3. Your first virtual machine

Creating a VM is like choosing the parts of a computer: you decide how many cores, how much memory and how big a disk it has. The difference is that you can change your mind in seconds. This page shows you the process in VirtualBox and the reasons behind each choice.

How to work with this page

Estimated time: about 60 minutes of reading and 60 of practice.

1. Read sections 1 and 2. Then try the lab tab New VM wizard.
2. Read sections 3 and 4 and play with the tabs Resource allocator and Network modes.
3. Do Activities 2 and 3 with the real program.

1. VirtualBox

Oracle VirtualBox is a Type 2 hypervisor that runs on Windows, Linux and macOS with an Intel processor. The program is free and open source, and it has a graphical interface (the VirtualBox Manager) and a command-line tool (VBoxManage).

Apple Silicon Macs

Macs with Apple chips use an Arm processor, so the guests must also be Arm systems. If you use one at home, check the current VirtualBox documentation or use another product made for those Macs.

Installing it

Host	How
Windows	Download the installer from the official VirtualBox website, run it and accept the default options. Windows may ask you to install network drivers: accept.
Ubuntu / Debian / LliureX	sudo apt install virtualbox, or add the repository of Oracle to get the newest version.
macOS (Intel)	Download the disk image from the official website and run the installer. You may have to allow the software in the system security settings.

You can check that it works with:

VBoxManage --version

On the official website you will also find the Extension Pack, a separate add-on with extra features (for example USB 2.0 and 3.0 support). It has a different licence from VirtualBox: it is free for personal, educational and evaluation use, but companies need to pay for it. Do not install it unless you need it.

A proprietary alternative

VirtualBox is not the only desktop option. VMware Workstation (Windows and Linux) and Hyper-V (included in some editions of Windows) are proprietary products for the same job. The ideas in this unit are the same; only the menus change.

The Manager

When you open VirtualBox, the window shows a list of your VMs on the left and the details and buttons on the right. The most important buttons are New, Settings, Start and, when a VM is selected, the Snapshots tool.

2. Creating a VM

You need three things: the name and type of the operating system, the amount of hardware, and an installation source (usually an ISO image, a file that contains the content of an installation disc).

Steps in the New wizard (VirtualBox 7; the texts can change a little in other versions):

1. Name and operating system. Write a descriptive name, such as SOM-U2-Ubuntu. Choose the ISO file, or choose the Type and Version yourself. This choice is important: VirtualBox uses it to select safe default settings. If you choose Ubuntu (64-bit) and then install Windows, it may work badly.
2. Hardware. Choose the memory (RAM) and the number of processors (vCPU). Look at the requirements of the guest (table below). The Enable EFI option makes the VM start with UEFI instead of the old BIOS.
3. Virtual hard disk. Choose the size of the disk and whether the space is reserved now or later (see section 3).
4. Summary. Check everything and finish.

You will see the new VM in the list. If you did not choose an ISO in the first step, open Settings → Storage, click the empty optical drive and select the ISO file. Then press Start.

Unattended installation

Version 7 of VirtualBox can install some operating systems automatically when you give it an ISO (it asks for user and password in the wizard). It is handy, but in this unit you should do the installation yourself at least once, to see the installer. If you do not want it, tick Skip Unattended Installation.

How much hardware does the guest need?

Always read the requirements of the guest system, then check that your host can give them. These are typical minimum values of the official documentation of each system:

Guest	vCPU	RAM	Disk	Other
Ubuntu 24.04 Desktop	2	4 GB	25 GB
Windows 11	2	4 GB	64 GB	UEFI with Secure Boot and TPM 2.0

For Windows 11, VirtualBox 7 can provide a virtual TPM and EFI. When you choose the type Windows 11 (64-bit), check in Settings → System that EFI and the TPM are enabled.

A VM compared with a physical computer

In a physical computer you choose…	In VirtualBox you choose…
The processor and cores	System → Processor: number of vCPUs
The memory modules	System → Motherboard: base memory
The disk	Storage: a virtual disk file
The firmware	System → Motherboard: EFI on or off
The graphics card	Display: video memory
The network card	Network: adapter and mode
The DVD drive	Storage: an ISO file or the host drive
The boot order	System → Motherboard: boot order

Do not over-assign

The host keeps working while the VMs run, so you must leave resources for it.

• Memory: give the guest what it needs, not more. As a rule, leave at least 2-3 GB for the host and never let the total of the running VMs eat the memory of the host. VirtualBox shows a green zone and a red zone on the memory slider: stay in the green.
• vCPU: do not give a VM more virtual processors than your host has cores. In most labs 1 or 2 vCPUs are enough. More vCPUs do not always make a VM faster; they can even make it slower.
• Disk: the virtual disk will live on the host disk, so check the free space.

3. Virtual disks

The disk of a VM is a file on the host. VirtualBox supports several formats:

Format	Origin	Comment
VDI	VirtualBox	The default format
VMDK	VMware	Also used in OVA packages
VHD	Microsoft	Used by Hyper-V and Windows

The format is not the most important decision; the allocation is:

	Dynamically allocated	Fixed size
Space used on the host	Starts small and grows as the guest writes data	All the space is reserved at creation
Creation	Instant	It takes some time
Speed	Usually a bit lower, mostly while it grows	More predictable
Risk	The host can run out of space if all disks grow	None, the space is already yours
Typical use	Classroom and tests	Servers, benchmarks

A dynamic disk of 50 GB that contains 8 GB of data uses about 8 GB on the host. But the file does not shrink by itself when the guest deletes files. To recover the space you have to compact the disk with a special tool.

4. Networking

Each VM has up to four virtual network adapters. Each one works in a mode, which decides who the VM can talk to. This is probably the setting that gives most problems, so learn it well.

Mode	What it does	Typical use
NAT	The VM shares the IP address of the host. It can go out to the Internet, but nobody from outside can start a connection to it (unless you add port forwarding). Each VM is alone in its own private network.	Default. Browsing and updates
Bridged adapter	The VM connects to the real network through the network card of the host and gets its own address, as if it were another physical computer.	A server that other computers must reach
Host-only adapter	A private network that only includes the host and the VMs. No Internet.	A safe lab where the host connects to the VMs
Internal network	A private network that only includes the VMs with the same network name. Even the host is outside.	Fully isolated labs
NAT Network	Like NAT, but several VMs share one private network and can talk to each other and to the Internet.	VMs that must see each other and also go out

Who can talk to whom:

Mode	VM → Internet	VM → host	Host → VM	VM ↔ VM	Rest of LAN → VM
NAT	Yes	Yes	Port forwarding	No	Port forwarding
Bridged	Yes	Yes	Yes	Yes	Yes
Host-only	No	Yes	Yes	Yes	No
Internal	No	No	No	Yes	No
NAT Network	Yes	Yes	Port forwarding	Yes	Port forwarding

Bridged mode in schools and Wi-Fi

Bridged mode needs the network to give a new address to the VM. Many school and public networks do not allow it, and some Wi-Fi adapters do not work well in this mode. If bridged does not work, use NAT or a different network.

The MAC address

Every network card has a MAC address that must be unique in the local network. When a VM is cloned or imported from an appliance, it may keep the same MAC as the original. Two machines with the same MAC in the same network cause strange connection problems. VirtualBox lets you generate new MAC addresses when you clone or import. Always use it in a classroom.

5. The VM and the host

A VM is not “inside” the host in a magical way. It is a set of files and a process:

• The configuration file (.vbox) describes the virtual hardware.
• The disk files (.vdi, …) contain the data.
• When the VM runs, VirtualBox is a process on the host that uses a part of its CPU and memory.
• If you close the window with the VM running, you can choose to save the state, send the shutdown signal or power off. Powering off is like pulling the plug: it can damage the guest file system.

You can control a VM from the terminal with VBoxManage:

VBoxManage list vms                           # all registered VMs
VBoxManage list runningvms                    # VMs that are running
VBoxManage showvminfo "SOM-U2-Ubuntu"         # full configuration
VBoxManage modifyvm "SOM-U2-Ubuntu" --memory 2048 --cpus 2
VBoxManage startvm "SOM-U2-Ubuntu"

The VM must be powered off to change the memory or the number of processors.

Check what you have learned

1. A classmate creates a Windows 11 VM with 2 GB of RAM and a 20 GB disk and it does not install. Why?

   Answer

   It does not meet the minimum requirements of Windows 11 (4 GB of RAM, 64 GB of disk, UEFI and TPM 2.0). The VM needs more memory and disk and EFI and TPM enabled.

2. What is the difference between a dynamically allocated disk and a fixed-size disk? Which one needs more space on the host at the beginning?

   Answer

   A dynamic disk grows as the guest writes data; a fixed-size disk reserves all its space at creation. The fixed-size disk needs more space at the beginning.

3. You want two VMs to talk to each other, but you do not want the host or the Internet to reach them. Which network mode do you use?

   Answer

   Internal network, with the same network name in both VMs.

4. Your VM must be visible to other computers of the LAN with its own IP address. Which mode do you choose, and what can go wrong?

   Answer

   Bridged adapter. It may not work on school or public networks that do not give addresses to unknown devices, or with some Wi-Fi cards.

5. Why should you reinitialize the MAC address when you import an appliance?

   Answer

   Because many students import the same file, and identical MAC addresses in the same network cause conflicts. A new MAC makes each VM unique.

Glossary

English	Castellano
VirtualBox Manager	administrador de VirtualBox
ISO image	imagen ISO
virtual disk	disco virtual
dynamically allocated	de tamaño dinámico (reserva dinámica)
fixed size	de tamaño fijo
NAT	traducción de direcciones de red (NAT)
bridged adapter	adaptador puente
host-only adapter	adaptador solo anfitrión
internal network	red interna
MAC address	dirección MAC
port forwarding	redirección de puertos
vCPU	procesador virtual
power off	apagado forzado

Credits

This page adapts the SOM course notes by Moisés Mas Rocamora, Daniel Álvarez Santamaría and Jana Taboada León, licensed under CC BY-NC-SA 4.0. This page is shared under the same licence.