I believe you can set the interface that apache listens on in httpd.conf file. For example:
# Change this to Listen on specific IP addresses as shown below to
# prevent Apache from glomming onto all bound IP addresses (0.0.0.0)
In the above example it would set apache to listen on 192.168.1.100 port 80. This makes the assumption that 192.168.1.100 is your inside interface.