
• ISP outage: Contact provider, check service status online from mobile device
• Bandwidth throttling: ISP limiting speeds; check service plan, contact provider
• Modem provisioning: ISP must activate modem using MAC address

9.2 Router Configuration

• WAN IP not assigned: Check modem connection, verify ISP settings (DHCP/static/PPPoE)
• NAT issues: Port forwarding misconfigured; verify NAT rules
• Firmware update: Outdated firmware causing issues; update from manufacturer website
• Factory reset: Restore default settings if misconfigured (hold reset button 10-30 seconds)
• DHCP server conflict: Multiple DHCP servers on network; disable one

9.3 Proxy and VPN Issues

• Incorrect proxy settings: Cannot access websites; verify proxy configuration or disable
• VPN connection failure: Check credentials, server address, firewall rules, ports
• Split tunneling: VPN routing only specific traffic; configure as needed
• VPN performance: Encryption overhead reduces speeds; expected behavior

10. Network Service Issues

10.1 DHCP Troubleshooting

• Verify DHCP service running: Check services.msc (Windows Server)
• Check DHCP scope: Sufficient addresses available, proper range configured
• Lease time: Too long causes scope exhaustion; too short increases overhead
• DHCP relay agent: Required for DHCP across subnets/VLANs
• Reservation conflicts: Static IP within DHCP scope; exclude or use reservation

10.2 DNS Troubleshooting

• Forward lookup failure: Cannot resolve hostname to IP; check DNS server, zone files
• Reverse lookup failure: Cannot resolve IP to hostname; check PTR records
• Zone transfer issues: Secondary DNS not updating; verify transfer settings
• Recursive query failure: DNS cannot query other servers; check forwarders
• Public DNS alternatives: Google (8.8.8.8, 8.8.4.4), Cloudflare (1.1.1.1), OpenDNS (208.67.222.222)

10.3 File and Print Sharing

• Cannot access shared folder: Check permissions, network discovery enabled, file sharing on
• Network path not found: Verify hostname/IP, firewall rules, sharing enabled
• Printer not found: Check printer sharing, driver installation, network connectivity
• SMB protocol versions: SMBv1 disabled by default (security); use SMBv2/v3

11. Advanced Troubleshooting Scenarios

11.1 Intermittent Connectivity

• Cable issues: Test cables, check connections, replace suspect cables
• Hardware overheating: Check temperature, clean dust, improve ventilation
• Switch port flapping: Port repeatedly going up/down; check spanning tree, cables, NIC
• Wireless roaming: Client switching between APs; adjust power levels, channel assignment
• ISP instability: Monitor connection over time, document outages, contact provider

11.2 VLAN Troubleshooting

• Cannot communicate between VLANs: Routing required; verify Layer 3 switch or router configuration
• Port VLAN assignment: Device on wrong VLAN; check port configuration
• Trunk port issues: VLAN traffic not passing; verify trunk configuration and allowed VLANs
• Native VLAN mismatch: Trunk ports must have same native VLAN

11.3 Performance Degradation

• Bandwidth saturation: Monitor utilization, identify bandwidth hogs, implement QoS
• Broadcast storms: Excessive broadcast traffic; check for loops, misconfigured devices
• Duplex mismatch: One side auto, other side fixed; results in collisions and slow speeds
• Network loops: Redundant connections without spanning tree; disable redundant links or enable STP
• MTU issues: Fragmentation causing performance issues; adjust MTU size (1500 bytes standard)

11.4 Email Connectivity Issues

• Cannot send email: Check SMTP settings, port 25/587, authentication, firewall
• Cannot receive email: Verify POP3 (port 110) or IMAP (port 143) settings, SSL/TLS
• Email stuck in outbox: Large attachment, server timeout, authentication failure
• Secure connections: POP3S (995), IMAPS (993), SMTP over TLS (587)

12. Documentation and Best Practices

12.1 Documentation Requirements

• Problem description: Initial symptoms, user reports, error messages
• Troubleshooting steps: Tests performed, results obtained, theories tested
• Solution implemented: Specific actions taken, configuration changes made
• Verification: Tests confirming resolution, user confirmation
• Follow-up actions: Preventive measures, recommendations, escalations

12.2 Network Documentation

• Network topology diagrams: Physical and logical layouts
• IP address scheme: Subnets, VLAN assignments, static IP reservations
• Equipment inventory: Make/model, serial numbers, warranty information
• Configuration backups: Router, switch, firewall configurations with version dates
• Cable management: Cable runs, patch panel documentation, labeling scheme

12.3 Best Practices

• Baseline performance: Document normal network behavior for comparison
• Change management: Document all configuration changes before implementation
• Backup configurations: Before making changes, save current settings
• Test in isolation: Change one variable at a time when troubleshooting
• Escalation procedures: Know when to escalate and who to contact
• Knowledge base: Maintain repository of common issues and solutions