IaaS – Infrastructure as a Service
😟 THE PROBLEM
An old app only installs on one specific version of Windows.
😊 THE SOLUTION
Rent a virtual machine (IaaS) and install exactly what you need.
- You need full control of the OS✓ IaaS gives you the whole virtual machine
- You must move 30 servers fast✓ Lift and shift them with no code changes
- Old hardware is about to fail✓ Microsoft runs the physical servers now
Quick answer
Infrastructure as a Service (IaaS) is the cloud service type where the provider gives you raw infrastructure (virtual machines, storage and networking) and you manage everything from the operating system upwards. It offers the most control and flexibility of the cloud service types, and the most management responsibility. Azure Virtual Machines are the classic example.
Exam objective: 1.3.1 Describe infrastructure as a service (IaaS). (Skills measured as of July 20, 2026)
Start with a question
You want to run an old program that only works on one specific version of Windows. Where do you put it in the cloud?
Most modern cloud platforms say “give us your code and we run it”. But this program needs a particular operating system, an installer that clicks through twenty screens, and a vendor’s monitoring agent.
You need a computer you control completely, without buying one. That is Infrastructure as a Service.
Explain it like I’m 10
The technical explanation
Infrastructure as a Service is the most flexible cloud service type. The provider supplies and manages the physical datacenter, physical network, physical hosts and the virtualisation layer. You get virtual machines, disks and virtual networks, and you are responsible for everything you put on them: the operating system, patches, middleware, runtimes, applications, data, and their configuration and security.
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1 Datacenter, power, cooling
Microsoft
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2 Physical servers, storage, network
Microsoft
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3 Virtualisation (hypervisor)
Microsoft
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4 OS, runtime, middleware
You
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5 Applications and data
You
IaaS on Azure
| Azure service | What you get |
|---|---|
| Azure Virtual Machines | Windows or Linux VMs in many sizes (general purpose, memory-optimised, GPU and more) |
| Virtual Machine Scale Sets | Groups of identical VMs that scale automatically |
| Managed Disks | Block storage disks for VMs |
| Azure Virtual Network | Your private network in Azure, with subnets and security rules |
Common IaaS use cases
- Lift-and-shift migration: move existing servers to Azure quickly, then modernise later.
- Test and development: build and tear down environments in minutes.
- Custom or legacy software that needs a specific OS, drivers or agents.
- High-performance computing (HPC): large numbers of powerful VMs for simulations and rendering.
- Website hosting where you need full control of the web server.
Architect level IaaS done well: what senior engineers add
- Availability: a single VM’s SLA depends on its disk type (highest with Premium SSD or Ultra Disk). Use availability zones or availability sets plus a load balancer for production.
- Automation: build images with Azure VM Image Builder, configure with cloud-init or Desired State Configuration, deploy with Bicep or Terraform. Never hand-build production VMs.
- Patching: Azure Update Manager can schedule and report patches across Azure and Arc-enabled servers.
- Security: no public IPs on management ports; use Azure Bastion or just-in-time VM access; enable Defender for Servers.
- Cost: right-size, autoscale with Scale Sets, use reservations or savings plans for baseline, and Spot VMs for interruptible batch work.
Real-world business scenario
A logistics company
- The problem
- Its warehouse management system runs on old servers whose support contract ends in three months. The vendor only certifies it on Windows Server with SQL Server installed locally.
- What they used
- Azure Virtual Machines in an availability zone pair, migrated with Azure Migrate, with Azure Backup and Azure Update Manager.
- Why that choice
- IaaS is the only option that keeps the vendor's supported configuration, and migration needs no code changes.
- The result
- Migrated before the hardware contract expired, with better availability than the single old server, and a plan to modernise in phase two.
When to use it and when not to
✓ Use it when…
- You need full control of the operating system and installed software.
- You are migrating existing workloads quickly with minimal changes.
- The software is not supported on PaaS (custom agents, drivers, legacy versions).
- Specialised compute such as GPUs or HPC clusters.
✕ Think twice when…
- A standard web app or API. PaaS (App Service) removes OS patching and scaling work.
- Event-driven code that runs occasionally. Serverless (Azure Functions) is simpler and cheaper.
- Your team has no capacity to patch, secure and back up servers.
↔ Alternatives to consider
- **PaaS** such as Azure App Service or Azure SQL Database when you just want to run code or data.
- **Containers** (Azure Container Apps, AKS) for portable, modern packaging.
- **Serverless** (Azure Functions) for event-driven tasks.
Interview preparation
Say your answer out loud first, then open the model answer. Interviewers care more about your reasoning than the exact words.
Beginner What is IaaS? Give an Azure example.
Model answer
IaaS means renting basic computing infrastructure, such as virtual machines, disks and networks, from a cloud provider. The provider manages the physical hardware; I manage the operating system, software and data. Azure Virtual Machines is the main example.
Interviewer follows up
What exactly do you manage on an Azure VM?
You
The OS and its patches, installed software and runtimes, the application, data, network configuration such as NSG rules, identities and access, and backup choices. Microsoft manages the physical server, storage hardware, network and datacenter.
Intermediate Why would you choose IaaS over PaaS?
Model answer
When I need full control of the OS (custom drivers, specific versions, agents), when migrating an existing app quickly without changes (lift and shift), when the software is not supported on a PaaS offering, or for workloads like custom databases or legacy line-of-business apps. The trade-off is that I take on patching, scaling and high-availability design.
Experienced developer You inherit 50 lifted-and-shifted VMs. How would you reduce the operational burden and cost?
Model answer
Right-size using Azure Advisor and actual utilisation, shut down or schedule non-production VMs, buy reservations or a savings plan for steady ones, and use Azure Hybrid Benefit for Windows and SQL licences. Automate patching with Azure Update Manager, protect with Azure Backup, and standardise with policy. Then plan a modernisation path, moving web tiers to App Service or containers and databases to Azure SQL, so the platform team stops managing OSes it does not need to.
Scenario A company runs a 15-year-old Windows application that only installs on a specific Windows Server version and needs a vendor agent. Which service type fits?
Model answer
IaaS, an Azure VM with that Windows Server image. PaaS would not allow the custom installation. The company can modernise later, but IaaS gets it into Azure now with full control of the OS.
Test yourself with exercises
4 original practice questions in the AZ-900 style. Pick your answers, then check them. You will see why each option is right or wrong. Score 75% or more and this topic is marked complete.
Quick revision
Key points
- IaaS = provider manages physical hardware, network, datacenter and virtualisation; you manage OS and above.
- Most control and flexibility, most management responsibility of the three service types.
- Azure examples include Virtual Machines, VM Scale Sets, Managed Disks and Virtual Networks.
- Typical use cases are lift-and-shift migration, test/dev, custom software, and HPC.
- You pay for VMs while they are running (allocated). Deallocated VMs stop compute charges.
Important terminology
- IaaS
- Infrastructure as a Service. Rent VMs, storage and networks; you manage the OS, runtime, apps and data.
- Virtual machine (VM)
- A software-based computer running on a physical host, with its own OS, CPU, memory and disks.
- Hypervisor
- Software that runs multiple isolated VMs on one physical server.
- Lift and shift (rehost)
- Moving an existing application to cloud VMs with minimal changes.
Common mistakes
- Thinking Microsoft patches the OS on your IaaS VMs. That is your job, though Azure offers tools to automate it.
- Choosing IaaS by default when a PaaS service would remove most of the work.
Exam tips
- "Full control over the operating system" = IaaS.
- "Migrate with minimal changes" or "lift and shift" = IaaS.
Five quick revision questions
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1.What does IaaS stand for?Show
Infrastructure as a Service.
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2.Who installs operating system updates on an Azure VM?Show
The customer (you), optionally using Azure automation tools.
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3.Which Azure service is the classic IaaS example?Show
Azure Virtual Machines.
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4.Which service type offers the most control?Show
IaaS.
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5.What is "lift and shift"?Show
Moving an application to cloud VMs with minimal changes.
Official references
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Next up: PaaS – Platform as a Service