HTTP Status Code Reference

Understanding HTTP Status Codes

HTTP status codes are three-digit numbers that a server returns in response to every HTTP request. They are part of the HTTP specification and provide a standardized way for servers to communicate the result of a request to the client. When you visit a web page, your browser sends an HTTP request to the server, and the server responds with a status code along with the requested content or an error message. Understanding these codes is fundamental to web development, API design, server administration, and debugging network issues.

Status codes are grouped into five classes based on their first digit. Each class represents a different type of response. The 1xx codes are informational, indicating that the request was received and processing is continuing. The 2xx codes signal success, meaning the request was received, understood, and accepted. The 3xx codes indicate redirection, telling the client to take additional action to complete the request. The 4xx codes represent client errors, where the request contains incorrect syntax or cannot be fulfilled. The 5xx codes indicate server errors, where the server failed to fulfill a valid request.

The Five Status Code Categories

1xx Informational Responses

Informational responses indicate that the server has received the request and is continuing to process it. The most notable is 100 Continue, which tells the client to proceed with sending the request body after sending only the headers. This is useful for large uploads where the client wants to check whether the server will accept the request before transmitting a potentially large payload. The 103 Early Hints is a newer addition that allows the server to send preliminary headers before the final response, enabling the browser to preload stylesheets and scripts while the server prepares the main response.

2xx Success Codes

Success codes confirm that the request was processed correctly. The 200 OK is the most common, returned for standard successful GET requests. The 201 Created is used when a new resource has been created, typically in response to a POST request. The 204 No Content indicates success without a response body, commonly used for DELETE operations and API actions that do not need to return data. The 206 Partial Content supports range requests, allowing clients to download large files in chunks or resume interrupted downloads.

3xx Redirection and 4xx/5xx Error Codes

Redirection codes instruct the client to look elsewhere for the requested resource. The distinction between 301 Moved Permanently and 302 Found is critical for SEO, as search engines treat them differently when passing link authority. Client error codes in the 4xx range cover everything from malformed requests (400) to authentication failures (401, 403), missing resources (404), and rate limiting (429). Server error codes in the 5xx range indicate problems on the server side, from general failures (500) to bad gateway responses (502) and service unavailability (503). Monitoring these codes helps operations teams identify and resolve issues quickly.

Common Status Codes in API Development

RESTful APIs rely heavily on status codes to communicate results. A well-designed API uses 200 for successful reads, 201 for successful creates, 204 for successful deletes, 400 for validation errors, 401 for missing authentication, 403 for insufficient permissions, 404 for resources that do not exist, 409 for conflicts like duplicate entries, 422 for semantically invalid requests, and 429 for rate-limited requests. Consistent use of status codes makes APIs predictable and easier for consumers to integrate with.

Frequently Asked Questions

What are HTTP status codes?

Three-digit numbers returned by web servers indicating the result of a request. They are grouped into five classes: 1xx Informational, 2xx Success, 3xx Redirection, 4xx Client Error, and 5xx Server Error.

What do 4xx status codes mean?

4xx codes indicate client errors. The request was malformed, unauthorized, or pointed to a missing resource. Common examples include 400 Bad Request, 401 Unauthorized, 403 Forbidden, 404 Not Found, and 429 Too Many Requests.

What is the difference between 301 and 302 redirects?

301 is a permanent redirect that tells browsers and search engines to update their URLs. 302 is a temporary redirect where the original URL should still be used for future requests. 301 passes SEO link equity; 302 does not.

What is HTTP status code 418?

An April Fools joke from RFC 2324 that defines the Hyper Text Coffee Pot Control Protocol. A teapot returns 418 when asked to brew coffee. Despite being humorous, it has been widely implemented as an Easter egg.

When should I use a 204 No Content response?

Use 204 for successful requests that do not need a response body, such as DELETE operations, successful updates where the client already has the data, and fire-and-forget API actions.