Infrastructure as a Service is one of the three delivery methods of cloud computing (the other two are Platform as a Service and Software as a Service).

Users of IaaS have the expertise to maintain operating systems and applications, but don't wish to purchase server, storage and networking hardware and a datacenter to house the hardware. The cloud provider provides these services from a shared pool. The cloud user will then use the virtual machines to fulfill their computing requirements and may install their own operating system and will install their own applications on the virtual machines.

Some characteristics of IaaS:

• Elasticity: you can provision (add) or de-provision (remove) cloud instances to scale up or down your environment.
• Firewalls: may be host-based on each cloud instance (virtual machine - see below) and/or an Internet firewall that offers the customers restrictions on who can access their cloud servers.
• Multi-tenancy: The cloud servers are hosted on a shared infrastructure. This means that your cloud instances co-exist on the same hardware as another customer's cloud instances. To understand multi-tenancy, think of an apartment building (or block of flats). The renters/tenants have their own apartment, but share an elevator or stairway, foundation and roof. The owner of the building rents out apartments as needed and is responsible for the plumbing etc while each tenant is responsible for their own furniture and interior decorations. Similarly: an IaaS customer is responsible for their own applications, the hosting provider is simply providing the infrastructure.(also see segmentation below).
• Networking: is provided at the TCP/IP layer and is usually simple networking between the customer's cloud servers and Internet access from the customer's access point(s).
• Segmentation/Isolation: While resources are drawn from a shared pool and cloud tenants co-exist, there are levels of segmentation to isolate one cloud tenant from another, just like there are walls to segment or isolate one apartment from another.
• Storage: is usually delivered as raw (block) storage.
• Utility billing: The cloud provider will bill the cloud-user for the resources used. Infrastructure as a Service is akin to a utility company providing and billing for electricity, water and natural-gas. You share electricity with everyone on the power grid provided by the power station, and only pay for what you use.
• Virtual Machines: The servers, also called "cloud instances", are delivered to customers as virtual machines. A virtual machine is a server or workstation, with operating system and applications that appears to the user as a physical server. For more on virtual machines, see wikipedia here

Infrastructure as a Service is typically offered in three forms:

1. Private cloud also called on-premise
2. Public cloud
3. Hosted private cloud

Private Cloud/ on-premise IaaS
An organization can build a private IaaS cloud and then provide infrastructure services to their internal departments or partners.

To build a private IaaS cloud, you need virtualization software to run a hypervisor.

Examples of hypervisor software are:

• KVM - Kernel-based Virtual Machine is available with most Linux distributions since kernel build 2.6 and is available as open-source software. Redhat is an example of KVM virtualization
• Xen - is another Linux based open-source hypervisor. Citrix licensed Xen and offers it as a commercial product. Rackspace and Amazon Web Services run Xen.
• VMware - the commercial company has an open-source offering as well.
• Microsoft Private Cloud

Once you have a virtualization or hypervisor layer, then you need cloud software to provide the on-demand and elasticity features of cloud computing as a Service.

Examples of IaaS cloud software are:

• Eucalyptus. Eucalyptus open-source and is available commercially.
• OpenStack. OpenStack is open-source and licensed as a commercial product by Rackspace, Nebula and many others

I will write tutorials in future blog posts, illustrating the steps to build an open-source private cloud IaaS offering.

Hosted private cloud
Hosted private cloud is a cloud offering for an organization's exclusive private use, but is not hosted in the organization's data center, but rather hosted by a hosting company. This provides an organization with the benefits of a private cloud, without the capital cost of setting it up.

Some hosted private cloud providers:

• Rackspace
• Gogrid
• Hosting.com

Public Cloud IaaS
Public cloud IaaS provides cloud instances to anyone with Internet access. Public IaaS is used by individuals or corporations who don't wish to use capital expenditure to purchase hardware and associated operating expenses. Unlike a hosted private IaaS cloud, the interface to a public IaaS cloud is open to anyone and access is restricted only by firewall and authentication credentials.

Some examples of public cloud:

• Amazon Elastic Compute Infrastructure (EC2), and Amazon Simple Storage Service (S3) - also called AWS (Amazon Web Services)
• Gogrid
• Rackspace
• SAVVIS
• Terremark
• Virtustream

Risks with Public cloud
Just as there are risks with living in an apartment building, example your upstairs neighbour's shower leaking into your living room, or neighbours eavesdropping on neighbour's conversations through the walls, so there are risks with public cloud. A neighbouring cloud customer could accidentally (or nefariously) gain access to your data on the shared storage or even view data on your cloud instance or access it directly.

Cloud management
The cloud-user may use a cloud management platform to manage public and private clouds. Cloud management is used to provision, automate, secure and govern public or private cloud instances.

Some examples of cloud management platforms:

• Rightscale
• Enstratus

• Introduction to Cloud Computing for Newbies