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HTTP Status Codes: The Complete Guide with Common Codes Explained

HTTP status codes are server responses that indicate request success, errors, or redirects—essential for web devs, SEO, and troubleshooting.

 
HTTP Status Codes: The Complete Guide with Common Codes Explained

Introduction

Every time you visit a website, submit a form, or click a link, your browser communicates with a web server using HTTP (Hypertext Transfer Protocol). Part of this communication involves something called HTTP status codes—a set of three-digit numbers that tell the browser or client how the request was handled by the server.

These status codes are essential not just for web developers, but also for cybersecurity professionals, SEO experts, system administrators, and anyone who deals with the web.

In this comprehensive article, we’ll dive deep into HTTP status codes—what they are, how they work, the different classes they fall into, and the most common codes you’re likely to encounter. We’ll also go over real-world use cases, troubleshooting tips, and how these codes affect SEO and user experience.

What Is an HTTP Status Code?

An HTTP status code is a three-digit response sent from the server to the client (typically a browser or an API consumer) after a request is made. These codes indicate whether a request was successful, encountered an error, or needs further action.

For example:

HTTP/1.1 200 OK
  • HTTP/1.1 is the protocol version.
  • 200 is the status code.
  • OK is a brief description of the code.

These codes help developers and clients understand the result of their request without needing to analyze the entire response.

Classes of HTTP Status Codes

HTTP status codes are grouped into five main categories:

Class Description
1xx Informational – The request was received and is continuing to be processed.
2xx Success – The request was successfully received, understood, and accepted.
3xx Redirection – Further action is needed to complete the request.
4xx Client Error – The request contains bad syntax or cannot be fulfilled.
5xx Server Error – The server failed to fulfill an apparently valid request.

Let’s explore each category and its most commonly used status codes.

1xx – Informational Responses

These are rarely seen by end users and mostly used internally by the server to manage ongoing processes.

Common 1xx Codes:

  • 100 Continue Informs the client that the initial part of the request has been received and it can continue with the rest.

  • 101 Switching Protocols Indicates that the server is switching to a different protocol as requested by the client.

  • 103 Early Hints Allows the server to suggest resources for preloading while the final response is still being prepared.

2xx – Successful Responses

These codes mean the server successfully processed the request.

Key 2xx Codes:

  • 200 OK The standard response for a successful request. Used for everything from loading a web page to responding to an API call.

  • 201 Created Indicates that a new resource has been created as a result of the request. Common in POST requests when creating records via an API.

  • 202 Accepted The request has been accepted for processing, but the processing has not been completed.

  • 204 No Content The server successfully processed the request but is not returning any content. Often used in APIs after a DELETE request.

3xx – Redirection Responses

Redirection codes tell the client that further action is needed to complete the request.

Common 3xx Codes:

  • 301 Moved Permanently Indicates that the requested resource has been permanently moved to a new URL. This is the preferred method for redirecting URLs in SEO.

  • 302 Found Temporarily redirects the client to a different URL. Unlike a 301, this tells search engines that the original URL will be restored.

  • 303 See Other Directs the client to retrieve the resource using a GET request on another URI.

  • 304 Not Modified The client’s cached version of the resource is still valid; no need to resend the data.

4xx – Client Error Responses

These codes are sent when there is a problem with the client’s request.

Widely Used 4xx Codes:

  • 400 Bad Request The server cannot or will not process the request due to a client-side error (e.g., malformed request syntax).

  • 401 Unauthorized Authentication is required. This code is returned when the request lacks valid credentials.

  • 403 Forbidden The client does not have access rights to the content; unlike 401, authentication will not help.

  • 404 Not Found The server cannot find the requested resource. This is the most commonly seen error on the web.

  • 405 Method Not Allowed The method used in the request is not supported for the resource.

  • 408 Request Timeout The server timed out waiting for the request.

  • 429 Too Many Requests The user has sent too many requests in a given amount of time. Used for rate limiting.

5xx – Server Error Responses

These codes indicate that the server failed to process a valid request.

Key 5xx Codes:

  • 500 Internal Server Error A generic error message indicating an unexpected condition was encountered.

  • 501 Not Implemented The server does not support the functionality required to fulfill the request.

  • 502 Bad Gateway A gateway or proxy received an invalid response from an upstream server.

  • 503 Service Unavailable The server is currently unavailable due to maintenance or overload.

  • 504 Gateway Timeout A gateway or proxy server did not receive a timely response from an upstream server.

Practical Use Cases

Status Code Scenario
200 OK Normal webpage loading or API success
201 Created User registration or new blog post via API
301 Moved Permanently Redirecting old URLs to a new domain
403 Forbidden Blocking access to sensitive admin areas
404 Not Found Handling broken links or missing pages
500 Internal Server Error Backend failure or misconfiguration
429 Too Many Requests API abuse or bot rate limiting

HTTP Status Codes and SEO

Search engines treat status codes as signals that influence how pages are crawled and indexed.

  • 301: Signals a permanent change. Good for maintaining SEO authority.
  • 302: Temporary redirect. Avoid overuse, as it doesn’t pass SEO value.
  • 404: Indicates missing content. Too many 404s can harm your site’s SEO.
  • 503: Tells search engines your site is temporarily down and they should come back later.

Troubleshooting Common Errors

Code Problem How to Fix
400 Malformed request Check request formatting and parameters
401 Missing/invalid credentials Ensure authentication is correct
403 Insufficient permissions Review file/folder permissions
404 Missing file Check URLs and routes
500 Server crash or error Inspect logs, debug code
503 Server overloaded Restart or scale services

Example HTTP Response Header

Here’s a sample server response for a successful page request:

HTTP/1.1 200 OK
Date: Sat, 03 May 2025 12:00:00 GMT
Server: Apache/2.4.41 (Ubuntu)
Content-Type: text/html; charset=UTF-8
Content-Length: 1024

Tools for Viewing HTTP Status Codes

  • Curl curl -I https://example.com

  • HTTPie http https://example.com

  • Wget wget --server-response https://example.com

  • Browser Developer Tools Use the "Network" tab to inspect status codes for every request.

Code Type Meaning
200 Success OK
201 Success Created
301 Redirect Moved Permanently
302 Redirect Found
304 Redirect Not Modified
400 Client Error Bad Request
401 Client Error Unauthorized
403 Client Error Forbidden
404 Client Error Not Found
429 Client Error Too Many Requests
500 Server Error Internal Server Error
502 Server Error Bad Gateway
503 Server Error Service Unavailable

Conclusion

HTTP status codes are an integral part of how the web works. They help browsers, servers, APIs, and search engines understand what’s happening during a request.

Whether you're building a web app, maintaining a website, running a server, or managing SEO, understanding these status codes is essential for diagnosing issues and creating a smoother user experience.