apache2's handling of IP version 6

[Satoru Otsubo <trnsnt@otip.jp>]

I'm Satoru Otsubo

I have some problems concerning apache2's handling of IP version 6.

My PC: Linux squeeze Japanese version clean install.
 I use the normal apache2 package prepared by debian maintainers, that is, I did not compile apache2.
 This is server use, that is, no Grafical User Interface.
(Current kernel version: 2.6.32-5-686)

I think apache2 is using IPv4-mapped addresses, that is, apache2 lets only IPv6's ports open, and apache2 handles both the IPv6 and IPv4 tcp packets from IPv6's ports.
Therefore I think apache2 normally has to listen IPv6's ports after booting PC.

But,
(i) After apache2 starts by booting my PC, apache2 listens only IPv4's 80 and 443 tcp ports.
(ii) Then after I execute
# /etc/init.d/apache2 restart,
apache2 becomes normal, that is, apache2 becomes to listen only IPv6's 80 and 443 tcp ports.

I have repeated many times.
But after booting, apache2 always listens only IPv4's 80 and 443 tcp ports.
And after 
# /etc/init.d/apache2 restart,
apache2 always becomes normal.

Why this phenoma happens ?
In what situations apache2 falls back to use IPv4's 80 and 443 tcp ports
instead of IPv6's 80 and 443 tcp ports ?
If I know these situations, I might let apache2 listen IPv6's 80 and 443 tcp ports after booting my PC, by repairing files, etc. of my PC.

Thanks