1. You can ping an IP but not the domain. What do you check?

Situation:
The system can reach an IP address but fails when using a domain name.

Task:
Identify why domain-based communication is failing.

Action:

  • Checked DNS configuration in /etc/resolv.conf
  • Used nslookup / dig to verify resolution
  • Verified DNS server reachability
  • Checked /etc/hosts for overrides

Result:
Identified a DNS misconfiguration. After correcting DNS settings, domain resolution worked successfully.

2. DNS resolves but HTTPS connection fails. What could be the issue?

Situation:
Domain resolves correctly, but HTTPS connection fails.

Task:
Determine why secure communication is not established.

Action:

  • Tested with curl -v and openssl s_client
  • Verified TLS certificate validity
  • Checked firewall rules on port 443
  • Confirmed TLS version and cipher compatibility

Result:
Found an expired SSL certificate. Renewing the certificate restored HTTPS connectivity.

3. Clients receive IP addresses but cannot access the internet. Why?

Situation:
Devices get IPs via DHCP but cannot access external networks.

Task:
Identify connectivity issue beyond IP assignment.

Action:

  • Checked default gateway configuration
  • Verified DNS settings
  • Tested NAT configuration on router
  • Inspected firewall rules

Result:
Discovered missing default gateway configuration. After fixing it, internet access was restored.

4. A website loads slowly during peak hours. What do you investigate?

Situation:
Website performance degrades under high traffic.

Task:
Identify bottleneck causing slow response.

Action:

  • Monitored CPU, memory, and network usage
  • Checked load balancer distribution
  • Analyzed database query performance
  • Reviewed CDN usage

Result:
Identified server overload. Implemented load balancing and caching, improving response time significantly.

5. A TLS handshake fails but TCP connects successfully. What does it mean?

Situation:
TCP connection is established, but TLS handshake fails.

Task:
Diagnose security layer failure.

Action:

  • Verified certificates and trust chain
  • Checked supported TLS versions and ciphers
  • Used openssl for debugging
  • Reviewed server configuration

Result:
Found TLS version mismatch. Updating configuration resolved the issue.

6. A host can reach external networks but not an internal subnet. Why?

Situation:
External connectivity works, but internal subnet is unreachable.

Task:
Find internal routing or filtering issue.

Action:

  • Checked routing table (ip route)
  • Verified subnet routes
  • Inspected firewall and ACL rules
  • Checked VLAN configuration

Result:
Missing route to internal subnet was identified and added, restoring connectivity.

7. Users cannot access the application after enabling IPv6. What do you check?

Situation:
Application access fails after IPv6 is enabled.

Task:
Identify dual-stack configuration issue.

Action:

  • Checked AAAA DNS records
  • Verified IPv6 routing
  • Tested IPv6 connectivity
  • Reviewed firewall rules for IPv6

Result:
Detected incomplete IPv6 routing. Fixing routing restored access.

8. How do you detect a routing loop?

Situation:
Network shows high latency and instability.

Task:
Identify if a routing loop exists.

Action:

  • Ran traceroute to detect repeated hops
  • Observed TTL exceeded messages
  • Monitored network traffic patterns

Result:
Confirmed routing loop. Corrected routing configuration, stabilizing the network.

9. One region can access an application while another cannot. How do you troubleshoot?

Situation:
Application is accessible from one region but not another.

Task:
Determine region-specific issue.

Action:

  • Compared DNS resolution across regions
  • Used traceroute from affected region
  • Checked CDN and load balancer routing
  • Reviewed firewall/geolocation rules

Result:
Identified incorrect routing in one region. Fixing it restored global accessibility.

10. What is the difference between “Connection refused” and “Connection timed out”?

Connection Refused

Situation:
Client receives immediate error.

Task:
Understand why connection is rejected.

Action:

  • Checked if service is running
  • Verified port listening using ss -tuln

Result:
Service was not running. Starting the service resolved the issue.

Connection Timed Out

Situation:
No response from server.

Task:
Identify why connection is not established.

Action:

  • Checked firewall rules
  • Verified network path using traceroute
  • Tested server availability

Result:
Firewall was blocking traffic. Allowing traffic resolved the issue.

11. Two servers in different VLANs cannot communicate. What could be wrong?

Situation:
Two servers are placed in different VLANs and cannot reach each other.

Task:
Identify why inter-VLAN communication is failing.

Action:

  • Verified VLAN configuration on switches
  • Checked if inter-VLAN routing is enabled (Layer 3 switch/router)
  • Validated trunk port configuration (802.1Q tagging)
  • Checked gateway configuration on both servers
  • Reviewed ACLs or firewall rules

Result:
Found that inter-VLAN routing was not configured on the Layer 3 switch. Enabling routing resolved the issue.

12. Production traffic fails after a firewall rule update. What steps would you take?

Situation:
After updating firewall rules, production traffic stops working.

Task:
Quickly identify and fix the misconfiguration with minimal downtime.

Action:

  • Reviewed recent firewall rule changes
  • Compared with previous working configuration
  • Checked blocked ports/services
  • Used logs to identify dropped packets
  • Temporarily rolled back rules for validation

Result:
Identified a rule blocking required ports. Correcting the rule restored production traffic.

13. A service works internally but not externally. What do you check?

Situation:
Service is accessible within the network but not from outside.

Task:
Find why external users cannot access it.

Action:

  • Checked NAT/port forwarding configuration
  • Verified firewall rules for inbound traffic
  • Ensured service is listening on correct interface (0.0.0.0)
  • Tested external connectivity using tools like curl

Result:
Port forwarding was missing. Adding correct NAT rules allowed external access.

14. SSH works but HTTP downloads are slow. Why?

Situation:
SSH connections are normal, but HTTP traffic is slow.

Task:
Identify performance degradation specific to HTTP.

Action:

  • Checked bandwidth utilization
  • Investigated QoS or traffic shaping policies
  • Analyzed HTTP server performance
  • Used packet capture to inspect delays
  • Checked MTU mismatch or fragmentation issues

Result:
Detected QoS limiting HTTP traffic. Adjusting policies improved performance.

15. There is intermittent packet loss to a remote server. How do you diagnose it?

Situation:
Connectivity to a server is unstable with packet loss.

Task:
Identify where packet loss is occurring.

Action:

  • Used ping and mtr to monitor packet loss
  • Ran traceroute to identify problematic hop
  • Checked network congestion and interface errors
  • Reviewed logs on intermediate devices

Result:
Found packet loss at an intermediate router. Fixing link congestion resolved the issue.

16. Users say the internet is slow but only for one application. What do you check?

Situation:
Only a specific application is slow; others work fine.

Task:
Determine application-specific bottleneck.

Action:

  • Tested application performance separately
  • Checked server-side performance
  • Verified DNS resolution for that application
  • Checked CDN or backend latency
  • Inspected application logs

Result:
Identified backend API latency. Optimizing the service improved user experience.

17. One host cannot reach the gateway but others can. What could be the issue?

Situation:
Only one machine cannot reach the gateway.

Task:
Identify host-specific issue.

Action:

  • Checked IP configuration (ip addr)
  • Verified subnet mask and gateway
  • Checked ARP table
  • Tested cable/interface
  • Reviewed local firewall

Result:
Incorrect subnet mask was configured. Fixing it restored connectivity.

18. DNS resolves but curl https://domain fails with a timeout. What do you check next?

Situation:
DNS resolution works, but HTTPS request times out.

Task:
Identify network or service issue.

Action:

  • Checked connectivity to port 443 using telnet or nc
  • Verified firewall rules
  • Checked load balancer or reverse proxy
  • Inspected server availability

Result:
Port 443 was blocked by firewall. Opening the port resolved the issue.

19. A container cannot communicate with another container in Docker. How do you debug?

Situation:
Two Docker containers cannot communicate.

Task:
Identify networking issue within container environment.

Action:

  • Checked Docker network configuration
  • Verified both containers are on same network
  • Tested connectivity using container IPs
  • Inspected Docker bridge network
  • Checked firewall rules

Result:
Containers were on different networks. Connecting them to same network resolved communication.

20. Kubernetes pods cannot reach a service. What checks do you perform?

Situation:
Pods are unable to access a Kubernetes service.

Task:
Identify cluster networking issue.

Action:

  • Checked service definition (kubectl get svc)
  • Verified endpoints (kubectl get endpoints)
  • Ensured pods are running and healthy
  • Checked network policies
  • Tested DNS resolution inside pod

Result:
Service selector mismatch was found. Fixing labels restored connectivity.

21. A server cannot resolve DNS names. What files do you inspect?

Situation:
A server cannot resolve domain names, though IP connectivity exists.

Task:
Identify why DNS resolution is failing.

Action:

  • Checked /etc/resolv.conf for correct nameserver entries
  • Verified /etc/hosts for conflicting entries

  • Tested DNS using:

    dig example.com
    nslookup example.com
  • Ensured DNS server is reachable

Result:
Incorrect DNS server IP was configured. Updating it restored name resolution.

22. How do you debug MTU-related issues?

Situation:
Applications experience fragmentation issues or connectivity problems.

Task:
Identify MTU mismatch along the path.

Action:

  • Used ping with DF (Don’t Fragment):

    ping -M do -s 1472 <destination>
  • Gradually reduced packet size to find working MTU

  • Checked interface MTU:

    ip link show
  • Verified Path MTU Discovery behavior

Result:
MTU mismatch identified between networks. Adjusting MTU fixed connectivity.

23. What tools help detect packet drops in a network?

Situation:
Users report intermittent connectivity and packet loss.

Task:
Identify where packet drops occur.

Action:

  • Used ping for basic loss detection
  • Used mtr for real-time monitoring
  • Ran traceroute to identify problematic hop
  • Captured packets using tcpdump
  • Checked interface errors (ip -s link)

Result:
Detected packet loss at an intermediate device. Fixing that node resolved the issue.

24. How do you identify bandwidth bottlenecks?

Situation:
Network performance degrades under load.

Task:
Identify where bandwidth is being constrained.

Action:

  • Monitored traffic using iftop, nload, or vnstat
  • Checked interface utilization
  • Used iperf for throughput testing
  • Reviewed QoS policies
  • Analyzed logs for congestion

Result:
Found link saturation during peak usage. Upgrading bandwidth or optimizing traffic resolved the issue.

25. A website shows an SSL certificate error. What could be wrong?

Situation:
Users see SSL warnings when accessing a website.

Task:
Identify certificate-related issue.

Action:

  • Checked certificate validity (expiry date)
  • Verified domain matches certificate (CN/SAN)
  • Checked certificate chain

  • Used:

    openssl s_client -connect example.com:443

Result:
Certificate was expired. Renewing and deploying it fixed the issue.

26. How do you troubleshoot NAT problems?

Situation:
Internal users cannot access external services or vice versa.

Task:
Identify NAT misconfiguration.

Action:

  • Verified NAT rules on router/firewall
  • Checked translation table
  • Ensured correct mapping of private to public IP
  • Tested connectivity from both sides
  • Reviewed firewall rules

Result:
Incorrect NAT rule identified. Fixing mapping restored communication.

27. What steps do you follow when a network suddenly goes down?

Situation:
Entire network or service becomes unavailable.

Task:
Quickly identify and restore connectivity.

Action:

  • Checked physical connectivity (cables, interfaces)
  • Verified device status (routers, switches)
  • Tested basic connectivity (ping, traceroute)
  • Reviewed recent configuration changes
  • Checked logs and alerts

Result:
Identified misconfiguration in core router. Rolling back changes restored network.

28. A switch port is up but there is no connectivity. What do you check?

Situation:
Port is active but devices cannot communicate.

Task:
Identify Layer 2 or configuration issue.

Action:

  • Checked VLAN assignment
  • Verified trunk/access mode
  • Checked MAC address table
  • Looked for STP blocking state
  • Tested cable and NIC

Result:
Port was assigned to wrong VLAN. Correcting VLAN restored connectivity.

29. Why might DHCP fail to assign IP addresses?

Situation:
Clients fail to obtain IP addresses automatically.

Task:
Identify DHCP failure.

Action:

  • Checked DHCP server status
  • Verified IP pool availability
  • Checked DHCP relay configuration
  • Inspected network connectivity to DHCP server
  • Used packet capture to analyze DHCP process

Result:
IP pool was exhausted. Expanding pool resolved the issue.

30. How do you troubleshoot intermittent connectivity problems?

Situation:
Users experience occasional network failures.

Task:
Identify root cause of instability.

Action:

  • Used mtr for continuous monitoring
  • Checked logs for errors
  • Inspected hardware (cables, ports)
  • Monitored network load
  • Checked DNS and routing stability

Result:
Identified faulty cable causing intermittent drops. Replacing cable fixed the issue.