Learning Objectives
By the end of this chapter, you will be able to:
- Define HTTP and its role in the World Wide Web.
- Explain the difference between HTTP and HTTPS.
- Identify the key protocols used for sending and receiving email, and compare POP3 and IMAP.
- Define FTP and its purpose.
- Match common application protocols to their port numbers and uses.
Key Internet Application Protocols
While TCP/IP is the foundational protocol for the internet, several other application-layer protocols run on top of it to provide specific services.
1. HTTP (Hypertext Transfer Protocol)
HTTP is the protocol that powers the World Wide Web. It defines how messages are formatted and transmitted, and what actions web servers and browsers should take in response to various commands. When you type a web address into your browser, your browser sends an HTTP request to the web server to retrieve the page.
- HTTPS: A secure version of HTTP that encrypts the communication between your browser and the web server, protecting your data from being intercepted.
sequenceDiagram
participant B as Browser
participant W as Web Server
B->>W: HTTP GET /page
W->>W: Find and prepare page
W->>B: HTTP 200 OK + HTML
B->>B: Render page
Figure 1: HTTP Web Page Request Flow
HTTP works on the client-server request-response model. Common request methods are GET (fetch a page) and POST (send form data, such as a login). The server replies with a status code:
| Status code | Meaning |
|---|---|
| 200 OK | The request succeeded; page delivered |
| 301 / 302 | The page has moved (redirect) |
| 404 Not Found | The page does not exist |
| 500 Internal Server Error | Something went wrong on the server |
| Aspect | HTTP | HTTPS |
|---|---|---|
| Security | Data sent as plain text | Data encrypted using SSL/TLS |
| Port | 80 | 443 |
| Browser sign | “Not secure” warning | Padlock icon |
| Use | Rare today | Login pages, payments, almost all websites |
2. Email Protocols
Sending and receiving email involves several different protocols:
- SMTP (Simple Mail Transfer Protocol): The standard protocol for sending email.
- POP3 (Post Office Protocol 3): A protocol for receiving email. It typically downloads the email from the server to your device and then deletes it from the server.
- IMAP (Internet Message Access Protocol): Another protocol for receiving email. Unlike POP3, IMAP syncs with the mail server, so your emails are available on multiple devices and any changes you make (like deleting an email) are reflected everywhere.
flowchart LR
A[Sender's email app] -->|SMTP| S1[Sender's mail server]
S1 -->|SMTP| S2[Receiver's mail server]
S2 -->|IMAP or POP3| B[Receiver's email app]
| Aspect | POP3 | IMAP |
|---|---|---|
| Where mail stays | Downloaded to one device | Stays on the server, synced to devices |
| Multiple devices | Poor; each device sees different mail | Good; same inbox on phone and laptop |
| Offline access | Yes, once downloaded | Yes for cached mail |
| Server storage | Little needed | More needed |
| Best for | One computer, limited server space | Modern use across phone, laptop, web |
Webmail such as Gmail in a browser uses HTTPS to show your mailbox, while the mail servers still use SMTP between them.
3. FTP (File Transfer Protocol)
FTP is a protocol used to transfer computer files from one host to another over a TCP-based network, such as the internet. It is often used for uploading files to a web server.
Plain FTP sends usernames and passwords unencrypted, so secure versions are preferred:
- SFTP (SSH File Transfer Protocol) and FTPS (FTP over SSL/TLS) encrypt the transfer.
- Popular FTP client programs include FileZilla and WinSCP.
Common Protocols and Ports
A port number tells the receiving computer which service a message is for, like an extension number in an office phone system.
| Protocol | Port | Use |
|---|---|---|
| HTTP | 80 | Web pages (unencrypted) |
| HTTPS | 443 | Secure web pages |
| SMTP | 25 / 587 | Sending email |
| POP3 | 110 | Receiving email (download) |
| IMAP | 143 | Receiving email (sync) |
| FTP | 20, 21 | File transfer |
| SSH/SFTP | 22 | Secure remote login and file transfer |
| DNS | 53 | Name lookup |
Business Scenario
A trading company in Birgunj sets up email on its own domain. Staff read mail on both office PCs and phones, so the IT officer configures IMAP rather than POP3, so that a message read or deleted on one device is updated everywhere. Outgoing mail uses SMTP with encryption. The company website is updated by uploading files using SFTP, and the site runs on HTTPS so customer enquiry forms are encrypted.
Summary
Several key application protocols run on top of TCP/IP to provide the internet services we use every day. HTTP is the foundation of the World Wide Web, enabling communication between browsers and web servers, and HTTPS adds encryption. A combination of protocols—SMTP for sending, and POP3 or IMAP for receiving—manage our email, with IMAP better suited to multiple devices. FTP is a specialized protocol used for transferring files between computers, with SFTP and FTPS as secure alternatives. Each protocol uses a standard port number.
Key Takeaways
- HTTP is the protocol of the World Wide Web; HTTPS encrypts it.
- SMTP is for sending email; POP3 and IMAP are for receiving it.
- IMAP keeps mail synced across devices; POP3 downloads to one device.
- FTP is used for transferring files; SFTP and FTPS are secure versions.
- Port numbers identify services, such as 80 for HTTP and 443 for HTTPS.
Discussion Questions
- What is the main difference between HTTP and HTTPS? Why is HTTPS important?
- What is the main advantage of using IMAP over POP3 for your email?
- Have you ever used an FTP client? What for?
Exam-Style Questions
Short answer
- Expand HTTP, SMTP, IMAP, and FTP.
- Differentiate between HTTP and HTTPS.
- What does a 404 status code mean?
Long answer
- Explain how an email travels from sender to receiver, naming the protocols used. Compare POP3 and IMAP.
- Describe HTTP, email protocols, and FTP, with their uses in a business.
