There are 5 classes of HTTP codes, divided into categories by the first digit.
- A first digit of 1, 2, or 3 represents a fully functional request.
- A first digit of 4 represents a client-side error. The most common codes span 400 to 404.
- A first digit of 5 represents a server-side error. Common codes span 500 to 510.
4xx Errors
Although these errors are technically the result of a problem with the client's (ie, the browser's) request, oftentimes they point to a problem on a website. For example, if you have a faulty link on your home page, and visitors click it, they may see a 404 error.
400 Bad Request
The user's request contains incorrect syntax.
401 Unauthorized
The requested file requires authentication (a username and password).
403 Forbidden
The server will not allow the visitor to access the requested file. If a visitor sees this code in error, check the file's permission settings.
404 Not Found
The server could not find the file that the visitor requested. This error commonly occurs when a URL is mistyped.
5xx Errors
These errors are caused by the server being unable to fulfill an apparently valid request from a visitor. Often, you will need the help of a server administrator in order to investigate these errors.
It is also important to consider that many times, a chain of servers is handling an HTTP request, so that it may not be the actual web server that is returning the error.
500 Internal Server Error
This error signifies that the server has encountered an unexpected condition. It is a “catch-all” error that will be displayed when no specific information can be gathered by the server regarding the condition. This error often occurs when an application request cannot be fulfilled due to the application being misconfigured.
501 Not Implemented
This signifies that the HTTP method sent by the client is not supported by the server. It is most often caused by the server being out of date. This error is very rare and generally requires that the web server be updated.
502 Bad Gateway
This error is usually due to improperly configured proxy servers. However, the problem may also arise when there is poor IP communication amongst back-end computers, when the client’s ISP is overloaded, or when a firewall is functioning improperly.
The first step in resolving the issue is to clear the client’s cache. This action should result in the a different proxy being used to resolve the web server’s content.
503 Service Unavailable
This error occurs when the server is unable to handle requests due to a temporary overload or due to the server being temporarily closed for maintenance. The error signifies that the server will only temporarily be down. It is possible to receive other errors in place of 503.
Contact the server administrator if this problem persists.
504 Gateway Timeout
This occurs when a server somewhere along the chain does not receive a timely response from a server further up the chain. The problem is caused entirely by slow communication between upstream computers.
To resolve this issue, contact the system administrator.
505 HTTP Version Not Supported
This error occurs when the server refuses to support the HTTP protocol that has been specified by the client computer. It can be caused by the protocol not being specified properly by the client computer; for example, if an invalid version number has been specified.
506 Variant Also Negotiates
This error indicates that the server is not properly configured. Please contact the system administrator to resolve this issue.
507 Insufficient Storage
This code indicates that the server is out of free memory. It is most likely to occur when an application being requested cannot allocate the necessary system resources for it to run.
To resolve the issue, the server’s hard disk may need to be cleaned of any unnecessary documents to free up more hard disk space, its memory may need to be expanded, or it may simply need to be restarted.
Please contact the system administrator for more information regarding this error message.
509 Bandwidth Limit Exceeded
This error occurs when the bandwidth limit imposed by the system administrator has been reached. The only fix for this issue is to wait until the limit is reset in the following cycle.
Consult the system administrator for information about acquiring more bandwidth.
510 Not Extended
This error occurs when an extension attached to the HTTP request is not supported by the web server.
To resolve the issue, you may need to update the server. Please consult the system administrator for more information.
