What is the minimal requirement to implement an IAAS Cloud?

The minimal requirement to implement are basically three things:
 OS to support hypervisor or a hypervisor.
- Preferably open-source software like Linux and Xen hypervisor
Networking topology and implementation.
- Public Network or Private network with Level 3 Switch
Selection of cloud model as per requirement or business.
- SaaS, Software as a Service
- PaaS, Platform as a Service
- CaaS, Communication as a Service