What is HTTP Status Code? Definition, examples and how it works

Q: What is HTTP Status Code?

A three-digit integer in an HTTP response that indicates the result of the server's attempt to process the request, grouped into five classes (1xx through 5xx).

An HTTP status code is a three-digit integer that the server includes in the first line of an HTTP response to communicate the result of processing the client's request. The first digit defines the class of response. There are five classes defined in the HTTP specification: 1xx (Informational), 2xx (Successful), 3xx (Redirection), 4xx (Client Error), and 5xx (Server Error). The remaining two digits do not carry any hierarchical meaning.

The status code is placed in the status line of the response, for example 'HTTP/1.1 200 OK'. The numeric code is intended to be processed automatically by clients, while the accompanying reason phrase (e.g., 'OK', 'Not Found') is a human-readable text that servers may customize, though the RFC recommends standard phrases for interoperability. The code defines semantics for both the client and any intermediate proxies. For instance, a 304 Not Modified response tells a cache that it can serve its stored copy; a 301 Moved Permanently tells a client to update its bookmark.

Status codes are extensible. The core set was defined in RFC 7231, which superseded earlier RFCs like 2616. Additional codes have been added by other RFCs, like 451 (Unavailable For Legal Reasons) from RFC 7725, and 103 (Early Hints) from RFC 8297. The HTTP specification does not limit total codes, but new codes must be registered with the IANA. Developers often see status codes as the first indicator of a request's fate. In practice, many web frameworks and APIs rely on a small subset, primarily 200, 201, 204, 301, 302, 400, 401, 403, 404, 500, and 502, but new services often use less common codes for fine-grained signaling.

Key facts

Status codes are grouped by first digit: 1xx (informational), 2xx (success), 3xx (redirection), 4xx (client error), 5xx (server error).
The HTTP specification (RFC 7231) defines the standard set; new codes are registered with IANA.
Clients use the numeric code for automatic handling; the reason phrase is optional and non-normative.
Codes are extensible, allowing new semantics without breaking older clients that ignore unknown codes.
Most common codes include 200, 301, 400, 404, and 500.

How it works in practice

When a browser requests https://example.com/page, the server may respond with 'HTTP/1.1 200 OK' if the page exists, along with the page body. If the page has moved permanently, the server responds with '301 Moved Permanently' and a Location header pointing to the new URL. If the page does not exist, the server responds with '404 Not Found'. If the server software crashes internally, the client sees '500 Internal Server Error'.

Related terms

HTTP response HTTP request HTTP header Content-Type RFC 7231

References

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