Packet Movement Through a Network: Lecture Notes

Introduction

• Video series finale by Ed from practicalnetworking.net
• Focus on how packets move through a network
• Recommended to read the previous article series for better understanding

Sample Topology

• Contains 4 hosts, 2 switches, and 1 router
• Routers facilitate communication between networks
• Each host has an IP address and a MAC address
• Example IP address for Host A: 11.11.11.10
• Router interfaces:
  • ETH1: 11.11.11.1 (connected to 11.11.11.0/24 network)
  • ETH2: 22.22.22.1 (connected to 22.22.22.0/24 network)
• Switches:
  • Switch X: Ports 1, 2, 3
  • Switch Y: Ports 4, 5, 6

Key Tables in Network Devices

1. ARP Table
   • Maps IP addresses to MAC addresses
   • Used by all Layer 3 devices
2. MAC Address Table
   • Maps switch ports to MAC addresses
   • Helps switches know which devices are connected to which ports
3. Routing Table
   • Maps IP networks to egress interfaces
   • Pre-populated before traffic arrives

Data Flow from Host A to Host D

1. Initial Set-Up

   • Host A knows Host D's IP address (22.22.22.40)
   • Creates a Layer 3 header with source IP (11.11.11.10) and destination IP (22.22.22.40)
   • Determines that Host D is on a foreign network, needs to send data to the router (default gateway)

2. ARP Request

   • Host A does not know router's MAC address, sends ARP request to discover it
   • ARP request reaches Switch X, which learns Host A's MAC address
   • Switch floods the ARP request to all ports except port 2
   • Router receives ARP request, learns Host A's MAC mapping
   • Router sends ARP response back to Host A

3. Forwarding Data

   • Host A receives ARP response, now knows router's MAC address
   • Host A creates Layer 2 header and sends data to the router
   • Switch X forwards the frame to the router based on MAC address table
   • Router receives the packet, strips Layer 2 header, consults routing table for destination
   • Router determines it needs to send the packet to Host D, requires Host D's MAC address

4. Second ARP Request

   • Router sends ARP request to discover Host D's MAC address
   • Switch Y receives the ARP request, floods it out to all ports
   • Host D receives the request, generates ARP response back to the router
   • Router receives the response, updates ARP table for Host D's MAC

5. Final Delivery

   • Router creates Layer 2 header for final delivery to Host D
   • Switch Y forwards the frame to Host D
   • Host D strips Layer 2 and Layer 3 headers, processes data

Response from Host D to Host A

• Much quicker due to populated tables
• Host D creates response data with destination IP of Host A
• Sends the packet back through the same process with existing ARP entries
• Final delivery back to Host A

Conclusion

• 30 steps needed to successfully transfer data from Host A to Host D and back
• For more details, refer to the packet traveling series at practicalnetworking.net.
• Suggestions for sharing the video if found useful.