Infrastructure as a Service
IaaS provides a customer full control of virtualized hardware, memory, and storage. Servers, firewalls, and routers are provided, and a network topology can be configured by the tenant.
This granular type of environment control is not provided by PaaS or SaaS.
Amazon Web Services, is an IaaS. Although, today they offer PaaS, and SaaS.
This is where multiple tenants use the same service over the cloud. They all share the same hardware, software, and services.
You pay as you go.
If you want to use the server for 40 hours a week, or 8 hours a day, you will only be charged for that amount.
Amazon Web Services is an example of a public cloud. It can also be a private cloud, depending on your needs.
The above graphic is an example of a public cloud with an infrastructure as a service.
The customer is responsible for the security of everything "in" the cloud.
The customer has full responsibility over the security of the data in a public cloud.
Amazon is responsible for securing the cloud itself i.e. the hardware, servers, underlying hypervisor maintenance, or the actual physical data center where everything is housed.
Amazon has full responsibility over the security of the hardware.
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