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Step-by-Step Internet Troubleshooting Guide

Nov 15, 2024

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.