By default ASP.net expects to find the Login.aspx page in the root directory of the application. When a request comes in for a protected resource and the user is not authenticated or authorized to access that resource it will automatically redirect the user to /Login.aspx so the user can login.
Personally I do not like to place all of my Membership pages in the root of the application. I like a little order to my madness and prefer to place the Login.aspx page in a folder like Registration which is publicly accessible. But if you try to place the Login.aspx page in any folder except the root of the application you will see an error stating the page cannot be found when you try to access a protected resource. This is because of the default settings discussed above.
To fix this you simply need to alter the default setup for Forms Authentication by adding some configuration entries to your web.config as follows.

    <forms loginUrl="/Registration/Login.aspx" 
           protection="All" 
           timeout="30" 
           name="AppNameCookie" 
           path="/FormsAuth" 
           requireSSL="false" 
           slidingExpiration="true" 
           defaultUrl="default.aspx"
           cookieless="UseCookies"
           enableCrossAppRedirects="false"/>

• loginUrl: allows you to alter the default location of your Login.aspx page

You can look up the rest of the attributes by going to the MSDN documentation for ASP.net Membership.