ASP.Net 1.1 Web Pages in Iframe control

I am Using ASP.NET 1.1 . I have used iframe in Index.aspx to show webpages in my project like Master page in .net 2.0.
Here is one valid url which works properly
http://intranet/Index.aspx?menu=Employee&submenu=PhoneBook.aspx&Tab=0
Every thing is working properly, but when I edit the url & type the name of Webpage like
 http://intranet/PhoneBook.aspx
browswe shows the web page but it doesen't show in Index.aspx page's iframe. I need to restrict the web page to be shown only in Index.aspx page's iframe so that it will look like MasterPage & ContentPage in .Net 2.0.
Note: I am using ASP.NET 1.1
Will anyone give me the guideline to solve this problem.
Thanks in advance.
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