HTTP Status Codes

The first line of an HTTP response message is the status line, consisting of the protocol version followed by a numeric status code and its associated textual phrase. This line indicates the result of the request. The codes fall into five different groups, according to the first digit of the code number.

1 - Informational

100 Continue - The response indicates that the request headers were received and that the client should continue sending the body.

101 Switching Protocols - The server understands and is willing to comply with the client request for a change in the application protocol being used on this connection.

2 - The request has succeeded

200 OK - Indicates that the request was successful and the response body contains the result of the request.

201 Created - Successful PUT request.

202 Accepted - The request has been accepted, but the processing has not been completed.

203 Non-Authoritative Information.

204 No Content - Server does not need to return an entity-body, might update meta information.

205 Reset Content - Server fulfilled request, the user agent should reset document view.

206 Partial Content - Partial GET request.

3 - The client is redirected to a different resource

300 Multiple Choices - User select a preferred representation, request is redirected to the location.

301 Moved Permanently - Redirects the browser permanently to a different URI.

302 Found - The requested resource resides temporarily under a different URI.

303 See Other - The response to the request can be found under a different URI.

304 Not Modified - Instructs the browser to use its cached copy.

305 Use Proxy - Resource must be accessed through the proxy given by the location field.

306 Unused - Code reserved.

307 Temporary Redirect - The requested resource resides temporarily under a different URI.

4 - Client Error

400 Bad Request - Invalid HTTP request.

401 Unauthorized - The request requires user authentication. If the request already included authorization credentials, then 401 indicates that authorization has been refused for those credentials.

402 Payment Required - Reserved for future use.

403 Forbidden - No one is allowed to access the requested resource, regardless of authentication.

404 Not Found - Requested resource does not exist.

405 Method Not Allowed - Method used in the request is not supported for the specific URI.

406 Not Acceptable - Resource only capable of generating response entities which have content characteristics not acceptable according to the accept headers sent in the request.

407 Proxy Authentication Required - Similar to 401, but indicates that the client must first authenticate itself with the proxy.

408 Request Timeout - Client did not produce a request within the time that the server was prepared to wait.

409 Conflict - With the current state of the resource.

410 Gone - Requested resource is no longer available at the server.

411 Length Required - Server refuses to accept the request without a defined content length.

412 Precondition Failed - The precondition given in one or more of the request-header fields evaluated to false when it was tested on the server.

413 Request Entity Too Large - Indicates that the body of the request was too large for the server.

414 Request URI Too Long - Similar to 413, this request indicates that the URI sent in the request is too long for the server to handle.

415 Unsupported Media Type - Format not supported by the requested resource for the requested method.

416 Requested Range Not Satisfiable.

417 Expectation Failed - The expectation given in an expect request header field could not be met by this server.

5 - Server Error

500 Internal Server Error - Indicates that the server encountered an error fulfilling the request.

501 Not Implemented - The server does not support the functionality required to fulfill the request.

502 Bad Gateway - While acting as a gateway or proxy, the server received an invalid response from the upstream server.

503 Service Unavailable - Although the server itself is functioning, the application accessed via the server is not responding.

504 Gateway Timeout - The server, while acting as a gateway or proxy, did not receive a timely response from the upstream server.

505 HTTP Version Not Supported - The server does not support the HTTP protocol version that was used in the request.

URI, URL, URN:
URI → Uniform Resource Identifier
URL → Uniform Resource Locator
URN → Uniform Resource Name

The URI is classified as a locator, a name, or both. A URL is a subset of the URI protocol (http://, ftp://, mailto:). All URLs are URIs. In technical documentation the more correct term URI is used.

The URI Standard (RFC3986) clarifies the distinction in section 1.1.3, "URI, URL, and URN":
A URI can be further classified as a locator, a name, or both. The term "Uniform Resource Locator" (URL) refers to the subset of URIs that, in addition to identifying a resource, provide a means of locating the resource by describing its primary access mechanism (e.g., its network "location"). The term "Uniform Resource Name" (URN) has been used historically to refer to both URIs under the "urn" scheme [RFC2141], which are required to remain globally unique and persistent even when the resource ceases to exist or becomes unavailable, and to any other URI with the properties of a name.