Application Layer

3.4. Application Layer#

In this section, you will learn about how applications such as web browsers, email clients, and file transfer programs communicate over the internet using specific protocols. These protocols work at the Application Layer of networking and are built on top of the Transmission Control Protocol (TCP). Understanding how these work is important because it shows you how different services (websites, email, file downloads) are delivered to your device.

By the end of this section, you will be able to explain:

  • How different application layer protocols like FTP, email, and HTTP work

  • The role of DNS in helping your device find websites and services

3.4.1. Application Layer and Protocols#

The Application Layer is the topmost layer in the networking model. It deals with the protocols that define how applications communicate over the internet. These protocols ensure that different types of data—whether it’s a file, an email, or a webpage are properly formatted and delivered.

There are many different application protocols, but we’ll focus on three major ones: FTP, Email (SMTP, POP, IMAP), and HTTP.

3.4.2. FTP (File Transfer Protocol)#

File Transfer Protocol (FTP) is used to transfer files from one computer to another over the internet. FTP allows users to upload files to a server or download files from a server. It’s commonly used for sharing large files, like software updates or media files.

For example, if you were to download a large game or software update, the company might use FTP to allow you to transfer the file from their server to your computer. You can think of FTP as a delivery truck that moves files from one location to another.

FTP typically uses port 21 for communication.

3.4.3. Email (SMTP, POP, IMAP)#

Email is one of the most common forms of communication on the internet, and it involves several protocols to send, receive, and manage messages.

  • SMTP (Simple Mail Transfer Protocol): SMTP is used to send emails. When you send an email, your email client (like Gmail or Outlook) uses SMTP to transfer the message to the recipient’s mail server. SMTP typically uses port 25.

  • POP (Post Office Protocol): POP is used to receive emails. When you open your email client, it connects to your mail server using POP to download any new messages. POP stores emails on your device and often removes them from the server. POP typically uses port 110.

  • IMAP (Internet Message Access Protocol): Like POP, IMAP is also used to receive emails, but it works differently. IMAP keeps emails on the server, allowing you to access them from multiple devices (like your phone and laptop). This is why you can open an email on your phone and still see it later on your computer. IMAP typically uses port 143.

3.4.4. HTTP (HyperText Transfer Protocol)#

HTTP (HyperText Transfer Protocol) is the protocol used by web browsers and web servers to communicate. When you type a website’s address into your browser (like google.com.au), your browser sends a request to that website’s server using HTTP. The server then sends back the webpage, which your browser displays.

For example, if you visit a website like wikipedia.org, your browser uses HTTP to request the page, and the server sends it back so you can view it.

When a website is using HTTPS, it means the data is encrypted for security, making it safe for activities like online shopping or banking. HTTP uses port 80, while HTTPS uses port 443 for secure communication.

Hint

We will learn about the details of HTTP in HTTP.

3.4.5. DNS (Domain Name System)#

You probably visit websites by typing their names into your browser, like google.com.au or youtube.com, but computers don’t understand these names. They need the IP addresses of the servers that host these websites. This is where the Domain Name System (DNS) comes in.

DNS is like the internet’s phonebook. When you type a domain name into your browser, your computer asks a DNS server to look up the corresponding IP address.

Here’s how it works:

  • When you type google.com.au into your browser, your computer sends a request to a DNS server.

  • The DNS server looks up the corresponding IP address (for example, 142.250.75.14).

  • Your computer then uses this IP address to communicate with Google’s server and load the website.

Without DNS, you would have to remember IP addresses for every website, which would be a lot harder than remembering domain names

3.4.6. Glossary#

Domain Name System#

DNS is like the internet’s phonebook, translating human-friendly domain names (like google.com) into IP addresses that computers use to find and communicate with websites.

DNS#

See Domain Name System

HTTP#

HTTP is the protocol used by web browsers to request and load web pages from servers, allowing you to browse websites on the internet.

IMAP#

IMAP is a protocol used to receive emails, allowing you to view your messages on multiple devices by keeping them stored on the email server.

POP3#

POP3 is a protocol used to receive emails, typically downloading messages from the server to your device and removing them from the server afterward.

SMTP#

SMTP is the protocol used for sending emails from your device to a mail server, which then forwards the message to the recipient’s server.

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