Licence Mobility and Virtualisation: Licensing VMs Without Overpaying

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Virtualisation licensing has two separate rules and mixing them is what costs money. Windows Server is licensed on the physical host and never travels with the guest; server applications such as SQL Server and Exchange Server can travel, but only through Licence Mobility with active Software Assurance.

1. Licence Windows Server on the host: every physical core, minimum 8 core licences per processor and 16 per server, whatever the guests are running.
2. Use Standard if the host will carry two Windows virtual machines; relicense all the cores again for each further pair, and compare that total against Datacenter.
3. Do not assume a guest carries its licence to another host. Core licences move between server farms no sooner than 90 days after the last assignment.
4. Buy Software Assurance or subscription licences if virtual machines must move freely, or if you want to licence individual virtual machines rather than whole hosts.
5. Skip Licence Mobility for Windows Server. It is not eligible; Azure Hybrid Benefit is the route into Azure and the Flexible Virtualization Benefit is the route to an outsourcer.
6. Never build Windows client virtual desktops on OEM Pro licences. Desktop virtualisation rights come from Windows Enterprise E3/E5, VDA, Windows 365 or Azure Virtual Desktop.

If your estate is one host with a couple of Windows guests, the host licence is the whole answer and you can stop. Below is what changes when machines move, when applications rather than the operating system are virtualised, and when the workload leaves your building.

Why it happens

Microsoft licenses virtualisation in two layers that people routinely collapse into one. The operating system layer is counted on the physical hardware: you licence every physical core in the server, and the edition then decides how many operating system environments you may run on it. The application layer – SQL Server, Exchange Server, SharePoint Server and the rest – is counted separately, either on the same physical cores or on the virtual cores assigned to a guest. A wrong bill almost always means someone applied the host rule to an application, or the application rule to the operating system.

On the operating system layer, the number of licences required equals the number of physical cores on the licensed server, subject to a minimum of eight per physical processor and sixteen per server. That minimum is why deliberately buying small hardware saves nothing: a four-core box and a sixteen-core box cost the same in Windows Server licences. Datacenter then permits the server software in any number of operating system environments on that server; Standard permits two, plus a third in the physical environment if that instance is used solely to host and manage the virtual ones.

Standard scales by relicensing rather than by adding: for each additional two operating system environments, the server must be relicensed for the same number of core licences. Four Windows guests on a sixteen-core host is two complete sets of sixteen. The core count does not move the Standard-versus-Datacenter crossover, because it sits on both sides of the sum and cancels; only the price ratio on your own quote does.

Full reference

Which rule applies to which layer

What the physical operating system environment costs you

Windows Server Standard entitles you to two operating system environments on a fully licensed server, and there is a third instance in the deal running in the physical environment – but only where that instance is used solely to host and manage the virtual ones. This is the clause that quietly consumes an allowance. Install a file share, a print queue, a backup agent that does more than back up the hypervisor, or a line-of-business service on the host, and that physical instance stops qualifying and starts counting against your two. On Datacenter the same rule exists but the ceiling above it is unlimited, so nobody notices.

Containers follow the same accounting. Windows Server containers without Hyper-V isolation are unlimited on both editions. Containers with Hyper-V isolation count as operating system environments, so Standard allows two of them and they compete with your virtual machines for the same allowance.

Licensing individual virtual machines instead of the host

There is a second model that most small estates never hear about. Rather than licensing every physical core in the box, you can licence the virtual machine on its virtual cores. Microsoft restricts it to customers with subscription licences or active Software Assurance, and it carries its own floors: a minimum of eight core licences per virtual machine and sixteen per customer for Windows Server, and a minimum of four per virtual OSE for SQL Server. It is the only model that lets licences follow workloads between servers in the same farm without waiting out the reassignment rule, which is why it exists at all.

Run the comparison honestly. On a large host carrying a handful of small guests, per-virtual-machine licensing is often cheaper than licensing sixty-four physical cores. On a densely packed host it is not, and the physical model wins comfortably. Microsoft also offers pay-as-you-go Windows Server billing through Azure Arc for customers without unlimited virtualisation rights who need to add capacity temporarily, which is worth knowing about before you buy a permanent set of licences for a workload that only exists for a quarter.

Licence Mobility, precisely

Licence Mobility through Software Assurance is a benefit attached to server application licences. It permits deployment in an Authorised Mobility Partner’s data centre or on Azure, which the base licence terms do not allow, and it removes the standard reassignment restriction so licences can move to another server within the same server farm at any time. Microsoft names Exchange Server, SharePoint Server and SQL Server among the eligible products and states plainly that Windows Server is not eligible. Deployment has to be registered with Microsoft through the licence verification form within ten days.

Since October 2022 the Flexible Virtualization Benefit sits alongside it and is broader: subscription licences and licences with active Software Assurance may be deployed with an Authorised Outsourcer on shared or dedicated hardware under the same terms as on-premises. The exclusion is the important part. An Authorised Outsourcer is a provider that is neither a Listed Provider nor using one as its data centre provider, and Microsoft publishes the Listed Provider list. Check your hosting partner against it before you plan a migration around either benefit.

Hypervisor choice does not change any of this

The Hyper-V role ships as part of Windows Server, so there is no separate hypervisor licence to buy. Equally, running your Windows Server guests on VMware, Proxmox, KVM or anything else changes nothing about the counting: the licences attach to the physical hardware and the operating system environments running on it, not to the product doing the virtualising. Linux guests do not consume the Windows Server allowance at all, so a Standard host running two Windows guests and a dozen Linux ones is correctly licensed.

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