What is the application layer of the OSI model?

Application Layer is the 7th, topmost layer of the OSI model, the one the user and applications work with directly.

Put simply: it is the layer where programs that use the network "live".

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#Why the application layer is needed

The application layer makes sure applications can use the network without worrying about:

• how bits are transmitted,
• how packets are routed,
• how connections are established.

It provides clear network services for programs.

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#What the application layer actually does

At the application layer:

• data is sent and received by applications
• work happens with high-level protocols
• the user interacts with the network

Example tasks:

• opening a web page
• sending an email
• downloading a file
• getting data from an API

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#Examples of application layer protocols

The ones most often asked about in interviews:

• HTTP / HTTPS - loading web pages and APIs
• FTP / SFTP - file transfer
• SMTP - sending email
• POP3 / IMAP - receiving email
• DNS - converting a domain name into an IP address

Important: a browser is not a protocol, it is a program, while HTTP is an application-layer protocol.

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#Real-life example

When you type https://example.com into a browser:

1. The application layer builds an HTTP request
2. The layers below it:

• split the data into packets
• deliver them across the network

3. The response comes back
4. The application layer passes the data to the browser, which displays the page

The user just sees the page. How exactly the data traveled across the network does not concern them.