Practical Cloud Knowledge: Install, Configure, and Manage - Deploying and Managing a Cloud Infrastructure - page 196

powered through a net driver installed on the host OS through the hypervisor or as a sepa-
rate plug-in.
A virtual NIC is implemented via a combination of code within the virtual machine
itself and the underlying hypervisor. All the communications, the IRQs, and the virtual I/O
ports as well as the virtual DMA transfers between the VM's memory and the adapter are
semantically equivalent to that of a real network adapter.
Virtual Network
At a basic conceptual level, a virtual network and a real physical network are not that differ-
ent. The reason for this is that a virtualized network is made so that it acts as a real network
and the virtual machines or guest operating systems believe it to be a real network, so all of
the interfaces and protocols being used are the same or made to act very similarly with minor
differences. For example, instead of data going directly out of the virtual NIC to a virtual
router and then to the outside network, it actually goes through the hypervisor interface,
which manages everything in the virtualized environment, before it is passed to the physical
NIC for actual delivery. A virtual network is simply made to mimic an actual network.
In most networking scenarios, for a machine to connect to a network, it needs a network
interface card (NIC), which enables it to interface with a network via a switch or router
that at the same time connects multiple machines and networks to form a larger network.
A virtualized network is made up of the same hardware, and the virtual machine must also
have a network interface controller, in this case a virtual one, in order for the guest OS
to communicate with a virtual switch or router. So in the strictest sense, everything is the
same except the fact that the other is made entirely in virtual space.
A virtual network also consists of multiple virtual machines that can send and receive
data to and from one another. Each of those virtual machines represents an actual computer
in the network, and the virtual switch or router can connect these computers to each other
and other virtual networks as well as with the physical network.
Connecting a virtual NIC to a virtual network is the most straightforward method and
does not require an Ethernet interface on the host. This can be done by creating on the host a
virtual network that functions even if the host itself is not connected to any type of network
at all. The virtual network, as well as all the connected virtual machines, is running within
the host privately. If desired, the host can perform IP masquerading or routing to connect the
virtual network to an external network, even to a non-Ethernet one.
IP Address
The IP address is, and always should be, unique for each NIC or device, and that remains
true for virtual NICs. It is important to make the distinction that the VM is a totally sepa-
rate computer even though it is residing on the host computer, and as such it cannot simply
communicate using the host's network adapter unless it's configured as bridged, meaning
the virtual NIC is bridged with the physical network adapter of the host. Another way is to
connect the VM in a virtual network with its own virtual gateway connection to an external
physical network. Or the entire ecosystem of virtual devices and networks could exist pri-
vately within the host as a local network without connection to outside networks.