Think of a busy theatre on opening night. Out front the audience sees actors, lights, and scenery. Backstage there is one person with a headset who never appears on stage but controls everything: when each light comes on, when the curtain moves, which prop goes where, who walks on next. Nothing reaches the audience without passing through that stage manager. ESXi is the stage manager of a physical server. The virtual machines are the performance everyone notices. ESXi is the quiet layer backstage that owns the hardware and decides who gets what, moment to moment, so the show runs without the actors ever bumping into each other.
In Part 2 we said a hypervisor sits between hardware and VMs. This part opens that box. By the end you will know what ESXi is made of, why it is built differently from the virtualization software on your laptop, how you talk to it, and the one installation detail that catches almost every beginner.
What ESXi actually is
ESXi is VMware’s hypervisor: the software you install onto a physical server to turn it into a host for virtual machines. You do not install Windows or Linux first and then add ESXi on top. ESXi is the operating system of that box. You boot the server from the ESXi installer, answer a few questions, and within minutes the machine’s only job is to run VMs.
A server running ESXi is called an ESXi host, or just a host. The current release you will meet at work is ESXi 8, part of VMware vSphere 8, now sold by Broadcom. One genuinely useful detail for learners: VMware brought back a free edition of ESXi with version 8.0 Update 3e, so you can install a real hypervisor in a home lab without paying for a licence.
ESXi is deliberately small. There is no desktop, no web browser, no general-purpose software cluttering it up. A smaller hypervisor means fewer bugs, fewer security holes, and more of the hardware left over for your VMs. That minimalism is a design choice, not a limitation.
Inside ESXi: the VMkernel
At the centre of ESXi is the VMkernel. This is the real engine. It is a small operating system VMware wrote specifically for virtualization, and it does the jobs any operating system does, scheduling CPU time, managing memory, talking to disks and network cards, but with one customer in mind: virtual machines. When a VM wants to run some code, the VMkernel finds it a physical core. When two VMs want memory, the VMkernel decides the split. The hardware device drivers live inside the VMkernel too, which is how it talks directly to the server’s network cards and storage controllers.
Around the VMkernel sit a few helper processes. Two worth knowing by name early: hostd, the agent that manages the host and answers when you log in directly, and vpxa, the agent that lets vCenter manage the host (vCenter gets its own part next). There is also the DCUI, the Direct Console User Interface, the yellow and grey text screen you see on the physical monitor of an ESXi host. It is intentionally bare: set the management network, set the root password, restart the agents, and not much else.
Type 1 versus Type 2: why ESXi is not like Workstation
If you have ever run VMware Workstation or VirtualBox on your laptop, you have used a hypervisor already, but a different kind. There are two families. A Type 1 hypervisor installs straight onto the hardware with nothing underneath it. ESXi is Type 1. A Type 2 hypervisor runs as an application on top of your normal operating system; Workstation and VirtualBox are Type 2, because Windows or macOS is still running underneath them.
The difference is not academic. With Type 2, every request from a VM has to pass through the host operating system before it reaches the hardware, which adds overhead and a whole extra layer that can crash or get patched on its own schedule. With Type 1, the hypervisor is the operating system, so there is nothing in the way. That is why production data centres run ESXi and not Workstation. Workstation is wonderful for learning and testing on a machine you already use. It is not what you run a hundred business VMs on.
| Aspect | Type 1 (ESXi) | Type 2 (Workstation, VirtualBox) |
|---|---|---|
| Runs on | Bare hardware | On top of an existing OS |
| Overhead | Very low | Higher (extra layer) |
| Best for | Production, data centres | Learning, dev, testing on a PC |
| Examples | ESXi, Hyper-V, KVM | VMware Workstation, VirtualBox |
How you actually talk to an ESXi host
There are three ways you will interact with a host, and knowing which is which saves a lot of early confusion.
DCUI. Plug a monitor into the server and you get the DCUI. It does the bare essentials: set the IP address of the management network, change the root password, and restart management agents if the host stops responding. You use it during setup and emergencies, not day to day.
Host Client. Point a web browser at the host’s IP address and you reach the VMware Host Client, a clean web console for that single host. You can create VMs, power them on and off, and check health. For a home lab with one host, this is all you need.
vCenter. Once a company has more than a couple of hosts, logging into each one separately gets painful. vCenter Server is the single console that manages many hosts together and gives you the features people associate with VMware, like moving a running VM between hosts. The host talks to vCenter through the vpxa agent we met earlier. vCenter is important enough that it is the whole of Part 4.
Boot media, where freshers get bitten
For ESXi 8, the host needs at least 8 GB of RAM just to boot, up from 4 GB in older versions, and a boot device of at least 8 GB. But read the next sentence carefully, because this is where old guides will lead you wrong. When ESXi installs to a normal local disk, SAN, or iSCSI target, VMware now wants at least a 32 GB device so it can create its system storage volumes, including the area called ESX-OSData where logs and crash dumps are written.
Gotcha
If your host fails to boot with a purple screen showing Fatal CPU mismatch on feature, that is not a broken install. ESXi found the physical CPUs do not match each other or do not expose a feature it expects, common on mixed or very old hardware and on some nested lab setups. The fix lives in BIOS settings or boot options, not in reinstalling ESXi over and over. The purple screen, called the PSOD or Purple Screen of Death, is ESXi’s version of the Windows blue screen, and the text on it is a clue, not just bad news.
Knowing the colour and what it means turns a panic moment into a five-minute lookup.
Stop booting production ESXi from a USB stick
For years it was normal, even recommended, to install ESXi onto a cheap USB flash drive or SD card and keep the real disks free for VMs. You will still find hundreds of blog posts telling you to do this. I would not, and neither does VMware any more. Modern ESXi writes to its system storage constantly, and USB sticks and SD cards have poor write endurance, so they wear out and the host starts throwing strange errors that are maddening to diagnose. For anything you care about, install ESXi onto a proper local disk of at least 32 GB, and ideally more. The old advice made sense when ESXi barely touched its boot device. It does not match how ESXi 8 behaves, and following it now is how you earn a 2am call.
Real interview question
What is the difference between a Type 1 and a Type 2 hypervisor, and which one is ESXi?
Answer: a Type 1 hypervisor runs directly on the physical hardware with no operating system beneath it, so it is leaner and used in production; ESXi is Type 1. A Type 2 hypervisor runs as an application on top of an existing operating system like Windows or macOS, which is fine for a laptop but adds overhead and an extra failure point; VMware Workstation and VirtualBox are Type 2. The strong version of the answer adds why it matters: with Type 1 there is no host OS to patch, crash, or slow things down, which is exactly what a data centre needs. That one extra sentence shows you understand the reason, not just the label.
Try it yourself
Download the free ESXi 8 installer ISO from Broadcom and install it, either onto an old physical PC that meets the 8 GB memory floor, or as a nested VM inside VMware Workstation on your laptop (give the nested host 8 GB of memory and a 40 GB virtual disk). Walk through the installer, set a root password, and set a static management IP in the DCUI.
How to check you got it right: from another device on the same network, open a browser to https://that-ip/ui and log in as root. If the VMware Host Client loads and shows your host’s CPU and memory, you have a working hypervisor. Bonus check: create one tiny VM and power it on.
Where ESXi sits in 2026
ESXi 8 is the hypervisor at the base of vSphere 8, and it carries modern security building blocks like UEFI Secure Boot and TPM 2.0 to make sure the host boots only trusted software. ESXi is also the foundation under the larger VMware Cloud Foundation stack, so the hypervisor you learn here is the same one running underneath the advanced material. Master ESXi as a single host first. Everything bigger, clusters, automation, software-defined networking, assumes you already understand this layer. When you want to go further than the fundamentals, the advanced VMware Cloud Foundation and NSX series picks up from here.
FAQ
Is ESXi free?
There is a free edition again. VMware brought back the free ESXi hypervisor with version 8.0 Update 3e, which is fine for learning and home labs. The paid vSphere licences add the management and clustering features most companies rely on, like vCenter-driven vMotion and HA.
What is the difference between ESX and ESXi?
ESX was the older version that included a Linux-based service console for management. ESXi replaced it years ago and removed that console, shrinking the hypervisor and reducing its attack surface. The i originally stood for the integrated, console-free design. Today you will only meet ESXi.
Can I run ESXi on my laptop or an old desktop?
Sometimes directly, often not, because consumer hardware can lack supported network or storage controllers. The reliable route for learning is nested virtualization: run ESXi as a VM inside VMware Workstation. It behaves like a real host for everything you need while studying.
What is a PSOD?
The Purple Screen of Death, ESXi’s equivalent of a Windows blue screen. It appears when the VMkernel hits a fault it cannot recover from. The purple screen prints the error and a memory dump location, which is what support uses to find the cause, often a driver, firmware, or hardware problem.
Do I need vCenter to use ESXi?
No. A single ESXi host runs and manages VMs perfectly well on its own through the Host Client. You add vCenter when you have several hosts and want to manage them as one, and to use features like vMotion, HA, and DRS, which we cover later in this series.
Your move
You now know the engine: ESXi is a Type 1 hypervisor whose VMkernel owns the hardware and serves your VMs, and you know the boot-media trap that catches beginners. Install the free ESXi this week using the Try It Yourself steps. Logging into your own Host Client for the first time is the moment this stops being theory. Next we step up a level and meet vCenter, the single control room for a whole fleet of hosts.
References
Broadcom TechDocs: ESXi Hardware Requirements (vSphere 8.0)
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DrJha