This is the fourth part of the Computer Networking series. Make sure to check the previous parts if you haven't. In this post, we’ll discuss an overview of the OSI model, TCP/IP model, and the Application Layer.

Table of Contents

1. Structure of Network
   • OSI Model
   • TCP/IP Model
2. Application Layer
   • Client-Server Architecture
   • Peer-to-Peer Architecture
3. Common Protocols in Networking
4. Important Networking Terminologies

Structure of Network

To understand network communication and data transfer, we rely on two fundamental models:

1. OSI Model
2. TCP/IP Model

OSI Model

The Open Systems Interconnection (OSI) model has seven layers:

1. Application Layer
2. Presentation Layer
3. Session Layer
4. Transport Layer
5. Network Layer
6. Data Link Layer
7. Physical Layer

Each layer has its own responsibilities and interacts with the layers directly above and below it. Each layer adds specific information to ensure the data reaches its destination correctly, and these steps are reversed upon arrival.

TCP/IP Model

The Transmission Control Protocol/Internet Protocol (TCP/IP) model has four layers:

1. Application Layer
2. Transport Layer
3. Network Layer
4. Network Access Layer

The Application Layer in the TCP/IP model combines the Application, Presentation, and Session layers of the OSI model. The Network Access Layer in the TCP/IP model combines the Data Link and Physical layers of the OSI model.

Application Layer

The Application Layer is where users interact with applications like web browsers, chat applications, and more. This layer resides on our devices and provides the interface for communication.

Architectures in the Application Layer

1. Client-Server Architecture
2. Peer-to-Peer Architecture

Client-Server Architecture

• The application has two parts: the client and the server, which communicate with each other as processes.
• Clients are users consuming resources. For example, when you make a request to Google, you act as a client.
• Servers control the websites and resources you access.
• A collection of servers forms data centers, which host services, maintain static IP addresses, and require high internet speeds for efficient operation.

Peer-to-Peer Architecture

• There’s no dedicated server in P2P architecture. Devices communicate directly with each other.
• Each device acts as both a client and a server, making this architecture highly scalable.

Common Protocols in Networking

1. HTTP (HyperText Transfer Protocol): Used for browsing websites.
2. DHCP (Dynamic Host Configuration Protocol): Allocates IP addresses dynamically.
3. FTP (File Transfer Protocol): Transfers files between devices.
4. SMTP (Simple Mail Transfer Protocol): Sends emails.
5. POP3 & IMAP: Receive emails.
6. SSH (Secure Shell): Securely connects to remote devices.
7. VNC (Virtual Network Computing): Enables remote desktop sharing.
8. Telnet: Allows connecting to remote hosts/devices using a Telnet client (port 23).
9. UDP (User Datagram Protocol): A lightweight, stateless connection protocol.

Important Networking Terminologies

Let’s explain networking terms with an example: WhatsApp.

1. Program: The application itself (e.g., WhatsApp).
2. Process: A running instance or a feature of the program. For instance, sending a message or recording a video.
3. Thread: A lightweight version of a process. For example, when you record a video, a thread handles the camera function.
4. Socket: The interface between a process and the internet.
5. Ports:
   • IP addresses identify the device making the request.
   • Ports identify the application on the device.

What about multiple processes?

If multiple processes of the same application are running (e.g., multiple Chrome tabs), ephemeral ports help distinguish which specific process made the request. These are temporary network endpoints that facilitate communication between client and server applications.

This concludes Part 4. Stay tuned for the next part, where we’ll explore more aspects of computer networking!

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