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HTTP

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HTTP

Summary: Hypertext Transfer Protocol — the request-response protocol used to transfer web pages, images, and other resources between a web browser (client) and a web server. HTTP is stateless; HTTPS adds SSL/TLS encryption for secure communication. Tags: igcse computer-science Created: 2026-05-08T14:03:00Z Last Updated: 2026-07-16

What is HTTP?

Hypertext Transfer Protocol (HTTP) is an application-layer protocol that governs how web browsers (clients) communicate with web servers. It follows a simple request-response model:

  1. The client (browser) sends an HTTP request to a server
  2. The server processes the request and sends back an HTTP response
  3. The connection is closed (in HTTP/1.0) or may be reused (in HTTP/1.1 onwards)

HTTP is stateless: each request-response pair is independent. The server does not retain information about previous requests from the same client. This is why websites use cookies and sessions to maintain state (e.g., keeping a user logged in).

The Client-Server Model

+---------+     HTTP Request (GET /index.html)    +----------+
| Browser | ----------------------------------->  |  Web     |
| (Client)|                                       |  Server  |
|         |  <----------------------------------- |          |
+---------+     HTTP Response (200 OK + HTML)     +----------+

HTTP Methods

HTTP defines methods (sometimes called verbs) that specify the action the client wants to perform:

MethodPurposeExample
GETRetrieve a resource from the serverRequesting a web page, image, or stylesheet
POSTSubmit data to the server (creates or updates)Submitting a login form, uploading a file, sending a comment
PUTReplace a resource on the serverUpdating a user profile
DELETERemove a resource from the serverDeleting a post
HEADLike GET but returns only headers, not bodyChecking if a page exists without downloading it

For IGCSE, the key methods to know are GET (retrieving data) and POST (submitting data).

HTTP Request Structure

An HTTP GET request for www.example.com/page.html looks like:

GET /page.html HTTP/1.1
Host: www.example.com
User-Agent: Mozilla/5.0
Accept: text/html
  • Request line: Method (GET), resource path (/page.html), HTTP version
  • Headers: Additional information (host, browser type, accepted formats)
  • Body (POST only): The submitted data (e.g., form values)

HTTP Response Structure

HTTP/1.1 200 OK
Content-Type: text/html
Content-Length: 1234

<html>...</html>
  • Status line: HTTP version, status code, reason phrase
  • Headers: Information about the response (content type, length)
  • Body: The actual content (HTML, image data, JSON, etc.)

Common HTTP Status Codes

CodeMeaningDescription
200OKRequest succeeded; response body contains the resource
301Moved PermanentlyResource has moved to a new URL
404Not FoundThe requested resource does not exist on the server
500Internal Server ErrorThe server encountered an error and cannot fulfil the request

HTTP vs HTTPS

HTTPS (Hypertext Transfer Protocol Secure) is HTTP with SSL/TLS encryption added. It uses port 443 instead of port 80.

FeatureHTTPHTTPS
EncryptionNone — data sent as plain textEncrypted via SSL/TLS
URL prefixhttp://https://
Default port80443
SecurityVulnerable to eavesdropping and man-in-the-middle attacksProtects against interception and tampering
Padlock iconNoYes (in browser address bar)
SEOGoogle deprioritises HTTP sitesPreferred by Google

HTTPS should always be used for:

  • Online banking and financial transactions
  • E-commerce (entering credit card details)
  • Login pages (passwords)
  • Any site handling personal/sensitive data

SSL/TLS: How HTTPS Works

SSL (Secure Sockets Layer) and its successor TLS (Transport Layer Security) provide:

  1. Encryption: Data is encrypted so that even if intercepted, it cannot be read
  2. Authentication: Digital certificates verify the server’s identity, preventing impersonation
  3. Data Integrity: Ensures data has not been modified in transit

Simplified TLS Handshake

  1. Browser connects to server and requests a secure connection
  2. Server sends its digital certificate (containing its public key, verified by a Certificate Authority)
  3. Browser verifies the certificate is valid and trusted
  4. Browser and server agree on encryption settings and exchange session keys
  5. All subsequent communication is encrypted with the session key

The padlock icon in the browser indicates an active HTTPS connection with a valid certificate.

Sources

  • BBC Bitesize GCSE Computer Science — HTTP and Web Protocols, BBC (free educational resource)
  • Cambridge IGCSE Computer Science 0478 — The Internet and Its Uses, Cambridge Assessment International Education
  • CK-12 Computer Science — Web Technologies, CK-12 Foundation (free, CC BY-NC 3.0)
  • HTTPS — Secure HTTP in detail
  • SSL-TLS — The encryption layer that makes HTTPS secure
  • Internet — The network infrastructure HTTP runs on
  • World Wide Web — The web of hyperlinked pages accessed via HTTP
  • URL — The address format used in HTTP requests
  • Web Browser — The client software that sends HTTP requests
  • DNS — Resolves domain names before HTTP connections
  • Cookie — How websites maintain state across HTTP requests
  • CS-Index

Common Misconceptions

MisconceptionReality
”HTTPS means the website is safe and trustworthy”HTTPS only means the connection is encrypted and the server’s identity is verified. A phishing website with a valid certificate would still have HTTPS.
”HTTP and HTTPS are completely different protocols”HTTPS is HTTP over an encrypted TLS connection. The protocol itself is the same; the transport layer underneath is different.
”GET and POST are interchangeable”GET requests include data in the URL (visible, length-limited, bookmarkable, cacheable). POST includes data in the request body (not visible in URL, no length limit, not bookmarkable, not cacheable). Use GET for retrieval, POST for submission.
”A 404 error means the server is down”A 404 means the specific resource was not found. The server itself is reachable and responding (if it were down, you would get no response at all or a timeout).

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