Certainly, let's break down the detailed process of communication between Computer 1 and Computer 2 across the networks A, B, and C. This scenario highlights how each layer of the networking model comes into play:

Please just watch the video. Too complex.

https://www.coursera.org/learn/computer-networking/lecture/BqSRb/all-the-layers-working-in-unison

1. Computer 1 (Client):

   • Computer 1 wants to access a webpage on 172.16.1.100 (Computer 2 - has a web server) using a web browser.

2. Networking Stack:

   • The web browser communicates with the local networking stack to establish a TCP connection to 172.16.1.100 Port 80.

3. Routing Decision:

   • Computer 1 checks its own subnet, realizes that 172.16.1.100 is on another network, and decides to route the data through its configured gateway 10.1.1.1.

   

4. ARP Resolution:

   • Computer 1 checks its ARP table for the MAC address of 10.1.1.1, doesn't find it, sends an ARP request, and gets the MAC address of its gateway (Router A).

   

5. TCP Segment Construction:

   • Computer 1 chooses an ephemeral port (e.g., 50000) for the source port, crafts a TCP segment with SYN flag set, calculates checksum, and constructs an IP datagram.

   

   

   

   

6. Ethernet Frame Construction:

   • An Ethernet frame is built with destination MAC as Router A's MAC, and source MAC of Computer 1, encapsulating the IP datagram.

   

   

7. Transmission through Physical Layer:

   • The frame is sent through the physical layer using electrical signals across a cable to a network switch.

8. Router A Processing:

   • Router A receives the frame, validates checksums, processes Ethernet frame, decrements TTL, updates checksum, encapsulates in a new Ethernet frame, and forwards to Router B.

   

   

   

   

   source is Network B interface MAC address, and destination is Router B interface MAC address

   

9. Router B Processing:

   • Router B performs similar processing, decrements TTL, encapsulates in a new Ethernet frame, and sends to Computer 2.

   

   

10. Computer 2 (Server):

    • Computer 2 receives the frame, validates checksums, processes Ethernet frame, extracts IP datagram, validates checksum, and processes TCP segment.

    

    

11. TCP Handshake and Data Transfer:

    • Computer 2 detects SYN flag, responds with SYN/ACK, waits for ACK, acknowledges, and prepares to establish a connection for data transfer.

12. Web Server Processing:

    • The web server (Apache) on Computer 2 responds with requested webpage data.

13. Data Return:

    • Data travels back through the layers in the reverse order, with routers making routing decisions, frames encapsulating data, and checksums ensuring integrity.

14. Web Browser Rendering:

    • Data is received, validated, and presented on the web browser of Computer 1.

This detailed process illustrates how data traverses the network layers, from application layer instructions to physical layer transmission, and back again through the reverse process for the response. Each layer contributes its specific functionality to ensure successful communication between two devices across multiple networks. It's this complex orchestration that enables seamless networking and communication.