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Network Virtualization

Networking is one of the fundamental elements of cloud computing and also one of the hazards to users of cloud computing. Network performance degradation and instability can greatly affect the consumption of cloud resources. Applications that are relatively isolated or are specially designed to deal with network disruptions have an advantage running in the cloud.

From a different perspective, network resources can be virtualized and used in cloud computing just as other resources are. In this section, we first discuss basic use of IP addresses in a cloud context and then cover virtual networks.

Delivery of cloud services takes place over networks at different levels using different protocols. This is one of the key differences in cloud models. In PaaS and SaaS clouds, delivery of services is via an application protocol, typically HTTP. In IaaS, cloud services can be delivered over multiple layers and protocols—for example, IPSec for VPN access and SSH for command-line access.

Management of the different layers of the network system also is the responsibility of either the cloud provider or the cloud consumer, depending on the type of cloud. In a SaaS model, the cloud provider manages all the network layers. In an IaaS model, the cloud consumer manages the network levels, except for the physical and data link layers. However, this is a simplification because, in some cases, the network services relate to the cloud infrastructure and some services relate to the images. The PaaS model is intermediate between IaaS and SaaS.

Table 1.5 summarizes the management of network layers in different cloud scenarios.

Table 1.5. Management for Network Layers

OSI Layer

Example Protocols




7 Application





6 Presentation





5 Session





4 Transport





3 Network





2 Data link

Ethernet, Fibre Channel




1 Physical

Copper, optical fiber




This table is a simplification of the many models on the market. However, it shows that an IaaS gives cloud consumers considerably more flexibility in network topology and services than PaaS and SaaS clouds (but at the expense of managing the tools that provide the flexibility).

IP Addresses

One of the first tasks in cloud computing is determining how to connect to the virtual machine. Several options exist when creating a virtual machine: system generated, reserved, and VLAN IP address solutions. System-generated IP addresses are analogous to Dynamic Host Control Protocol (DHCP)–assigned addresses. They are actually static IP addresses, but the IaaS cloud assigns them. This is the easiest option if all you need is a virtual machine that you can log into and use.

Reserved IP addresses are addresses that can be provisioned and managed independently of a virtual machine. Reserved IP addresses are useful if you want to assign multiple IP addresses to a virtual machine.

IPv6 is an Internet protocol intended to supersede IPv4. The Internet needs more IP addresses than IP v4 can support, which is one of the primary motivations for IPv6. The last top-level block of IPv4 addresses was assigned in February 2011. The Internet Engineering Task Force (IETF) published Request for Comments: 2460 Internet Protocol, Version 6 (IPv6), which was the specification for IPv6 in 1998. IPv6 also provides other features not present in IPv4. Network security is integrated into the design of IPv6, which makes IPSec a mandatory part of the implementation. IPv6 does not specify interoperability with IPv4 and essentially creates an independent network. Today usage rates of IPv6 are very low, and most providers operate in compatibility/tolerance mode. However, that could change.

Network Virtualization

When dealing with systems of virtual machines and considering network security, you need to manage networks. Network resources can be virtualized just like other cloud resources. To do this, a cloud uses virtual switches to separates a physical network into logical partitions. Figure 1.13 shows this concept.

Figure 1.13

Figure 1.13. Physical and virtual networks in a cloud

VLANs can act as an extension of your enterprise’s private network. You can connect to it via an encrypted VPN connection.

A hypervisor can share a single physical network interface with multiple virtual machines. Each virtual machine has one or more virtual network interfaces. The hypervisor can provide networking services to virtual machines in three ways:

  • Bridging
  • Routing
  • Network address translation (NAT)

Bridging is usually the default mode. In this mode, the hypervisor works at the data link layer and makes the virtual network interface externally visible at the Ethernet level. In routing mode, the hypervisor works at the network layer and makes the virtual network interface externally visible at the IP level.

In network address translation, the virtual network interface is not visible externally. Instead, it enables the virtual machine to send network data out to the Internet, but the virtual machine is not visible on the Internet. Network address translation is typically used to hide virtualization network interfaces with private IP addresses behind a public IP address used by a host or router. The NAT software changes the IP address information in the network packets based on information in a routing table. The checksum values in the packet must be changed as well.

NAT can be used to put more servers on the network than the number of virtual machines you have. It does this by port translation. This is one reason IPv6 is still not in wide use: Even though the number of computers exceeds the number of IP addresses, you can do some tricks to share them. For example, suppose that you have a router and three servers handling HTTP, FTP, and mail, respectively. You can assign a public IP address to the router and private IP addresses to the HTTP, FTP, and mail servers, and forward incoming traffic (see Table 1.6).

Table 1.6. Example of Network Address Translation

Public IP


Private IP

80, 443 (HTTP server) (router)

21 (FTP server)

25 (mail server)

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