[TYPO3] Access not inherited by sub pages

[Bing Du]

Hi,

Typo3 4.0.6

The page tree is like the following.

parent page
  subpage1 (pid: 111)
  subpage2
  subpage3

Login should be required for accessing all the pages.  I assigned certain
groups to 'parent page' in its access box in the page properties.  All the
subpages are hidden and have no extra access setting.  I anticipated all
the subpages should be protected by the access setting of the parent page,
that is, the subpages should be accessible to the same certain groups. 
However, looks like if I know its pid or realurl, I can access any
subpages without login at all.  For instance, I have no problem seeing
subpage1's content by going directly to
http://www.some.edu/index.php?id=111 or
http://www.some.edu/parent-page/subpage1.  Why is that?

Thanks in advance,

Bing