Troubleshooting Internet Access Issues

Introduction

• Problem: User unable to access the internet (e.g., Facebook.com).
• Focus on step-by-step troubleshooting process.

Step-by-Step Process

1. Name to IP Translation

• User types a URL (e.g., facebook.com).
• DNS (Domain Name System) resolves the domain name to an IP address.
  • Possible DNS servers:
    • Local DNS server
    • ISP's DNS server
    • Public DNS (e.g., Google DNS: 8.8.8.8)

2. TCP Handshake

• After successful DNS resolution, a TCP three-way handshake occurs, followed by HTTP message exchange.

Scenario: No Internal DNS Server

• User's computer connects to external DNS (e.g., Google DNS).
• Request sent to the DNS server over the internet, indicating the need for routing.

Network Configuration Details

• User's machine IP: 10.0.0.100
• Gateway IP: 10.0.0.1
• Destination DNS server IP: 8.8.8.8
• Since source and destination are on different networks, the request is sent to the gateway.
• Data frames sent to the switch, which forwards the request to the gateway after checking its MAC address table.
• Gateway checks its routing table for a route to 8.8.8.8.

Common Troubleshooting Steps

1. Check Connectivity to the Gateway

• First step: Ping the gateway IP address.
• If unreachable, check physical connectivity (cables, switch ports).

2. Check Switch Status

• Use commands like show interfaces status on manageable switches to verify port status.
• Check if the correct ports are enabled (e.g., GigabitEthernet0/2).

3. Check IP Configuration

• Use ipconfig /all to verify IP settings.
• Ensure the gateway is configured on the user's machine.

4. Test DNS Reachability

• Attempt to ping the DNS server (8.8.8.8).
• If unsuccessful, investigate routing issues.

Identifying DNS Issues

• If unable to ping DNS but can reach the gateway, there may be routing issues.
• Check the routing table on the gateway and ensure there are routes to external networks.

5. Configure Static Routes (If Necessary)

• If no route exists for the DNS server, configure a static route.
• Example command: ip route <destination> <subnet mask> <next hop>.

Final Steps

1. Configure DNS on Client

• Ensure the client's DNS settings point to the reachable DNS server (e.g., 8.8.8.8).

2. Test Connectivity to External Websites

• After configuration, attempt to ping or access websites (e.g., facebook.com).
• Successful IP resolution indicates DNS is functioning properly.

3. Default Route Configuration

• Consider configuring a default route for all unknown destinations.
• Command: ip route 0.0.0.0 0.0.0.0 <gateway IP>.

Conclusion

• Successfully troubleshooting the user's connectivity issue.
• Next class will focus on security and firewall considerations.