When visitors coming to a site request pages that the web server cannot find, the web server generates and displays a standard HTML page with an error message. You may want to create your own error pages and use them for your sites or individual virtual directories. The following error messages are the ones customized most often:

• 400 Bad File Request. Usually means the syntax used in the URL is incorrect (for example, uppercase letter should be lowercase letter; wrong punctuation marks).
• 401 Unauthorized. Server is looking for some encryption key from the client and is not getting it. Also, wrong password may have been entered.
• 403 Forbidden/Access denied. Similar to 401; a special permission is needed to access the site – a password or username, if it is a registration issue.
• 404 Not Found. Server cannot find the requested file. File has either been moved or deleted, or the wrong URL or document name was entered. This is the most common error.
• 500 Internal Server Error. Could not retrieve the HTML document because of server configuration problems.
• 503 Service Temporarily Unavailable. The site is temporarily unavailable due to maintenance.