What Is IaaS?
A cloud provider offers a service where customers can provision virtual machines, storage, and networks on-demand through a web portal. The customer is responsible for patching the guest operating system. Which cloud service model best describes this offering?
Quick Answer
Infrastructure as a Service is the model here because the customer both provisions the foundational resources — virtual machines, storage, networking — and retains responsibility for patching the guest operating system running on top of them. That second detail is the deciding factor: in IaaS, the provider's responsibility stops at the physical hardware, hypervisor, and underlying network, while everything from the OS upward belongs to the customer, including security updates, configuration, and installed software. If the provider were instead responsible for OS patching, the scenario would describe PaaS, where the platform layer is managed for the customer and only application code and data remain the customer's concern. On this exam, OS patching responsibility is one of the most reliable single details for distinguishing IaaS from PaaS in a scenario question — whichever party is described as handling operating system updates tells you immediately which service model the question is describing.
⚠ Common exam trap
Candidates often confuse PaaS with IaaS because both involve 'platform' or 'infrastructure' terms, but the key differentiator is who patches the guest OS — in PaaS, the provider patches it, while in IaaS, the customer does.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Infrastructure as a Service (IaaS)
(Infrastructure as a Service) because the customer provisions fundamental compute, storage, and networking resources on-demand, and retains control over the guest OS, including patching. In IaaS, the provider manages only the physical infrastructure (hypervisor, networking, storage hardware), while the customer is responsible for the OS and applications, matching the scenario exactly.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Software as a Service (SaaS)
Why it's wrong here
SaaS delivers finished applications, so the provider manages the entire stack including the operating system; customers never patch guest OSes. It is tempting because SaaS removes nearly all operational burden, and it would be correct if the requirement were simply consuming a hosted application such as email or CRM.
- ✗
Platform as a Service (PaaS)
Why it's wrong here
PaaS abstracts the operating system entirely, so the provider handles guest OS patching; the customer deploys only code and data. It is tempting because PaaS removes OS management, and it would be correct if the scenario required managed runtimes or app hosting without infrastructure control.
- ✓
Infrastructure as a Service (IaaS)
Why this is correct
IaaS delivers virtualised compute, storage and networking on demand, while the customer retains responsibility for the guest operating system, including patching. That split of responsibility matches the scenario exactly, unlike PaaS or SaaS, where the provider manages the OS layer.
- ✗
Desktop as a Service (DaaS)
Why it's wrong here
DaaS delivers virtual desktops, not provisionable virtual machines, storage and networks with customer-managed guest OS patching. It is tempting because both are cloud-hosted compute, but DaaS would be correct when the provider manages the desktop image and the user simply accesses a hosted workspace.
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Same concept, more angles
2 more ways this is tested on MS-900
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. A cloud provider offers virtual machines, but customers must install, configure, and maintain the operating system and applications. Which cloud service model does this describe?
easy- ✓ A.IaaS (Infrastructure as a Service)
- B.PaaS (Platform as a Service)
- C.SaaS (Software as a Service)
- D.FaaS (Function as a Service)
Why A: This scenario describes Infrastructure as a Service (IaaS), where the cloud provider supplies virtualized computing resources such as virtual machines, storage, and networking, but the customer retains full control over the operating system, middleware, and applications. In IaaS, the provider manages only the physical infrastructure (hypervisor, servers, storage, and network), while the customer is responsible for OS installation, configuration, patching, and application management. This aligns directly with the question's description of customer-managed OS and applications on provider-hosted VMs.
Variation 2. A company wants to migrate their on-premises file servers to Microsoft 365. Which cloud service model describes this approach?
easy- A.Platform as a Service (PaaS)
- ✓ B.Software as a Service (SaaS)
- C.Desktop as a Service (DaaS)
- D.Infrastructure as a Service (IaaS)
Why B: Migrating on-premises file servers to Microsoft 365 means moving to a fully managed, cloud-hosted application service, which is the definition of Software as a Service (SaaS). Microsoft 365 delivers applications like SharePoint, OneDrive, and Exchange Online where Microsoft manages the infrastructure, platform, and application layers. The customer only manages data and user access, which is characteristic of SaaS.
JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This MS-900 practice question is part of Courseiva's free Microsoft certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the MS-900 exam.