Why object's name modified when there is a master page in asp.net projects?

I am creating a asp.net pr开发者_如何学JAVAoject that have one master page for all pages. Why my object's name for example my div with ID="div1" is changed to ctl00_ div1 when this object is in master page?

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It is to ensure uniqueness of the id.

The Master Page can include content from multiple sources, so it prepends to each id to ensure it is unique across all sources. The value prepended represents the content section the content is within.