Unit 9 · Network Troubleshooting
9.5Common faults and fixes
| Symptom | Likely cause | Fix |
|---|
| 169.254.x.x address | DHCP server unreachable | Check cable/Wi-Fi, DHCP service and scope; renew lease |
| Can ping IP but not name | DNS problem | Check client DNS settings, DNS server status, nslookup, flush cache |
| Can reach local hosts, not Internet | Wrong or missing gateway, ISP fault | Correct gateway, check router and WAN link |
| Intermittent connection | Bad cable, Wi-Fi interference, duplex mismatch | Replace cable, change channel, set speed/duplex |
| IP conflict message | Two devices with same IP | Use DHCP reservations, find the device with arp -a |
| Website down, server pings fine | Web service stopped or port blocked | systemctl status, check firewall and port 80/443 |
| Cannot SSH or RDP | Service off, firewall, wrong port or credentials | Check service, ss -tulpn, firewall rules, logs |
| Certificate warning | Expired or mismatched certificate, wrong date on client | Renew certificate, check name and system clock |
| Server slow | CPU, RAM or disk exhausted, too many connections | top, df -h, find the process, add resources |
| Server will not boot | Bad disk, corrupt update, wrong boot order | Boot recovery media, check disk health, restore from backup |
| Application crashes after update | Incompatible version, missing dependency | Read logs, roll back, reinstall dependency |
| Cannot send email | Wrong MX/SMTP settings, port 25 blocked, SPF failure | Check DNS records, mail logs, firewall |
Frequent faults and fast checks
| Fault | Clue | Fix direction |
|---|
| Bad/wrong cable or port | No link light | Replace cable, other port |
| Wrong VLAN | Link up, no DHCP, cannot ping gateway | Correct access VLAN |
| Duplicate IP | Intermittent reachability, ARP warnings | Find clash; reserve DHCP |
| Wrong mask/gateway | Local OK or not; Internet fails | Correct IP plan |
| DNS wrong | IPs ping, names fail | Fix DNS servers / records |
| DHCP down | APIPA addresses | Repair DHCP service/relay |
| Firewall blocking | Ping OK, app port closed | Adjust host/network rules |
| Expired VPN/cert | Only remote users fail | Renew cert; check VPN logs |
Remember
- Match clue → likely layer → single corrective action → retest.