You'll not be able to directly access your guest virtual machine in Oracle's VirtualBox if it's under a NAT network. It is because it's in a completely different private network than the host machine.
For this, you'll need to forward the network traffic from a specific port of your host IP to a port on your VM's IP. It will allow you to access your virtual machines from your host machine via protocols such as SSH or RDP.
To do this, you'll need to configure port forwarding for your VirtualBox VMs in the NAT network.
In this example, TCP packets on port 2222 in the host machine will forward to port 22 of the guest machine whereby 10.0.2.15 is the default IP for guest VM under NAT network.
It's more convenient to leave the Host IP field blank which will default to 127.0.0.1 and Guest IP defaults to whatever the IP address is assigned to the guest VM. This simplifies the configuration and avoid the issue of the Guest IP changing.
user@host:~ $ ssh -p2222 firstname.lastname@example.org Warning: Permanently added '[127.0.0.1]:2222' (ECDSA) to the list of known hosts. email@example.com's password: Welcome to Ubuntu 18.04 LTS (GNU/Linux 4.15.0-20-generic x86_64) * Documentation: https://help.ubuntu.com * Management: https://landscape.canonical.com * Support: https://ubuntu.com/advantage System information as of Wed Sep 5 16:52:59 +08 2018 System load: 0.0 Processes: 91 Usage of /: 18.8% of 8.80GB Users logged in: 1 Memory usage: 6% IP address for enp0s3: 10.0.2.15 Swap usage: 0% 138 packages can be updated. 72 updates are security updates. Last login: Wed Sep 5 16:47:16 2018 from 10.0.2.2 To run a command as administrator (user "root"), use "sudo <command>". See "man sudo_root" for details. user@host:~$
The above example uses SSH as an example and SSH by default runs on port 22. In this case, the host's port 2222 forwards to the guest's port 22.
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