The 4xx HTTP status codes are purposed to indicate what you or your client agent may have done wrong. In any case, your client agent is required to display the error to you especially when it is related to your actions. When a HEAD request is submitted, the server does not display the error code. Instead, it acts on the request.

400 - Bad Request

This is normally due to erred syntax or inclusion of characters that the server would not understand. This request will be displayed for your purposeful response.

401 - Unauthorized

This is similar to 403, which means "forbidden", except that 401 appears when you need authentication in order to access the desired resource. The response is included with a WWW-Authenticate header, which has the authentication protocol you need to follow to access the URL.

402 - Payment Required

It was initially intended to restrict access to a URI when a payment is needed first. The Error response is 'Payment Required'. It has since not been used, although You Tube generates the error when you make excessive requests, at which point, the error requires you to enter a CAPTCHA image so as to continue accessing the service. It has also been generated on Apple's Mobile Me, when an account has issues not yet sorted to allow access to the service.

403 - Forbidden

This usually indicates that the URI access is 'forbidden', and there are no authorization protocols that will allow you access. The resource may be restricted for access using http protocols, and may be in use for other proprietary or secure access protocols.

404 - Not Found

This indicates that the resource you seek cannot be found, with a possibility that it may be temporarily unavailable. You are allowed to make more than one request to the same URL as it might be availed at a later time.

405 - Method Not Allowed

This error indicates that the wrong request was made, such as to use a PUT request for a read-only resource or to use a GET request to a resource which is supposed to present the data through POST.

406 - Not Acceptable

This indicates that the resource will not be availed by responding to the Accept headers in the client agent request.

407 - Proxy Authentication Required

Some resources need authentication to access them, which is why 'Proxy Authentication Required' error pops up. The resources will only be accessible when the proxy authenticates the request.

408 - Request Timeout

This indicates that the resource you requested did not respond in time, hence the 'Request Timeout'. It could also mean that the request was not understood or received by the target server due to infrastructural limitations and it would be prudent to repeat.

409 - Conflict

A conflict error code, that indicates the request, presents a conflict to the server, which could mean anything from request translation or a lack of clarity from the request.

410 - Gone

This error indicates that the resource being sought is permanently unavailable, and has been purged, and you should not look for it in the same location again, hence the 'Gone' response.

411 - Length Required

Indicates that the request did not show the length in the header as required by the resource. Correcting the length delimits can resolve this.

412 - Precondition Failed

The client agent may put conditions for the server, and should it fall short, this error will appear, indicating 'Precondition Failed'.

413 - Request Entity Too Large

This indicates that the request has exceeded the content volume the server is able to process, for example, requesting for a large file.

414 - Request-URI Too Long

The target server rejects the request due to the length of the URI.

415 - Unsupported Media Type

This code indicates that the resource you are attempting to upload is not supported by the target server.

416 - Requested Range Not Satisfiable

This indicates to you that the range of a file you requested from the server cannot be processed, and you might need to indicated a more palatable request

417 - Expectation Failed

This indicates to you that the server cannot respond to the expect request-header field.