The way VirtualBox configures networking in guests is by not only emulating the networking hardware associated with the VM, but it also creates a layer in between Host
and Guest
where it emulates a router.
The "Router" that VirtualBox emulates has settings which can be configured using the Virtual Machine Manager
and when set to use its own DHCP server, the emulated router pushes settings to the client, as if the client was situated on a physical network using DHCP.
When toggling the Guest
On/Off switch, it only applies to the Guest
as it would if you were to setup CentOS on a physical machine, enabling and disabling the interface.