WHAT IS PUBLIC CLOUD? 

The most recognisable model of cloud computing to many consumers is the public cloud model, under which cloud services are provided in a virtualised environment, constructed using pooled shared physical resources, and accessible over a public network such as the internet.

A public cloud is one based on the standard cloud computing model, in which a service provider makes resources, such as virtual machines (VMs), applications or storage, available to the general public over the internet. Public cloud services may be free or offered on a pay-per-usage model.

The main benefits of using a public cloud service are:

• it reduces the need for organizations to invest in and maintain their own on-premises IT resources;

• it enables scalability to meet workload and user demands; and

• there are fewer wasted resources because customers only pay for the resources they use.

Public cloud architecture

Public cloud is a fully virtualized environment. In addition, providers have a multi-tenant architecture that enables users -- or tenants -- to share computing resources. Each tenant's data in the public cloud, however, remains isolated from other tenants. Public cloud also relies on high-bandwidth network connectivity to rapidly transmit data.

Public cloud storage is typically redundant, using multiple data centers and careful replication of file versions. This characteristic has given it a reputation for resiliency.

Public cloud architecture can be further categorized by service model. Common service models include:

• software as a service (SaaS), in which a third-party provider hosts applications and makes them available to customers over the internet;

• platform as a service (PaaS), in which a third-party provider delivers hardware and software tools -- usually those needed for application development -- to its users as a service; and

• infrastructure as a service (IaaS), in which a third-party provider offers virtualized computing resources, such as VMs and storage, over the internet.

Public cloud pros and cons

In general, the public cloud is seen as a way for enterprises to scale IT resources on demand, without having to maintain as many infrastructure components, applications or development resources in house.
The pay-per-usage pricing structure offered by most public cloud providers is also seen by some enterprises as an attractive and more flexible financial model. For example, organizations account for their public cloud service as an operational or variable cost rather than as a capital or fixed cost. In some cases, this means organizations do not require lengthy reviews or advanced budget planning for public cloud decisions.
However, because users typically deploy public cloud services in a self-service model, some companies find it difficult to accurately track cloud service usage, and potentially end up paying for more cloud resources than they actually need. Some organizations also just prefer to directly supervise and manage their own on-premises IT resources, including servers.
In addition, because of the multi-tenant nature of public cloud, security is an ongoing concern for some enterprises evaluating the cloud. While public cloud providers offer security technologies, such as encryption and identity and access management tools, some organizations -- especially those with strict regulatory or governance requirements -- choose to keep workloads on premises.

Public cloud providers and adoption

The public cloud market is led by a few key players, including Amazon Web Services (AWS), Microsoft Azure and Google Cloud Platform. These providers deliver their services over the internet and use a fundamental pay-per-usage approach. Each provider offers a range of offerings oriented toward different workloads and enterprise needs

Examples of Public Cloud

The most salient examples of cloud computing tend to fall into the public cloud model because they are, by definition, publicly available. Examples include: 

• Cloud storage services
• Online software applications
• Cloud hosting, including website hosting
• Cloud based development environments 

To some extent they can be defined in contract to private clouds which ring-fence the pool of underlying computing resources, creating a distinct cloud platform to which only a single organisation has access. Public clouds, on the other hand, provide services to multiple clients using the same shared infrastructure. Public clouds are used extensively in offerings for private individuals who are less likely to need the level of infrastructure and security offered by private clouds. However, enterprises can still utilise public clouds to make their operations significantly more efficient, for example, with the storage of non-sensitive content, online document collaboration and webmail.

WHAT IS PRIVATE CLOUD? 

A private cloud is a particular model of cloud computing that involves a distinct and secure cloud based environment in which only the specified client can operate.

Private cloud refers to a model of cloud computing where IT services are provisioned over private IT infrastructure for the dedicated use of a single organization. A private cloud is usually managed via internal resources.

The terms private cloud and virtual private cloud (VPC) are often used interchangeably. Technically speaking, a VPC is a private cloud using a third-party cloud provider's infrastructure, while a private cloud is implemented over internal infrastructure.
Private clouds may also be referred to as enterprise clouds. 

A virtual private cloud (VPC) is an on-demand configurable pool of shared computing resources allocated within a public cloud environment, providing a certain level of isolation between the different organizations (denoted as users hereafter) using the resources. The isolation between one VPC user and all other users of the same cloud (other VPC users as well as other public cloud users) is achieved normally through allocation of a private IP subnet and a virtual communication construct (such as a VLAN or a set of encrypted communication channels) per user. In a VPC, the previously described mechanism, providing isolation within the cloud, is accompanied with a VPN function (again, allocated per VPC user) that secures, by means of authentication and encryption, the remote access of the organization to its VPC cloud resources. With the introduction of the described isolation levels, an organization using this service is in effect working on a 'virtually private' cloud (that is, as if the cloud infrastructure is not shared with other users), and hence the name VPC.

VPC is most commonly used in the context of cloud infrastructure as a service. In this context, the infrastructure provider, providing the underlying public cloud infrastructure, and the provider realizing the VPC service over this infrastructure, may be different vendors.

Techopedia explains Private Cloud

There is some controversy around the very idea of a private cloud. The central idea of cloud computing is an organization should not need to build out and manage computing infrastructure itself. By utilizing cloud vendors, an organization should lower costs while receiving services and applications that are on par or better than what could be done in-house. Given this, a private cloud would seem to be going backwards. An organization would still need to build out and manage the private cloud infrastructure and not get any benefits from the economies of scale that should come with cloud computing.

The flip side of this argument is that not all organizations can give up control to third-party vendors. A proponent of private clouds would argue there are still significant benefits to private clouds in the sense that a private cloud is a way to centralize large installations of IT infrastructure in a highly virtualized manner while avoiding exposure to the unknowns of an outside cloud vendor.

As with other cloud models, private clouds will provide computing power as a service within a virtualised environment using an underlying pool of physical computing resource. However, under the private cloud model, the cloud (the pool of resource) is only accessible by a single organisation, therefore providing that organisation with greater control and privacy.


Features and Benefits of Private Clouds:

1. HIGHER SECURITY AND PRIVACY
   While public cloud services offer a certain level of security, private clouds are the more secure option. This is achieved using distinct pools of resource with access restricted to connections made from one organisation’s firewall, dedicated leased lines and on-site internal hosting
2. MORE CONTROL
    As a private cloud is only accessible by a single organisation, that organisation will have the ability to configure and manage it inline with their needs to achieve a tailored network solution
3. COST AND ENERGY EFFICIENCY
    Implementing a private cloud model can improve the allocation of resources within an organisation by ensuring that the availability of resources to individual departments/business functions can directly and flexibly respond to their demand. They make more efficient use of the computing resource than traditional LANs and can also reduce an organisation’s carbon footprint
4. IMPROVED RELIABILITY
   Even where resources (servers, networks etc.) are hosted internally, the creation of virtualised operating environments means that the network is more resilient to individual failures across the physical infrastructure. Virtual partitions can, for example, pull their resource from the remaining unaffected servers
5. CLOUD BURSTING
   Some providers may offer the opportunity to employ cloud bursting, within a private cloud offering, in the event of spikes in demand. This service allows the provider to switch certain non-sensitive functions to a public cloud to free up more space in the private cloud for the sensitive functions that require it 

Private Cloud Characteristics

Private cloud services can vary considerably and so it is hard to define what constitutes a private cloud from a technical aspect. Instead such services are usually categorised by the features that they offer to their client. Traits that characterise private clouds include:

• Ring fencing of a cloud which has multiple clients accessing virtualised services, which all draw their resource from a distinct pool of physical computing. These may be hosted internally or externally and may be accessed across private leased lines or secure encrypted connections via public networks
• Additional security, which is ideal for enterprises that need to store and process private data or carry out sensitive tasks. For example, a private cloud service could be utilised by a financial company that is required to store sensitive data internally and who will still want to benefit from some of the advantages of cloud computing, such as on-demand resource allocation