Network Protocols

The Key Network Protocols

There are many network protocols, but for GCSE Computer Science you need to know seven in detail. Each protocol has a specific purpose, uses specific port numbers, and is designed for a particular type of network communication.

HTTP — HyperText Transfer Protocol

HTTP is the protocol used to transfer web pages from a web server to your browser. Every time you type a web address into your browser or click a link, HTTP is working behind the scenes to request and deliver the page.

How HTTP Works

HTTP operates on a request-response model:

1. You type a URL (such as http://www.example.com/page.html) into your browser.
2. Your browser sends an HTTP request to the web server at that address, asking for the specified page.
3. The web server receives the request, finds the page, and sends it back as an HTTP response containing the HTML, CSS, images, and other content.
4. Your browser receives the response and renders (displays) the web page on your screen.

Key Facts about HTTP

• Port: 80
• Purpose: Requesting and delivering web pages and web content
• Data: Not encrypted — data is sent in plain text, so anyone intercepting the traffic can read it
• Used by: Web browsers (Chrome, Firefox, Edge, Safari) when accessing websites

HTTPS — HyperText Transfer Protocol Secure

HTTPS is the secure, encrypted version of HTTP. It works in exactly the same way as HTTP (request-response for web pages), but all data is encrypted using SSL (Secure Sockets Layer) or its successor TLS (Transport Layer Security) before being transmitted. This means that even if someone intercepts the data, they cannot read it because it is scrambled.

How HTTPS Provides Security

• Encryption: All data exchanged between the browser and the server is encrypted using SSL/TLS. This protects sensitive information such as passwords, credit card numbers, and personal data from being intercepted and read.
• Authentication: The web server presents a digital certificate to prove its identity. This ensures you are communicating with the real website (e.g., your actual bank) and not an imposter. The browser verifies this certificate before establishing the connection.
• Data integrity: HTTPS ensures that data has not been tampered with during transmission. If anyone modifies the data in transit, the encryption detects the change and rejects the data.

Key Facts about HTTPS

• Port: 443
• Purpose: Secure, encrypted transfer of web pages and web content
• Visual indicator: A padlock icon appears in the browser’s address bar when HTTPS is in use
• Used for: Online banking, online shopping, logging into accounts, any site handling sensitive data
• Modern standard: Most websites now use HTTPS by default, not just banking and shopping sites

FTP — File Transfer Protocol

FTP is a protocol specifically designed for uploading and downloading files to and from a server. While HTTP can also transfer files (when you download a file from a website), FTP is purpose-built for managing files on a remote server and is much more efficient for large file transfers.

How FTP Works

FTP uses a client-server model:

1. The user connects to an FTP server using an FTP client application (such as FileZilla) and authenticates with a username and password.
2. Once connected, the user can browse the server’s file system, just like browsing folders on their own computer.
3. The user can upload files from their computer to the server, download files from the server to their computer, or delete, rename, and move files on the server.

Key Facts about FTP

• Ports: 20 (data transfer) and 21 (control/commands)
• Purpose: Uploading and downloading files to/from a remote server
• Used by: Web developers uploading website files to a web server, businesses transferring large files, backing up data to a remote server
• Security: Standard FTP sends data (including usernames and passwords) in plain text. SFTP (Secure FTP) and FTPS (FTP Secure) are encrypted versions that protect data in transit.

SMTP — Simple Mail Transfer Protocol

SMTP is the protocol used for sending emails. When you compose an email and click “Send,” SMTP is the protocol that pushes your email from your device to the mail server and then forwards it between mail servers until it reaches the recipient’s mail server.

How SMTP Works

1. You compose an email in your email client (Outlook, Gmail, Thunderbird, etc.) and click Send.
2. Your email client uses SMTP to send the email to your outgoing mail server (also called the SMTP server).
3. Your SMTP server looks up the recipient’s domain (e.g., gmail.com) using DNS to find the recipient’s mail server.
4. Your SMTP server forwards the email to the recipient’s mail server using SMTP.
5. The email is stored on the recipient’s mail server, waiting for the recipient to collect it (using POP3 or IMAP).

Key Facts about SMTP

• Port: 25 (original, server-to-server) or 587 (modern, client-to-server with authentication)
• Purpose: Sending emails from a client to a server, and forwarding emails between mail servers
• Direction: SMTP only sends (pushes) emails — it does not retrieve (pull) them. A separate protocol (POP3 or IMAP) is needed to receive emails.
• Used by: All email clients and all email servers for outgoing email

POP3 — Post Office Protocol version 3

POP3 is a protocol used to download emails from a mail server to a single device. When you check your email, POP3 retrieves the emails from the server and saves them on your local device.

How POP3 Works

1. Your email client connects to the mail server using POP3.
2. POP3 downloads all new emails from the server to your device (laptop, phone, etc.).
3. By default, POP3 then deletes the emails from the server after downloading them. The emails now exist only on your local device.
4. You can read, organise, and manage your emails offline on that device.

Key Facts about POP3

• Port: 110
• Purpose: Downloading emails from the server to a local device
• Behaviour: Downloads emails and deletes them from the server (by default)
• Best for: Users who access email from a single device only
• Advantage: Emails are stored locally, so you can read them offline without an internet connection. Also uses less server storage because emails are removed from the server.
• Disadvantage: If you check email on your laptop, those emails will not appear on your phone because they were downloaded to the laptop and deleted from the server. This makes POP3 poor for users with multiple devices.

IMAP — Internet Message Access Protocol

IMAP is a protocol used to synchronise emails across multiple devices. Unlike POP3, IMAP keeps emails stored on the server and synchronises them with every device that connects. This is the protocol used by most modern email services.

How IMAP Works

1. Your email client connects to the mail server using IMAP.
2. IMAP synchronises your email — it shows you the emails stored on the server without downloading and deleting them.
3. Emails remain on the server. Any changes you make (reading, deleting, moving to folders) are reflected on the server and therefore on all your other devices.
4. If you read an email on your phone, it will also appear as read on your laptop, tablet, and any other device.

Key Facts about IMAP

• Port: 143
• Purpose: Synchronising emails across multiple devices, keeping emails on the server
• Behaviour: Keeps emails on the server and syncs across all devices
• Best for: Users who access email from multiple devices (phone, laptop, tablet, work computer)
• Advantage: Emails are synchronised — read an email on your phone and it appears as read on your laptop. Delete on one device and it is deleted everywhere. This is ideal for modern use where people check email on multiple devices.
• Disadvantage: Requires a constant internet connection to access emails (since they are stored on the server, not locally). Also uses more server storage because emails are not deleted from the server.

DNS — Domain Name System

DNS is the system that translates human-readable domain names into IP addresses. Computers identify each other using numerical IP addresses (like 142.250.187.206), but humans find it much easier to remember names (like www.google.com). DNS acts as the “phone book” of the internet, converting names into numbers so your computer can find the right server.

Why DNS Is Needed

Every device on the internet has a unique IP address — a numerical identifier like 142.250.187.206 (IPv4) or 2a00:1450:4009:820::200e (IPv6). These numbers are how computers locate each other on the internet. However, humans cannot easily remember long strings of numbers for every website they visit. DNS solves this by allowing you to type www.google.com instead of 142.250.187.206. The DNS system automatically looks up the corresponding IP address.

How a DNS Lookup Works (Step by Step)

When you type a domain name into your browser, the following process occurs:

1. Browser cache check: Your browser first checks its own cache (local memory) to see if it has looked up this domain recently. If it has the IP address stored from a previous visit, it uses that immediately and skips the remaining steps.
2. Operating system cache check: If the browser cache does not have it, the request goes to the operating system, which checks its own DNS cache.
3. DNS resolver (recursive resolver): If neither cache has the answer, your computer sends the request to a DNS resolver. This is usually a server operated by your Internet Service Provider (ISP). The resolver’s job is to track down the correct IP address by querying other DNS servers.
4. Root server: The resolver contacts a root DNS server. There are 13 sets of root servers worldwide. The root server does not know the IP address of www.google.com, but it knows which server handles .com domains and directs the resolver there.
5. TLD (Top-Level Domain) server: The resolver contacts the TLD server for .com. This server does not know the exact IP address either, but it knows which authoritative name server is responsible for google.com and directs the resolver there.
6. Authoritative name server: The resolver contacts the authoritative name server for google.com. This server has the definitive record and returns the IP address (e.g., 142.250.187.206) to the resolver.
7. IP address returned: The resolver sends the IP address back to your computer. Both the resolver and your computer cache (store) this result for future use, so the next lookup will be much faster.
8. Browser connects: Your browser now uses the IP address to connect directly to Google’s web server using HTTP or HTTPS, and the web page is loaded.

  You type: www.google.com

  [Your Browser]
       |
       v
  [DNS Resolver (ISP)]
       |
       v
  [Root Server]  ---->  "I don't know, but try the .com TLD server"
       |
       v
  [.com TLD Server]  ---->  "I don't know, but try Google's name server"
       |
       v
  [Authoritative Name Server for google.com]
       |
       v
  Returns IP: 142.250.187.206
       |
       v
  [Your Browser connects to 142.250.187.206 via HTTP/HTTPS]
       |
       v
  Web page loads!
        

Key Facts about DNS

• Port: 53
• Purpose: Translating domain names (e.g., www.bbc.co.uk) into IP addresses (e.g., 151.101.0.81)
• Analogy: DNS is the “phone book” of the internet — you look up a name and get a number
• Caching: Results are cached at multiple levels (browser, OS, resolver) to speed up future lookups
• Without DNS: You would have to memorise the IP address of every website you want to visit

Test Your Knowledge

Try these interactive exercises to check your understanding of network protocols. Read each question carefully before answering.