vhosts entry and visibility only behind firewall

Hi,
We have a number of machines behind a firewall. All port 80 traffic is
coming to a single internal IP where on the basis of vhosts
redirection the requests are sent forward. This is working perfectly
fine if a particular web server is seen externally or internally.
Is there anyway by which we can make some of the servers visible only
from inside by "Allow from 172.16.0.0/16" entry in the httpd-
vhosts.conf file as follows:
<VirtualHost *:80>
ServerAdmin (E-Mail Removed)
ServerName enigma.a.b.c
ErrorLog /var/log/httpd-webmail-error.log
ProxyRequests Off
<Proxy *>
Order deny,allow
Allow from 172.16.0.0/16
</Proxy>
ProxyPass / http://172.17.4.5/
ProxyPassReverse / http://172.17.4.5/
</VirtualHost>

As I expected it to work such that the server called enigma.a.b.c is
visible only from IP rages 172.16.0.0/16 (internal behind the
firewall) and the server is hosted on 172.17.4.5 but this entry is not
working, the site becomes visible from outside as well?
Any help is welcome.
Thanks
-Karmath