This would help you, you shouls use two master pages, like name first master page Paren.master, name second master page Child.master.
Parent.master file:
<%@ Master Language="C#" %>
<HTML>
<BODY>
----------Some Tags---------
<asp:ContentPlaceHolder ID="MainContent"
runat="server" />
----------Some Tags---------
</BODY>
</HTML>
Child.master file:
<%@ Master Language="C#" MasterPageFile="Parent.master"%>
<asp:Content id="Content1" ContentPlaceholderID="MainContent" runat="server">
--------------Some Tags--------
<asp:ContentPlaceHolder ID= "ChildContent" runat="server" />
--------------Some Tags----------
<asp:ContentPlaceHolder ID="ChildFooter" runat="server" />
-----------Some Tags------------
</asp:Content>
Now just make a content page name child.aspx, give it the MasterPageFile=Child.master.
Child.aspx file:
<%@ Page Language="C#" MasterPageFile="Child.Master"%>
<asp:Content id="pageContent" ContentPlaceholderID="ChildContent" runat="server">
----------Some Tags---------
</asp:Content>
<asp:Content id="footerContent" ContentPlaceholderID="ChildFooter" runat=server>
----------Some Tags---------
</asp:Content>