Application Layer In OSI Reference Model

• Application layer is the topmost layer in the OSI model.

• It provides services to the user.

• It serves as the interface between the user and the network.

• It facilitates communication, data sharing and resource access over a network.

• It is responsible for providing network services directly to end user applications.

• Application programs are based on client server model.

Services provided by the Application layer:

→Transferring files.

→Accessing files.

→Managing files.

→Addressing files(From source to destination).

→Mail services.

→Directory service (To access global information).

Application layer functions includes:

*Identification of communication partners.

*Determining resource availability.

*Synchronizing communication.

*Application services.

*Data translation and Presentation.

*Communication Initiation & Termination.

*User authentication.

*Data formatting.

*Error handling and Data recovery.

Application layer protocols

1. HTTP / HTTPS.

2. FTP.

3. SMTP(Simple Mail Transfer Protocol).

4. DNS(Domain Name System).

5. Telnet & SSH(Secure Shell).

6. SNMP(Simple Network Management Protocol).

1. HTTP (Hypertext Transfer Protocol)

• Purpose: HTTP is used for transferring web pages and resources (such as images, videos, scripts) over the World Wide Web (WWW).

• Use Case: Accessing websites via web browsers.

• Ports: Default port 80 for HTTP, port 443 for HTTPS (secure version of HTTP).

• Example: When you visit a website, your browser sends an HTTP request to the server, and the server responds with an HTTP response containing the requested web page.

2. HTTPS (Hypertext Transfer Protocol Secure)

• Purpose: HTTPS is an extension of HTTP that includes security features like encryption via SSL/TLS.

• Use Case: Secure communication over the internet, used by online banking, e-commerce, and any site requiring privacy and data protection.

• Ports: Default port 443.

• Example: When you access a secure website, such as an online store or your bank's website, the communication between the server and your browser is encrypted with HTTPS.

3. FTP (File Transfer Protocol)

• Purpose: FTP is used for transferring files between computers on a network.

• Use Case: Uploading/downloading files to/from web servers, managing files on remote systems.

• Ports: Default ports are 20 and 21.

• Example: You can use an FTP client to upload files to a website's server or download large files from an FTP server.

4. SMTP (Simple Mail Transfer Protocol)

• Purpose: SMTP is used for sending and routing emails across networks.

• Use Case: Sending emails from a client (e.g., Outlook or Gmail) to an email server or between email servers.

• Ports: Default port 25 for unencrypted communication, 587 for encrypted communication (STARTTLS).

• Example: When you send an email, SMTP handles the process of sending your email to its destination.

5.DNS (Domain Name System)

• Purpose: DNS translates human-readable domain names (e.g., www.example.com) into IP addresses, allowing browsers and other software to connect to the appropriate server.

• Use Case: Resolving domain names to IP addresses to access websites or services.

• Ports: Default port53.

• Example: When you type a URL into your web browser, DNS queries the appropriate server to get the IP address of the website you want to visit.

6.TELNET

• Purpose: TELNET is used for accessing remote devices and servers over a network, typically in a command-line interface.

• Use Case: Remote access and management of network devices or servers.

• Ports: Default port 23.

• Example: TELNET can be used to log into remote servers to configure network equipment or troubleshoot systems. However, it is insecure, so SSH is often preferred.

7.SSH (Secure Shell)

• Purpose: SSH is a cryptographic network protocol used for secure communication between devices, allowing secure remote login and other network services.

• Use Case: Secure remote access to servers and devices, typically used for system administration.

• Ports: Default port 22.

• Example: System administrators use SSH to remotely access and manage servers over a secure channel.

8.SNMP (Simple Network Management Protocol)

• Purpose: SNMP is used for managing and monitoring network devices, such as routers, switches, and servers.

• Use Case: Collecting performance data, checking device status, and configuring devices remotely.

• Ports: Default port 161 for sending requests and port 162 for receiving notifications (traps).

• Example: Network administrators use SNMP to monitor the performance and status of routers and switches.