How Do You Get Oracle VirtualBox Guest Servers to Communicate with Each Other?

Problem scenario
You want to allow for TCP/IP connectivity between two VMs running Linux in Oracle VirtualBox. What do you do?

Solution
The network settings for the VMs should be as follows:

• The "Attached to:" setting hould be "Host-only Adapter"
• The "Adapter type:" setting should be "Paravirtualized Network"
• The "Cable connected" option should be checked.

Optional Reading
How Do You Connect to a VM in Oracle VirtualBox from a Desktop?