In what instances do file extensions not appear in website URLs?

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File extensions do not appear in website URLs when the website uses dynamically generated pages or server-side scripts to deliver content. In such cases, the URL only shows the page or script name, along with any necessary parameters, but does not include the actual file extension. Some examples of such server-side technologies include PHP, ASP, Ruby on Rails, and Django. Additionally, some websites may use URL rewriting techniques to create user-friendly URLs without file extensions.