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“Web server codes” usually means HTTP response status codes: three-digit numbers that tell you how a server handled a request. The first digit identifies the broad category—information, success, redirection, client error or server error—while the full number gives the specific meaning. A code is a useful clue, but it does not by itself explain the underlying cause.
What does a web server code mean?
An HTTP status code is a three-digit integer in the range 100–599. It describes the result of a request and the meaning of the response, including whether the request succeeded and whether the response contains content. The definition comes from the IETF’s RFC 9110: HTTP Semantics.
The first digit tells you which of five classes the response belongs to. The remaining digits identify a more specific status; they do not form additional classes. A status code reports how the request was handled, not necessarily why the server or client reached that result.
| Class | Meaning | What it tells you |
|---|---|---|
| 1xx | Informational | The request was received and processing is continuing. One or more interim responses may precede the final response. |
| 2xx | Successful | The request was successfully received, understood and accepted. The result still depends on the HTTP method. |
| 3xx | Redirection | Further action is needed, such as requesting another URL or using a cached response. |
| 4xx | Client error | The request has a problem or cannot be fulfilled, often because of its syntax, credentials or access permissions. |
| 5xx | Server error | The server failed to fulfill an apparently valid request or cannot perform the requested method. |
These class meanings are defined in RFC 9110. A client is expected to understand the class from the first digit; if it encounters an unrecognized code, it treats it as the corresponding class’s “x00” status. For example, an unrecognized 471 is handled as a 4xx-class response. Values outside 100–599 are not valid HTTP status codes. The IANA HTTP Status Code Registry lists registered, unassigned, obsolete and temporary values.
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What do common HTTP status codes mean?
| Code | Plain-language meaning | Important distinction |
|---|---|---|
| 200 OK | The request succeeded. | With GET, the response normally includes the requested resource. With HEAD, it includes representation headers but no response body. |
| 301 Moved Permanently | The resource has moved permanently; the response supplies its new URL. | A permanent redirect. User-agent behavior may change the request method. |
| 302 Found | The resource is temporarily available at another URL. | A temporary redirect; the client should use the original URI for future requests. User-agent behavior may change the method. |
| 401 Unauthorized | The request lacks valid authentication credentials. | Despite “Unauthorized,” this is an authentication issue. A 401 response must include a WWW-Authenticate challenge. |
| 403 Forbidden | The server refuses the request. | The client may be identified, but access is refused. This differs from 401, which indicates missing valid authentication credentials. |
| 404 Not Found | The server cannot find the requested resource. | A server may also use 404 instead of 403 to avoid revealing whether a protected resource exists. |
| 429 Too Many Requests | The client has sent too many requests in a given period. | Commonly associated with rate limiting. |
| 500 Internal Server Error | The server encountered an unexpected condition that prevented it from fulfilling the request. | The code identifies a server-side failure, not its specific cause. |
| 502 Bad Gateway | A gateway or proxy received an invalid response from an upstream server. | The upstream response was invalid. |
| 503 Service Unavailable | The server is temporarily unable to handle the request, often because of maintenance or overload. | A Retry-After header may suggest when to try again. |
| 504 Gateway Timeout | A gateway or proxy did not receive a timely response from an upstream server. | The upstream server did not respond in time, rather than returning an invalid response as with 502. |
The meanings and distinctions in this table follow RFC 9110 and, for reader-friendly explanations, MDN’s HTTP response status codes reference.
How are 301, 302, 307 and 308 redirects different?
301 and 308 indicate permanent redirection; 302 and 307 indicate temporary redirection. The key method distinction is that 307 and 308 preserve the request method. With 301 and 302, user-agent behavior may change the method. That matters when a request is something other than a simple page fetch, such as a form submission: a redirect that changes the method can alter what the client sends to the new URL.
What is the difference between 401 and 403?
A 401 response means the request does not include valid authentication credentials and challenges the client to authenticate. A 403 response means the server refuses the request; knowing who the client is does not necessarily grant permission to access the resource. In practical terms, 401 points to authentication, while 403 points to refusal of access. A server may return 404 instead of 403 if it does not want to disclose that a protected resource exists.
What is the difference between 502 and 504?
Both codes involve a server acting as a gateway or proxy communicating with an upstream server. A 502 means the gateway received an invalid upstream response. A 504 means it did not receive a response in time. The status identifies the type of gateway failure, but not the underlying reason the upstream system failed or responded too slowly.
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What does a 503 error mean, and should you retry?
A 503 means the server is temporarily unable to handle the request. Maintenance and overload are common circumstances described by RFC 9110. Check the response for a Retry-After header: when present, it can indicate when a retry is appropriate. The code itself does not guarantee that a retry at any particular time will succeed.
Are all numbers from 100 to 599 assigned?
No. The range defines valid HTTP status-code values, but not every number in it has an assigned meaning. The IANA registry includes codes specified by RFC 9110 and extensions defined elsewhere, including WebDAV-related statuses, 429 for rate limiting, 451 for legal restrictions and 511 for network authentication. It also identifies unassigned, unused and obsolete values.
The IANA registry page reported a last update of 2025-09-15. At that point, code 104, “Upload Resumption Supported,” was listed as a temporary registration, registered on 2024-11-13, with an expiry date of 2026-11-13. Because temporary assignments can change, consult the live IANA registry for its current status rather than assuming that listing remains unchanged.
Quick Recap
How to use a status code when troubleshooting
- Start with the first digit. It quickly identifies whether the response is informational, successful, a redirect, a client error or a server error.
- Check the exact code next. The distinction between 401 and 403, or between 502 and 504, changes what the response tells you.
- Consider the request method. A 200 response to HEAD has no response body, and redirect behavior can affect methods other than GET.
- Read the response headers and context. A 401 challenge or a 503
Retry-Aftervalue adds information that the status number alone does not provide. - Use the code as a clue, not a complete diagnosis. The same status can arise in different circumstances; resolving the cause may require server logs, application details or information about an upstream service.
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