Common problems - Learn Tech From Zero

4.5 Given a scenario, troubleshoot network connectivity issues.

📘CompTIA Server+ (SK0-005)

Network connectivity problems happen when a device (server, computer, or network appliance) cannot communicate with other devices, the internet, or specific resources on a network. Troubleshooting these issues requires a methodical approach to identify the root cause and fix it.

Here are the common network connectivity problems and how to understand and troubleshoot them:

1. Lack of Internet Connectivity

What it means: The device cannot access the internet at all.

Causes:

Network cable unplugged or damaged
Incorrect IP settings (IP address, subnet mask, gateway)
Router or switch failure
Firewall blocking traffic
ISP (Internet Service Provider) outage

Troubleshooting steps:

Check if the network cable is connected and the link light on the NIC (Network Interface Card) is on.
Verify IP configuration using commands like:
- ipconfig (Windows)
- ifconfig or ip addr (Linux)
Test local network connectivity:
- Ping the gateway (ping 192.168.1.1) to see if you can reach the router.
Test external connectivity:
- Ping an external IP (like 8.8.8.8) to check if the internet works.
Check firewall settings or antivirus that may block connections.
Contact ISP if all internal checks are fine.

2. Resource Unavailable

What it means: A specific server, share, or service cannot be accessed.

Causes:

The target device is offline
Service is stopped on the target server
Incorrect permissions
Network route to the device is broken

Troubleshooting steps:

Check if the device hosting the resource is powered on.
Verify that the service is running (like DNS, HTTP, or file sharing service).
Check permissions and access rights.
Use ping or tracert to see if the path to the resource is reachable.

3. Receiving Incorrect DHCP Information

What it means: Your device is getting wrong IP settings from DHCP, causing connectivity problems.

Causes:

DHCP server misconfigured
Multiple DHCP servers on the same network
IP address conflicts

Troubleshooting steps:

Check the IP address and subnet mask received:
- ipconfig /all (Windows)
- cat /etc/network/interfaces or ip addr (Linux)
Release and renew IP from DHCP:
- ipconfig /release then ipconfig /renew
Verify that only the intended DHCP server is active.
Check for IP conflicts using arp -a.

4. Non-functional or Unreachable Network

What it means: The device is connected but cannot reach other devices.

Causes:

Network cable or port failure
Switch or router failure
Incorrect VLAN assignment
IP misconfiguration

Troubleshooting steps:

Verify NIC status and link lights.
Check IP configuration.
Test connectivity to immediate network devices (gateway, switch) using ping.
Verify VLAN assignment and switch port configuration.
Replace cables or test different ports if needed.

5. Destination Host Unreachable

What it means: When you ping a device, the system reports that the destination cannot be reached.

Causes:

Device is powered off or disconnected
Routing issues
Firewall blocking ICMP (ping)
Incorrect subnet mask or gateway configuration

Troubleshooting steps:

Check if the target device is online.
Verify the routing table:
- route print (Windows)
- ip route (Linux)
Ensure firewalls allow ICMP for testing.
Verify subnet masks on both source and target.

6. Unknown Host

What it means: The hostname or FQDN cannot be resolved to an IP address.

Causes:

DNS server unavailable
Typo in hostname
Incorrect DNS configuration
Missing DNS record

Troubleshooting steps:

Test with ping hostname or nslookup hostname.
Check DNS server settings in IP configuration.
Verify that the DNS server has the correct record.
Test DNS resolution with a public DNS (like 8.8.8.8) to isolate if it’s local DNS or network-wide.

7. Unable to Reach Remote Subnets

What it means: Devices on a local network cannot communicate with devices on a different subnet.

Causes:

Missing or incorrect routes
Router or firewall blocking traffic
Wrong gateway configuration

Troubleshooting steps:

Check default gateway settings.
Use tracert or traceroute to identify where the packets fail.
Verify routing tables on routers and firewalls.
Confirm subnet masks and network addresses are correctly set.

8. Failure of Service Provider

What it means: The ISP or upstream service that provides network access is down.

Causes:

ISP outage
Fiber/cable cut or degraded signal
ISP DNS issues

Troubleshooting steps:

Check connectivity to ISP’s gateway.
Test with external IP addresses (ping 8.8.8.8) to see if the issue is DNS or full internet.
Contact ISP if the problem is outside your network.

9. Cannot Reach Server by Hostname/FQDN

What it means: You can access a server by IP, but not by its hostname.

Causes:

DNS resolution failure
Hosts file misconfigured
Cache issues

Troubleshooting steps:

Test with ping hostname and ping IP.
Check DNS server configuration.
Flush DNS cache:
- ipconfig /flushdns (Windows)
- systemd-resolve --flush-caches (Linux)
Confirm that the hostname is correctly registered in DNS.

Tips for Exam Success

Always start troubleshooting from the client device toward the server or service.
Use commands like ping, tracert/traceroute, ipconfig/ifconfig, and nslookup.
Understand the difference between physical issues (cables, NICs, switches) and logical issues (IP settings, DNS, routing).
Remember key error messages like “Destination host unreachable” or “Unknown host” and what they indicate.