When an HTTP client requests a URL that points to a directory structure instead of an actual web page within the directory, the web server will generally serve a default page, which is often referred to as a main or "index" page.
A Uniform Resource Locator, commonly informally termed a web address is a reference to a web resource that specifies its location on a computer network and a mechanism for retrieving it.
A web server is a computer system that processes requests via HTTP, the basic network protocol used to distribute information on the World Wide Web.
In computing, a directory is a file system cataloging structure which contains references to other computer files, and possibly other directories.
A common filename for such a page is index.html, but most modern HTTP servers offer a configurable list of filenames that the server can use as an index.
If a server is configured to support server-side scripting, the list will usually include entries allowing dynamic content to be used as the index page even though it may be more appropriate to still specify the html output, as this should not be taken for granted.
Server-side scripting is a technique used in web development which involves employing scripts on a web server which produce a response customized for each user's request to the website.
An example is the popular open source web server Apache, where the list of filenames is controlled by the DirectoryIndex directive in the main server configuration file or in the configuration file for that directory.
Open-source software is computer software with its source code made available with a license in which the copyright holder provides the rights to study, change, and distribute the software to anyone and for any purpose.
A.htaccess file is a directory-level configuration file supported by several web servers, used for configuration of site-access issues, such as URL redirection, URL shortening, Access-security control, and more.
The Apache HTTP Server, colloquially called Apache, is the world's most used web server software.
It is possible to make do without file extensions at all, and be neutral to content delivery methods, and set the server to automatically pick the best file through content negotiation.
Content negotiation are mechanisms defined as a part of HTTP that make it possible to serve different versions of a document at the same URI, so that user agents can specify which version fit their capabilities the best.
If the server is unable to find a file with any of the names listed in its configuration, it may either return an error or generate its own index page listing the files in the directory.
It may also return a 403 Index Listing Forbidden.
A web server may or may not return a 403 Forbidden HTTP status code in response to a request from a client for a web page or resource to indicate that the server can be reached and understood the request, but refuses to take any further action.
Usually this option is also configurable.