KVM/Virtualbox Virtualization with no physical network

I recently tried KVM as virtualization platform. I also use VirtualBox occasionally.

In every instance of deploying guest OSes, I always do one of these two: 1) set up network as NAT, guest not needed to be visible outside the virtualized environment. 2) set up network as bridged, guest needed to be visible. IP usually handed by DHCP from DHCP server in the physical network.

My question is, how to set up the environment so that the host OS and guest OSes can see each other given the condition that no physical network present (the host OS is not coected to any network) thus, no DHCP for both guests and host in option 2, or no DHCP for host in option 1 (virtual OSes will do just fine amongst themselves). I am interested in KVM on RHEL/CentOS, but if there is information for VirtualBox, it would be very nice.

This is actually so that I can set the host os as a platform (mainly CentOS/RHEL) and then I can set the guest as Fedora or Windows, and allows me to tinker deep to the inside of the OS without changing the host OS. For example, in CentOS I should add EPEL to acquire nginx and test something. However I don't need the EPEL to be constantly be contacted during yum update. Keeping base/host OS as minimal as possible (using the default repository whenever possible) and using the guests as testing ground (including adding new repos) therefore adds convenience that whenever I issue yum update on the host OS, it only pulls from the default repository. I don't need to update the guest OSes since it will sometimes be rebuilt from zero for practice.