I had the same issue with Oracle running on the Virtual Machine provided by Denodo for the Lab exercises, and solved it in the following manner:
* Make sure you followed the steps in the "Denodo Training Quickstart Virtual Machine - user manual"
* Login to the virtual machine
* Check the ip address of the virtual machine by entering the statement "ip addr". In my case it was 192.168.56.102 . Denodo has configured it to be 192.168.100.100, but somehow that didn't work and the 192.168.56.102 ip was given to the machine
* Go to the "Host Network Manager" of VirtualBox and enter the following settings:
* IP: 192.168.56.1 (this has to be the same IP as the IP in the Virtual machine, but ending with 1)
* DHCP: 192.168.56.2 (same IP as above, but now ending with 2)
* Lowest address: 192.168.56.3 (same IP as above, but now ending with 3)
* Highest address: 192.168.56.254 (same IP as above, but now ending with 254)
* Stop and start the virtual machine
* You are now able to reach Oracle by using jdbc:oracle:thin:@192.168.56.102:1521:xe