For a nice VM I needed a Linux server, but no need for a GUI, so went for CentOS.
Installed fine, but no networking. How do I install that? Turns out, it is installed, just disabled by default:
Find the line ONBOOT and change to yes:
UUID=some guid string
Then just restart networking at you should be up and running:
service network restart
Test by entering ifconfig and you should see the loopback address.
If you need to install Guest Additions on VirtualBox:
yum install gcc make kernel-devel
mount /dev/scd0 /media/cdrom