or less, does; these sections are by any means neither exhaustive nor complete. Error message A web server program may reply to a client request message with many kinds of error messages, anyway these errors are divided mainly in two categories: HTTP client errors, due to the type of request message or to the availability of requested web resource; HTTP server errors, due to internal server errors. When an error response / message is received by a client browser, then if it is related to the main user request (e.g. an URL of a web resource such as a web page) then usually that error message is shown in some browser window / message. URL authorization A web server program may be able to verify whether the requested URL path: can be freely accessed by everybody; requires a user authentication (request of user credentials, e.g. such as user name and password); access is forbidden to some or all kind of users. If the authorization / access rights feature has been implemented and enabled and access to web resource is not granted, then, depending on the required access rights, a web server program: can deny access by sending a specific error message (e.g. access forbidden); may deny access by sending a specific error message (e.g. access unauthorized) that usually forces the client browser to ask human user to provide required user credentials; if authentication credentials are provided then web server program verifies and accepts or rejects them. URL redirection A web server program may have the capability of doing URL redirections to new URLs (new locations) which consists in replying to a client request message with a response message containing a new URL suited to access a valid or an existing web resource (client should redo the request with the new URL). URL redirection of location is used: to fix a directory name by adding a final slash '/'; to give a new URL for a no more existing URL path to a new path where that kind of web resource can be found. to give a new URL to another domain when current domain has too much load. Example 1: a URL path points to a directory name but it does not have a final slash '/' so web server sends a redirect to client in order to instruct it to redo the request with the fixed path name. From: