Basically master pages enable you to create a consistent layout for the pages in your application. A ASP.Net master page defines the look and feel and standard behavior of your Main Page that you want for all of the pages (or a group of pages) in your application. later You can create individual content pages that contain the content you want to display.After creating Content Page when users request the content pages, they merge with the master page to produce output that combines the layout of the master page with the content from the content page.
you should navigate
MSDN-[ASP.NET Master Pages][
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Advantages of Master Pages in ASP.NET
* It allow you to centralize the common functionality of your pages so that you can make updates in just one place.
* It can make it easy to create one set of controls and code and apply the results to a set of pages.
* It give you fine-grained control over the layout of the final page by allowing you to control how the placeholder controls are rendered.
* It provide an object model that allows you to customize the master page from individual content pages.
Reference Link :- Why do I need ASP.NET Master Pages ?[
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