redirect to login page if user try to copy the url in another sub tab

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ASP.NET

I am using forms authentication on ASP.NET. If I try to access a page by copying the query string and pasting it into the browser, it allows me access to the page.

How can this be prevented? I want the user to always have to login.

i am using web.config file below code it is not working...plz help me

<configuration>
  <connectionStrings>
    <add name="sugarcon" connectionString="server=(local);database=DSugar_Stores;User Id=sa;Password=adila" providerName="System.Data.SqlClient"/>
  </connectionStrings>
    <system.web>
            <compilation debug="true" targetFramework="4.0">
            <assemblies>
                <add assembly="System.Design, Version=4.0.0.0, Culture=neutral, PublicKeyToken=B03F5F7F11D50A3A"/>
                <add assembly="System.Web.Extensions.Design, Version=4.0.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35"/>
                <add assembly="System.Windows.Forms, Version=4.0.0.0, Culture=neutral, PublicKeyToken=B77A5C561934E089"/></assemblies>
        </compilation>

    <authentication mode="Forms">
      <forms name="Authen" protection="All" timeout="60" loginUrl="login.aspx"/>
    </authentication>
    <authorization>
      <deny users="?"/>
    </authorization>

    </system.web>
</configuration>

And i am using login button code

protected void btnlogin_Click(object sender, EventArgs e)
   {

       objrg = new registration();
       objrg.User_Name = txt_name.Text;
       objrg.Pass_Word = txt_pwd.Text;
       objrg.Department = ddl_dept.SelectedItem.Text;
       try
       {
           if (objdal.validateuser(objrg))
           {
               if (ddl_dept.SelectedItem.Text.Equals("Stores"))
               {
                 Response.Redirect("Home2.aspx");

               }
               else if (ddl_dept.SelectedItem.Text.Equals("Engineering"))
               {
                   Response.Redirect("Engineeringdepartment.aspx");
               }
               else
               {
                   Response.Write("<script> alert('select valid department....');</script>");
               }

           }
           else
           {
               Response.Write("<script> alert('invalid user name and password....');</script>");
           }
       }
       catch(Exception ex)
       {
           Response.Write("<script> alert('"+ex.Message.ToString()+"');</script>");
       }
       finally
       {

       }
   }