You just need to add content in above and below to define Header content as above content and footer contain in below to your ContentPlaceHolder in Body tag.
Refer the below sample exapmle for your reference.
HTML
MasterPage.master
<%@ Master Language="C#" AutoEventWireup="true" CodeFile="MasterPage.master.cs" Inherits="MasterPage" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
<title></title>
<asp:ContentPlaceHolder ID="head" runat="server">
</asp:ContentPlaceHolder>
</head>
<body>
<form id="form1" runat="server">
<div id="dvheader">
<h1>
This is header content
</h1>
</div>
<div style="min-height: 500px">
<asp:ContentPlaceHolder ID="ContentPlaceHolder1" runat="server">
</asp:ContentPlaceHolder>
</div>
<div id="dvfooter">
<h1>
This is footer content
</h1>
</div>
</form>
</body>
</html>
Default.aspx
<%@ Page Title="" Language="C#" MasterPageFile="~/MasterPage.master" AutoEventWireup="true"
CodeFile="Default.aspx.cs" Inherits="_Default" %>
<asp:Content ID="Content1" ContentPlaceHolderID="head" runat="Server">
</asp:Content>
<asp:Content ID="Content2" ContentPlaceHolderID="ContentPlaceHolder1" runat="Server">
<br />
<h3>This is Default Page</h3>
<br />
This is test Line 1
<br />
This is test Line 2
<br />
This is test Line 3
<br />
This is test Line 4
<br />
This is test Line 5
<br />
This is test Line 6
<br />
This is test Line 7
<br />
This is test Line 8
<br />
This is test Line 8
<br />
This is test Line 10
</asp:Content>
Screenshot
