Where the viewstate is stored after the page postback?

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ViewState is stored in a hidden field on the page at client side. ViewState is transported to the client and back to the server, and is not stored on the server or any other external source.

In ASP.NET, after a page postback, the ViewState is stored in the __VIEWSTATE hidden field. This hidden field is automatically generated by ASP.NET and is sent to the client along with the page’s HTML. Upon postback, the contents of the ViewState are sent back to the server, allowing the server-side code to access and modify the state of the controls on the page.