If you can’t ping between two guest OS in VirtualBox (two Debian 9.1 in my example) try:
Increase an ext4 partition with Gparted
In this post, I explain how to resize and increase the size of an ext4 partition in a Linux Machine.
How To Resize a Virtual Machine’s Disk in VirtualBox
If you need to resize a Virtual Machine’s Disk in VirtualBox, you can do it with one command line.