Network Troubleshooting Guide

This guide provides a systematic approach to diagnosing and resolving common network issues. We'll cover a range of problems, from basic connectivity to complex performance bottlenecks.

Common Symptoms and Initial Checks

Before diving deep, it's essential to identify the symptoms and perform quick, basic checks.

1. No Connectivity

• Check Physical Connections: Ensure all cables (Ethernet, Wi-Fi) are securely plugged in. Look for indicator lights on network interfaces and routers.
• Verify IP Address: Confirm your device has a valid IP address, subnet mask, and default gateway. Use commands like ipconfig (Windows) or ifconfig/ip addr (Linux/macOS).
• Ping Gateway and External Address: Try pinging your default gateway first. If that works, try pinging a well-known external address like 8.8.8.8 (Google DNS).

2. Slow Network Performance

• Bandwidth Test: Use online speed test tools or tools like iperf3 to measure actual download and upload speeds.
• Check for Bandwidth Hogs: Identify which applications or devices are consuming the most bandwidth. Task Manager (Windows) or Activity Monitor (macOS) can help.
• Router/Switch Health: Check logs on your network devices for errors or excessive traffic. Consider rebooting your router or modem.

3. Intermittent Connectivity

• Check for Interference: If using Wi-Fi, ensure there's no significant radio frequency interference from other devices.
• Monitor Network Load: High traffic volumes or poorly configured network devices can cause intermittent drops.
• Hardware Issues: Faulty network cables, network interface cards (NICs), or failing switches can lead to instability.

Diagnostic Tools and Techniques

Leverage these powerful tools to pinpoint the root cause of your network problems.

1. Ping

The ping command is fundamental for checking reachability and latency between two hosts.

ping google.com
ping 192.168.1.1

2. Traceroute (or Tracert)

traceroute (Linux/macOS) or tracert (Windows) maps the path packets take to a destination, showing each hop and its latency. This is invaluable for identifying where a connection is failing or slowing down.

traceroute google.com
tracert 8.8.8.8

3. Nslookup (or Dig)

nslookup (Windows/Linux/macOS) or dig (Linux/macOS) queries Domain Name System (DNS) servers to resolve hostnames to IP addresses. Problems here can prevent you from reaching websites by name.

nslookup microsoft.com
dig google.com

4. Netstat

netstat displays network connections, routing tables, interface statistics, and more. It helps identify which ports are open and which processes are listening.

netstat -an
netstat -p tcp -a -n

5. Wireshark

Wireshark is a powerful network protocol analyzer. It captures and displays network traffic in detail, allowing for deep packet inspection. This is typically for advanced troubleshooting.

Troubleshooting Specific Scenarios

DNS Resolution Issues

If you can ping IP addresses but not hostnames, it's likely a DNS problem.

• Flush your DNS cache:
  • Windows: ipconfig /flushdns
  • macOS: sudo dscacheutil -flushcache; sudo killall -HUP mDNSResponder
  • Linux: Varies by distribution; often involves restarting nscd or systemd-resolved.
• Check your DNS server settings. Try temporarily using public DNS servers like Google's (8.8.8.8, 8.8.4.4) or Cloudflare's (1.1.1.1, 1.0.0.1).
• Use nslookup or dig to query your configured DNS servers directly.

Firewall Blocking

Firewalls (both on your device and network) can block legitimate traffic.

• Temporarily disable your software firewall (e.g., Windows Defender Firewall) to see if the issue is resolved. Remember to re-enable it afterward.
• Check network firewall logs for any blocked connections.
• Ensure necessary ports are open for the application or service you are using.

Network Adapter Issues

Problems with your network adapter can manifest in various ways.

• Update your network adapter drivers.
• Disable and re-enable the network adapter in your system's network settings.
• Try using a different network cable or Wi-Fi connection.

Advanced Network Configuration

For more complex environments, understanding routing, subnets, and protocols is crucial.

• Subnetting: Ensure devices are on the correct subnets and that routing between subnets is configured correctly.
• Gateway Configuration: Verify that default gateways are correctly set on all network devices.
• VLANs: If using Virtual LANs, ensure ports are assigned to the correct VLANs and that inter-VLAN routing is functioning.