Whenever you browse the Internet, your web browser and the web server communicate with each other using HTTP (HyperText Transfer Protocol). Every time you open a webpage, click a link, submit a form, upload a file, or use an API, your browser sends an HTTP Request to the server. The server processes that request and sends back an HTTP Response.

Along with the response, the server also sends a three-digit HTTP Status Code. This status code tells the browser what happened to the request. It informs whether the request was successful, whether the requested resource has moved, whether the client made an error, or whether the server encountered a problem while processing the request.

You can think of an HTTP Status Code as a communication language between the Client (Browser or Application) and the Server. Without these status codes, the browser would not know whether to display a webpage, redirect the user, ask for authentication, or show an error message.

For web developers, website owners, API developers, and SEO professionals, understanding HTTP Status Codes is very important. They help identify website problems, improve user experience, troubleshoot server issues, optimize website performance, and maintain good search engine rankings.

HTTP Status Codes are important because they provide clear information about the result of every HTTP Request. They help both users and developers understand what happened during the communication between the browser and the server.

• They inform whether a request was successful or failed.
• They help browsers decide how to handle the server response.
• They help developers troubleshoot website and API problems.
• They improve website maintenance and debugging.
• They play an important role in Search Engine Optimization (SEO).
• They help improve website security and performance.
• They support browser caching and resource optimization.

The communication between a browser and a web server follows a simple process known as the HTTP Request-Response Cycle.

1. The user enters a website URL or clicks a hyperlink.
2. The browser sends an HTTP Request to the web server.
3. The server receives and processes the request.
4. The server generates an HTTP Response.
5. The response contains the requested resource along with an HTTP Status Code.
6. The browser reads the status code and decides what action to perform.

For example, if the server returns 200 OK, the browser displays the webpage. If it returns 301 Moved Permanently, the browser automatically redirects the user to the new URL. If it returns 404 Not Found, the browser displays an error page because the requested resource does not exist.

Every HTTP Status Code belongs to one of five categories. The category is determined by the first digit of the status code. Even if you have never seen a particular status code before, you can understand its general meaning simply by looking at its first digit.

Each category represents a different type of response. Understanding these categories makes it easier to diagnose website problems, develop web applications, and prepare for technical interviews and competitive examinations.

Suppose you visit the website https://example.com/login.

• If the login page opens normally, the server returns 200 OK.
• If the login page has permanently moved to another URL, the server returns 301 Moved Permanently, and the browser automatically redirects you.
• If you enter a wrong URL such as /loginn, the server may return 404 Not Found.
• If the login page requires authentication before access, the server may return 401 Unauthorized.
• If the server crashes while processing your request, it may return 500 Internal Server Error.

In every case, the browser uses the HTTP Status Code to determine how the request should be handled and what information should be shown to the user.

Correct Status Code: 415 Unsupported Media Type

The server should return 415 Unsupported Media Type because the uploaded file format violates the server's accepted content types.

Why 415 is the Right Choice

• Client error category: 4xx codes indicate the problem is on the client side. The request was understood but cannot be processed due to invalid content.
• Specific to content type: 415 explicitly signals that the server refuses to accept the payload because its media type is not supported, unlike generic 400 which covers all bad requests.
• Content-Type header mismatch: The client likely sent a Content-Type header (e.g., image/heic or application/x-bittorrent) that the server does not allow for document uploads.

Response Example:

HTTP/1.1 415 Unsupported Media Type

Content-Type: application/json

{

"error": "Unsupported file format",

"message": "Only PDF, JPG, and PNG files are accepted",

"received": "image/webp",

"allowed_types": ["application/pdf", "image/jpeg", "image/png"]

}

Why Not Other Codes?

Client Action:
The mobile app should read the 415 response, display the allowed formats to the user, and prompt them to select a supported file type before retrying the upload.

HTTP Status Code Categories

i. 2xx (Success)

• Indicates that the request was successfully received and processed.
• Example: 200 OK → Request successful
• Example: 201 Created → Resource successfully created

ii. 4xx (Client Error)

• Indicates an error from the client side (wrong request).
• Example: 400 Bad Request → Invalid request format
• Example: 404 Not Found → Requested resource not found

iii. 5xx (Server Error)

• Indicates an error from the server side.
• Example: 500 Internal Server Error → Server failed to process request
• Example: 503 Service Unavailable → Server temporarily unavailable

[Source:Mozilla]

HTTP Status Codes are standard response codes sent by a web server to a client (browser or application) after receiving an HTTP request.

These codes indicate whether the request was successful, failed, redirected, or caused a server error.

HTTP status codes are represented by three-digit numbers.

Purpose of HTTP Status Codes

• Inform the client about request status.
• Help in debugging network or website problems.
• Control browser and application behavior.
• Support communication between client and server.

Categories of HTTP Status Codes

HTTP status codes are divided into five categories:

1. Informational Status Codes (100–199)

These codes indicate that the request has been received and processing is continuing.

• 100 Continue: Client can continue sending the request.
• 101 Switching Protocols: Server agrees to switch protocols.

2. Success Status Codes (200–299)

These codes indicate successful request processing.

• 200 OK: Request completed successfully.
• 201 Created: New resource created successfully.
• 204 No Content: Request successful but no content returned.

3. Redirection Status Codes (300–399)

These codes indicate that additional action is needed.

• 301 Moved Permanently: Resource has permanently moved to another URL.
• 302 Found: Resource temporarily moved.
• 304 Not Modified: Cached version can be used.

4. Client Error Status Codes (400–499)

These errors occur due to problems in the client request.

• 400 Bad Request: Invalid request syntax.
• 401 Unauthorized: Authentication required.
• 403 Forbidden: Access denied.
• 404 Not Found: Requested resource not found.

5. Server Error Status Codes (500–599)

These errors occur when the server fails to process the request.

• 500 Internal Server Error: General server-side error.
• 502 Bad Gateway: Invalid response from another server.
• 503 Service Unavailable: Server temporarily unavailable.
• 504 Gateway Timeout: Server response timeout.

Commonly Used HTTP Status Codes

HTTP Status Codes are three-digit numeric codes returned by a web server to indicate the result of a client's HTTP request. They tell the client whether the request was successful, redirected, invalid, or failed due to a server error.

Importance:

They provide a standard way for clients and servers to communicate, help browsers take appropriate actions, simplify debugging, and improve the reliability of web communication.

HTTP Status Codes act as a standard communication mechanism between clients and servers. They indicate the outcome of every HTTP request, allowing browsers and applications to display content, redirect users, or report errors appropriately. They also support efficient troubleshooting and application development.

First, the browser sends an HTTP request to the web server. The server processes the request and returns an appropriate HTTP Status Code. The browser interprets the code and performs the required action, such as displaying a webpage, following a redirect, or showing an error message.

HTTP 2xx status codes indicate that the client's request was successfully received, understood, and accepted by the server. Examples include 200 OK (request successful), 201 Created (resource created), and 204 No Content (successful but no content to return).

HTTP 3xx status codes indicate redirection. They inform the client that further action is needed to complete the request, usually by accessing a different URL. Examples include 301 Moved Permanently (resource moved to a new URL permanently), 302 Found (temporary redirect), and 307 Temporary Redirect.

HTTP 4xx status codes are client error codes. They indicate that the request contains bad syntax or cannot be fulfilled by the server due to a mistake made by the client. Examples include 400 Bad Request, 401 Unauthorized, 403 Forbidden, and 404 Not Found.

HTTP 5xx status codes are server error codes. They indicate that the server failed to fulfill a valid request due to an internal server problem. Examples include 500 Internal Server Error, 502 Bad Gateway, 503 Service Unavailable, and 504 Gateway Timeout.

HTTP 400 Bad Request indicates that the server cannot process the request because the client sent malformed syntax or an invalid request message. The error is due to incorrect request formatting.

HTTP 404 Not Found indicates that the server understood the request but could not find the requested resource at the specified URL. The error is due to the resource not existing on the server.

HTTP 401 Unauthorized means the client must authenticate itself to get the requested response. The server requires valid login credentials (username/password or token) that were missing or incorrect.

HTTP 403 Forbidden means the server understood the request and the client is authenticated, but the client does not have permission to access the requested resource. Access is denied regardless of authentication.

HTTP 404 Not Found means the requested resource could not be found on the server, but it does not specify whether the resource was permanently removed or might be available again in the future.

HTTP 410 Gone means the requested resource was intentionally and permanently removed from the server, and the server knows it will not be available again. It is more specific than 404.

HTTP 500 Internal Server Error is a generic error indicating that the server encountered an unexpected condition that prevented it from fulfilling the request. It usually points to a bug or misconfiguration in the server-side code.

HTTP 503 Service Unavailable means the server is currently unable to handle the request due to temporary overload or scheduled maintenance. The server is operational but temporarily unavailable, and the client should retry later.

HTTP 502 Bad Gateway means the server, acting as a gateway or proxy, received an invalid response from an upstream server it accessed to fulfill the request. The upstream server sent a malformed or unexpected response.

HTTP 504 Gateway Timeout means the server, acting as a gateway or proxy, did not receive a timely response from the upstream server within the expected time frame. The upstream server took too long to respond.

HTTP 404 Not Found should be returned because the requested resource (webpage) does not exist on the server. This status code informs the client that the server could not find the requested page, allowing the browser to display an appropriate error message to the user.

HTTP 401 Unauthorized should be returned because the request lacks valid authentication credentials. The server requires the user to provide login credentials (such as username/password or an authentication token) before granting access to the protected resource.

HTTP 403 Forbidden should be returned because the customer is authenticated (logged in) but does not have the necessary permissions to access another customer's confidential data. The server understands the request but refuses to authorize it due to insufficient privileges.

HTTP 301 Moved Permanently should be returned because the resource has been permanently relocated to a new URL. This status code tells the client (browser) to update its bookmarks and redirect all future requests to the new URL. It also helps with SEO by transferring link equity to the new location.

HTTP Status Codes provide a standardized language for clients and servers to communicate. They clearly indicate whether a request succeeded, failed, or requires further action. This standardization enables browsers to respond appropriately (display content, redirect, show errors), helps developers debug issues efficiently, and ensures consistent behavior across different systems and platforms.

Security: Codes like 401 and 403 enforce authentication and authorization, preventing unauthorized access. Performance: Codes like 301, 302, and 304 (Not Modified) enable caching and redirection, reducing unnecessary data transfer and server load. Maintainability: Clear status codes help developers quickly identify and fix issues, monitor application health, and implement proper error handling, making the codebase easier to maintain and scale.

HTTP 503 Service Unavailable should be returned because it indicates that the server is temporarily unable to handle requests due to maintenance or overload. It informs the client that the unavailability is temporary and the service will be restored soon, allowing clients to retry after some time.

HTTP 504 Gateway Timeout should be returned because the gateway or proxy server did not receive a timely response from the upstream server within the allowed time limit. This status code indicates a communication delay between servers, not a client error.