I don't know apache in details so I can't help much, but just to remind you that by default the installation might have set up the server with dhcp. If you don't cancel it during install, that's what it does.
You need to configure static IP on your server so that you know which IP it has always. Look into /etc/network/interfaces. If the eth0 interface is something like:
you need to edit it to something like:
iface eth0 inet dhcp
Of course, use the IP address you want and it needs to be outside of the routers dhcp range. And for gateway use the routers IP. For nameservers you can use which ever you want, either some global ones or your ISP nameservers.
iface eth0 inet static
dns-nameservers x.x.x.x y.y.y.y
After changing the settings restart networking with:
After that you know which static IP your server has.
sudo /etc/init.d/networking restart