Network Troubleshooting
Key Takeaways
- The "bottom-up" approach (Physical → Data Link → Network → Transport → Application) is the most systematic way to troubleshoot network issues.
- Common command-line diagnostic tools: ping (test connectivity), tracert/traceroute (trace the path), ipconfig/ifconfig (check IP configuration), nslookup (test DNS), netstat (view connections), pathping (combined ping+tracert).
- If a user can access websites by IP address but not by domain name, the problem is DNS resolution — check DNS server settings or try flushing the DNS cache.
- Intermittent connectivity often points to physical layer issues: damaged cables, loose connections, interference (wireless), or failing network hardware.
- When troubleshooting wireless issues, check signal strength, channel overlap/interference, SSID visibility, correct security credentials, and whether airplane mode is enabled.
Last updated: March 2026
Network Troubleshooting
Network Diagnostic Commands
ping
Tests basic connectivity to a host:
ping 8.8.8.8— Tests connectivity to Google DNS (tests internet connectivity)ping 127.0.0.1— Tests local TCP/IP stack (loopback)ping gateway-ip— Tests connectivity to default gateway- Request timed out — Host is unreachable or blocking ICMP
- Destination host unreachable — No route to host, local network issue
tracert (Windows) / traceroute (Linux/macOS)
Shows the path packets take to reach a destination:
- Each hop shows a router along the path with round-trip time
- Helps identify where in the network path a problem occurs
- Timeouts (* * *) at a specific hop indicate that router is the bottleneck or blocking ICMP
ipconfig (Windows) / ifconfig (Linux/macOS)
Displays and manages IP configuration:
ipconfig— Shows IP address, subnet mask, default gatewayipconfig /all— Shows MAC address, DHCP server, DNS servers, lease infoipconfig /releasethenipconfig /renew— Gets a new DHCP addressipconfig /flushdns— Clears cached DNS entries
nslookup / dig
Tests DNS name resolution:
nslookup example.com— Resolves domain to IP addressnslookup example.com 8.8.8.8— Tests resolution using a specific DNS server- If nslookup fails but ping by IP works, the issue is DNS
netstat
Shows active network connections and listening ports:
netstat -an— Shows all connections with numerical addressesnetstat -b(Windows) — Shows which process owns each connection- Useful for identifying malware connections or confirming service is listening
pathping (Windows)
Combines ping and tracert:
- Shows route like tracert, then measures packet loss at each hop
- Takes longer but provides more detailed diagnosis
Common Network Problems and Solutions
No Connectivity
| Check | Action |
|---|---|
| Physical connection | Is the Ethernet cable plugged in? Is the link light on? |
| Wi-Fi | Is Wi-Fi enabled? Is airplane mode off? Correct SSID/password? |
| IP address | Run ipconfig — do you have a valid IP or 169.254.x.x (APIPA)? |
| Gateway | Can you ping the default gateway? |
| DNS | Can you ping 8.8.8.8 but not google.com? → DNS issue |
| Firewall | Is the firewall blocking the connection? |
Slow Network Performance
| Possible Cause | Solution |
|---|---|
| Network congestion | Check bandwidth usage, implement QoS |
| Duplex mismatch | Verify both ends are set to same speed/duplex (auto-negotiate) |
| Damaged cable | Replace cable, test with cable tester |
| Wi-Fi interference | Change channel, move AP, reduce distance |
| DNS issues | Switch to faster DNS (1.1.1.1 or 8.8.8.8) |
| Malware | Run antimalware scan, check netstat for suspicious connections |
Intermittent Connectivity
| Possible Cause | Solution |
|---|---|
| Loose cable | Reseat all connections, replace suspect cables |
| Failing NIC | Test with different NIC, check link light |
| Wi-Fi interference | Check for competing signals, microwave interference |
| IP conflict | Two devices with the same IP → use DHCP or reassign static IPs |
| Overheating switch/router | Check for proper ventilation, clean dust |
Wireless-Specific Troubleshooting
| Issue | Troubleshooting Steps |
|---|---|
| Can't see the network (SSID) | Check if SSID broadcast is enabled, verify frequency band (some devices only support 2.4 GHz), move closer to AP |
| Connected but no internet | Check if AP has internet connectivity, verify DNS settings, check for captive portal |
| Slow Wi-Fi | Check signal strength, change to less congested channel, move closer to AP, check for interference |
| Frequent disconnects | Update wireless driver, check for firmware updates on AP, reduce distance, check for interference |
| Wrong password error | Verify password, check for special characters, ensure correct security type (WPA2/WPA3) |
| Limited connectivity | Check DHCP server, verify IP configuration, ensure AP is connected to network |
Wi-Fi Interference Sources
- Microwaves (2.4 GHz band)
- Cordless phones (2.4 GHz band)
- Bluetooth devices (2.4 GHz band)
- Baby monitors
- Other Wi-Fi networks on overlapping channels
- Thick walls, metal surfaces, water (signal absorption)
- Fluorescent lighting (minor)
Test Your Knowledge
A user can access websites by IP address (e.g., http://142.250.80.46) but cannot access any websites by name (e.g., www.google.com). What is the MOST likely problem?
A
B
C
D
Test Your Knowledge
Which command traces the path that packets take through the network to reach a destination?
A
B
C
D
Test Your Knowledge
A user reports intermittent Wi-Fi disconnections that occur primarily around lunchtime. What is the MOST likely cause?
A
B
C
D
Test Your KnowledgeMulti-Select
Which of the following commands can be used to check a computer's current IP address configuration? (Select TWO)
Select all that apply
ipconfig
tracert
ifconfig
nslookup
pathping