Causes of common problems - Learn Tech From Zero

4.5 Given a scenario, troubleshoot network connectivity issues.

📘CompTIA Server+ (SK0-005)

When a server or device cannot communicate on a network, there can be many causes. Troubleshooting means finding the root cause and fixing it. The exam expects you to know common causes and how to resolve them.

Here’s a breakdown of each cause:

1. Improper IP Configuration

Every device on a network must have a valid IP address, subnet mask, and gateway to communicate.

Symptoms:

Cannot reach other devices
“No network access” or limited connectivity

Common mistakes:

Two devices have the same IP → conflict
Incorrect subnet mask → devices can’t communicate even if they are “on the same network”
Wrong default gateway → device cannot reach other networks, including the Internet

Troubleshooting steps:

Check IP settings with ipconfig (Windows) or ifconfig / ip addr (Linux)
Ensure the IP, subnet, and gateway match the network plan
Use ping to test connectivity

2. IPv4 vs. IPv6 Misconfigurations

Networks can use IPv4 (e.g., 192.168.1.10) or IPv6 (e.g., 2001:db8::1). Mixing them incorrectly causes communication issues.

Symptoms:

Device shows “Connected” but cannot access other devices
Services only reachable via IPv4 or IPv6

Common mistakes:

Device only has IPv6 but network uses IPv4
Static IPv6 configured incorrectly
Dual-stack issues (both IPv4 and IPv6 exist but routing is wrong)

Troubleshooting steps:

Confirm which protocol the network uses
Check IP version configuration
Test with ping -4 or ping -6 to force IPv4 or IPv6

3. Improper VLAN Configuration

VLANs (Virtual LANs) segment networks for security and efficiency. A misconfigured VLAN prevents devices from communicating.

Symptoms:

Cannot reach devices on another VLAN
No access to network resources

Common mistakes:

Device assigned to wrong VLAN
Switch port misconfigured (tagged vs untagged)
Trunk port misconfiguration

Troubleshooting steps:

Check switch port VLAN assignment
Ensure correct VLAN tagging for servers and access ports
Use show vlan or switch management interface to verify

4. Network Port Security

Some switches use port security to restrict which devices can connect. If a device is blocked, it can’t access the network.

Symptoms:

Device loses connectivity after connecting
Port shows “error-disabled”

Common mistakes:

MAC address not allowed on port
Too many devices connected to a secure port

Troubleshooting steps:

Check switch port security settings
Add MAC address if needed or adjust port policy
Reset the port if “error-disabled”

5. Component Failure

Network connectivity depends on hardware like NICs, switches, routers, and cables.

Symptoms:

Intermittent connectivity
No connectivity at all

Common failures:

NIC malfunction
Switch or router failure
Power issues or overheating

Troubleshooting steps:

Replace suspected hardware
Test device on another port
Check logs on switches/routers

6. Incorrect OS Route Tables

Operating systems use routing tables to determine where to send network traffic. Wrong routes prevent devices from reaching other subnets or the Internet.

Symptoms:

Cannot reach devices outside local network
Specific destinations unreachable

Common mistakes:

Missing default route
Manual static routes misconfigured

Troubleshooting steps:

Check routes: route print (Windows), ip route (Linux)
Add correct default gateway or static routes

7. Bad Cables

Sometimes connectivity issues are just physical, not software.

Symptoms:

Intermittent connectivity
Link light off or flickering on NIC or switch port

Troubleshooting steps:

Replace the cable
Test with a known working cable
Ensure correct cable type: straight-through vs crossover

8. Firewall Misconfiguration, Hardware, or Software Failure

Firewalls control which traffic is allowed or blocked. Misconfiguration or failure can block legitimate traffic.

Symptoms:

Cannot access services or servers
Some devices can connect, others cannot

Common mistakes:

Firewall rules blocking IPs or ports
Firewall service stopped or failed
Hardware firewall malfunction

Troubleshooting steps:

Review firewall rules
Test connectivity with firewall temporarily disabled
Check firewall logs for blocked traffic

9. Misconfigured NIC (Network Interface Card)

NIC settings can affect connectivity.

Symptoms:

Device can’t connect
Speed or duplex mismatch

Common mistakes:

Wrong speed/duplex settings (e.g., 1 Gbps vs 100 Mbps)
NIC disabled in OS

Troubleshooting steps:

Check NIC status in OS
Match speed/duplex with switch port
Update drivers if needed

10. DNS and/or DHCP Failure

DHCP gives devices IP addresses, DNS resolves domain names to IP addresses.

Symptoms:

Cannot reach Internet by hostname
IP address not assigned
Intermittent network access

Common mistakes:

DHCP server down or misconfigured
DNS server unreachable or incorrect
Static IP conflicts with DHCP

Troubleshooting steps:

Test DHCP: release/renew IP (ipconfig /release and ipconfig /renew)
Test DNS: nslookup or ping using hostname
Check server logs

11. DHCP Server Misconfigured

If DHCP settings are wrong, devices get wrong or no IP addresses.

Symptoms:

APIPA addresses (169.254.x.x)
Cannot connect to network

Common mistakes:

Wrong IP range or scope
No available addresses in DHCP pool
Incorrect default gateway or DNS in DHCP options

Troubleshooting steps:

Check DHCP scope
Verify lease availability
Ensure DHCP server is authorized and running

12. Misconfigured Hosts File

The hosts file is a local way to map domain names to IP addresses. If incorrect, it can override DNS.

Symptoms:

Cannot reach certain websites or servers
Other devices can reach the destination, but this one cannot

Common mistakes:

Wrong IP for a server in hosts file
Outdated entry

Troubleshooting steps:

Open hosts file (C:\Windows\System32\drivers\etc\hosts or /etc/hosts)
Remove or correct incorrect entries
Test connectivity after changes

✅ Key Exam Tips

Know how to identify symptoms vs. cause.
Always check physical layer first (cables, NICs) before complex configuration.
Use built-in OS commands for testing:
- ping, ipconfig, nslookup, tracert (Windows)
- ping, ifconfig, ip addr, dig, traceroute (Linux)
Understand how IP, VLAN, DHCP, DNS, firewall, and routing interact.
Memorize common troubleshooting steps for each issue.