ITM 301 Chapter Notes - Chapter 10: Virtual Network Computing, Hypervisor, Network Virtualization

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Chapter 10 Summary
Virtualization is the emulation of a computer, operating system
environment, or application on a physical system. One host computer can
support many VMs (virtual machines). VMs, also called guests, share the
physical computer’s CPU, hard disk, memory, and network interfaces. Yet
each functions independently, with its own logically defined hardware
resources, operating system, applications, and network interfaces.
VMs exist as files on the hard disk of the physical computer. These files
contain the operating system, applications, data, and configurations for the
The software that allows you to define VMs and manages resource
allocation and sharing among them is known as a virtual machine
manager, or, more commonly, a hypervisor. Hypervisors are part of all
virtualization programs, of which VMware is the most popular. Other
virtualization programs include Hyper-V, KVM (Kernel-based Virtual
Machine), and VirtualBox.
Advantages of virtualization include efficient use of resources; cost and
energy savings, which can contribute to sustainability; fault and threat
isolation; and simple backups, recovery, and replication.
Potential disadvantages of virtualization include compromised
performance, increased complexity, increased licensing fees, and a single
point of failure.
To connect to a network, a virtual machine requires a virtual adapter, or
vNIC (virtual NIC). Just like a physical NIC, a vNIC operates at the Data
Link layer and provides the computer with network access. Each VM may
have several vNICs, no matter how many NICs the host machine has.
A virtual switch is a logically defined device that operates at the Data Link
layer. Ports on a virtual switch connect vNICs with a network, whether
virtual or physical, through the host’s physical NIC. A virtual switch allows
VMs to communicate with each other and with nodes on a physical LAN or
Virtual switches reside in the RAM of the physical computers that act as
their hosts, while their configuration resides in a separate file on the host’s
hard disk. One host can support multiple virtual switches. The hypervisor
controls the virtual switches. In Hyper-V, a virtual switch is called a virtual
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When you configure a vNIC, you are asked to identify what type of network
connection or networking mode the adapter will use. The most frequently
used network connection types include bridged, NAT, and host-only.
In bridged mode, a vNIC accesses a physical network using the host
machine’s NIC and obtains its own IP address, default gateway, and
netmask from a DHCP server on the physical LAN. When connected using
bridged mode, a VM appears to other nodes as just another client or
server on the network. Bridged mode is best used for VMs that must be
available at a specific address, such as mail servers or Web servers.
In the NAT networking mode, a VM relies on the host machine to act as a
NAT device. It obtains IP addressing information from the DHCP service in
the host’s virtualization software. A vNIC operating in NAT mode can still
communicate with other nodes on the network and vice versa. However,
other nodes communicate with the host machine’s IP address to reach the
VM; the VM itself is invisible to other nodes. NAT networking mode is
appropriate for clients that do not need to be addressed directly and at a
specific address by other nodes.
In host-only networking mode, VMs on one host can exchange data with
each other and with their host, but they cannot communicate with any
nodes beyond the host to create an isolated, all-virtual network. In host-
only mode, as in NAT mode, VMs use the DHCP service in the host’s
virtualization software to obtain IP address assignments. Host-only
networking mode is best used for test environments.
A virtual appliance is an image that includes the appropriate operating
system, software, hardware specifications, and application configuration
necessary for the package to run properly. Popular uses for virtual
appliances include firewall and other security measures, network
management, e-mail solutions, and remote access.
VLANs are subnets logically defined on a physical switch that allow
network administrators to separate network traffic for better performance,
customized address management, and security. On a network that uses
virtual machines, VLANs will typically include those VMs.
To add VMs to a VLAN defined on a physical network, you modify a virtual
switch’s configuration. In other words, VMs are not added to a preexisting
VLAN on the physical switch that manages that VLAN
In VMware, vNICs are associated with port groups, which can be assigned
to VLANs. Multiple vNICs can be assigned to a single port group. Also, a
single vNIC can be assigned to multiple port groups. In other virtualization
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programs, vNICs are assigned to VLANs by associating them directly with
a VLAN number or with a bridge that is, in turn, associated with a VLAN.
For VLANs to include vNICs, the host machine’s physical NIC must be
configured to operate in trunking mode. In other words, it must be capable
of carrying the traffic of multiple VLANs. Virtualization software refers to the
physical NIC, acting as an interface for VLANs, as a trunk.
As a remote user, you can connect to a LAN or WAN in one of several
ways: dial-up networking, connecting to a remote access server, remote
virtual computing, or through a VPN (virtual private network).
Dial-up networking involves a remote client dialing into a remote access
server and connecting via a PSTN, X.25, or ISDN connection. The client
must run dial-up software to initiate the connection, and the server runs
specialized remote access software to accept and interpret the incoming
Remote access servers accept incoming connections from remote clients,
authenticate users, allow them to log on to a LAN or WAN, and exchange
data by encapsulating higher-layer protocols, such as TCP and IP in
specialized protocols such as PPP. The Microsoft RRAS (Routing and
Remote Access Service) is the remote access software that comes with
the Windows operating systems.
To exchange data, remote access servers and clients must communicate
through special Data Link layer protocols, such as PPP (Point-to-Point
Protocol) or SLIP (Serial Line Internet Protocol), that encapsulate higher-
layer protocols, such as TCP and IP. PPP is the preferred protocol. When
PPP is used on an Ethernet network, as is the case with most modern
broadband Internet connections, it is called PPP over Ethernet, or PPPoE.
Remote virtual computing uses specialized client and host software to
allow a remote user to connect via modem to a workstation that is part of a
LAN. Once the connection is made, the remote user can control that
workstation, performing functions just as if she were directly connected to
the LAN.
Remote Desktop is a remote virtual computing client and server package
that comes with Windows operating systems. VNC (Virtual Network
Computing) refers to an open source system that enables a remote client
(or viewer) workstation to manipulate and receive screen updates from a
host. ICA (Independent Computing Architecture) provides the basis for
Citrix Systems’ proprietary remote virtual computing software.
By creating a VPN (virtual private network), you can construct a WAN from
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