Question about networking in CentOS-guest running under VirtualBox

Question

virtual box

the situation is as follows:
1. in the VirtualBox settings of the network, I disable DHCP (the default VBox HostOnly adapter remains under 192.168.56.1)
2. I set the guest CentOS 64 minimal
3. in ifconfig some muddy eth0 is started by default (for a virtual machine it is NAT) which does not lets the host into the WAN, in other words, the Internet does not plow.
4. I do ifdown eth0 && ifup eth0 - and WAN (Internet) in the guest OS starts working!
What kind of nonsense? Of course, I can write this into the startup script, but I would like to understand if this is a problem of VirtualBox, host, guest, or crooked hands?