Using Server Side Includes
Server Side Includes statements can only be used inside a Server Side Includes File. A server side includes file has a .SHTML file extension.
Normal HTML files end with an extension of ".HTM" or ".HTML". A server side include file is simply an HTML file that may include server side include statements. The server identifies these types of files by the file name extension ".SHTML". When the server is sending a file to the client with this extension, it knows to search for server side include statements in the file before sending it. Therefore to use server side includes, you should first rename the HTML file that will contain the statements to a ".SHTML" file. Just because the file name indicates it is a Server Side Includes file, does not mean it must contain Server Side Includes statements, but you would not want to change your HTML extensions to SHTML extensions unless necessary due to the extra burden placed on the server.
To use Server Side Includes, one or more lines like the following must be included on the HTML page.