Network Troubleshooting Guide

Overview

This guide provides a systematic approach to diagnosing and resolving common network connectivity issues in enterprise environments. Whether dealing with LAN, WAN, VPN, or wireless problems, following these procedures will help isolate and fix issues efficiently.

Prerequisites

• Basic understanding of TCP/IP networking
• Access to network diagnostic tools (ping, traceroute, ipconfig/ifconfig)
• Administrative credentials for network devices
• Physical access to relevant network equipment

Step 1: Initial Assessment

Before diving into technical troubleshooting, gather basic information about the issue:

1. Scope: Is the problem affecting a single user, multiple users, or the entire network?
2. Timing: When did the problem start? Is it intermittent or constant?
3. Recent Changes: Were there any recent changes to the network, hardware, or software?
4. Error Messages: Document any specific error messages or symptoms

Step 2: Physical Layer Verification

Check all physical connections before moving to software diagnostics:

Cable Connections

• Verify Ethernet cables are firmly connected
• Check for damaged or bent cables
• Ensure cables are plugged into correct ports
• Look for link lights on network adapters and switches

Hardware Status

• Confirm network adapters are enabled in Device Manager
• Check router/switch status lights
• Verify power to all network equipment
• Test with known-good cables if available

Step 3: Basic Connectivity Tests

Use command-line tools to test basic connectivity:

Windows Commands

# Check IP configuration
ipconfig /all

# Release and renew DHCP lease
ipconfig /release
ipconfig /renew

# Flush DNS cache
ipconfig /flushdns

# Test connectivity to gateway
ping 192.168.1.1

# Test connectivity to external server
ping 8.8.8.8

# Trace route to destination
tracert google.com

Linux/Mac Commands

# Check IP configuration
ifconfig
# or
ip addr show

# Test connectivity to gateway
ping -c 4 192.168.1.1

# Test connectivity to external server
ping -c 4 8.8.8.8

# Trace route to destination
traceroute google.com

Step 4: DNS Resolution Testing

Many connectivity issues are actually DNS problems in disguise:

# Test DNS resolution (Windows)
nslookup google.com

# Test DNS resolution (Linux/Mac)
dig google.com

# Test with alternative DNS server
nslookup google.com 8.8.8.8

Step 5: DHCP Troubleshooting

If the device isn't getting an IP address:

1. Check if the device has a valid IP address (not 169.254.x.x)
2. Verify DHCP service is running on the server/router
3. Check DHCP scope for available addresses
4. Ensure DHCP is not being blocked by firewall
5. Try assigning a static IP address temporarily to isolate the issue

Step 6: Wireless-Specific Issues

For wireless connectivity problems:

• Verify wireless adapter is enabled
• Check signal strength (should be above -70 dBm for good performance)
• Ensure correct SSID and password
• Test on 2.4GHz vs 5GHz bands
• Check for interference from other devices
• Verify AP is not at capacity (device limit reached)

Step 7: VPN Troubleshooting

For VPN connectivity issues:

• Verify VPN client is up-to-date
• Check user credentials and certificates
• Ensure VPN server is reachable (ping or telnet to VPN port)
• Verify firewall isn't blocking VPN protocols
• Check split tunneling configuration
• Review VPN logs for specific error messages

Step 8: Advanced Diagnostics

If basic troubleshooting doesn't resolve the issue:

Packet Analysis

• Use Wireshark to capture and analyze network traffic
• Look for dropped packets, retransmissions, or errors
• Verify proper TCP handshakes are completing

Port Testing

# Test if specific port is open (Windows)
Test-NetConnection -ComputerName server.com -Port 443

# Test if specific port is open (Linux)
telnet server.com 443
# or
nc -zv server.com 443

Documentation and Follow-up

Once the issue is resolved:

1. Document the problem, steps taken, and solution in your ticketing system
2. Update network diagrams if physical changes were made
3. Consider if similar issues might affect other users
4. Schedule follow-up with user to ensure stability

Common Issues Quick Reference

No Connectivity at All

• Check physical connections and cables
• Verify network adapter is enabled
• Check if device has valid IP address

Intermittent Connectivity

• Check for cable damage or loose connections
• Look for wireless interference
• Verify network equipment isn't overheating
• Check for IP address conflicts

Slow Performance

• Test bandwidth using speed test tools
• Check for network congestion
• Verify QoS settings if applicable
• Look for malware or bandwidth-heavy applications