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Understanding Windows Server Client Access Licenses (CALs)


Whether you are a customer or a reseller, it is important to understand the ins and outs of Windows Server Client Access Licenses (CALs) to ensure license compliance is met.

Today, we will explain CALs: what they are, the different types, and why they are important!

Windows Server 2016 Core-Based Licensing
First, let’s start with some basic Windows Server licensing information, we’ll call it 'Windows Server licensing 101'. Starting with Windows Server 2016 Standard edition and Datacenter edition, Microsoft introduced a new licensing model. This new model is based on the cores in the server, (where as previous Windows Server licensing models were based on processors). Although the licensing model has changed, one thing has stayed consistent… the need for CALs!
It is important to note that CALs are only required for Windows Server 2016 Standard and Datacenter editions. For Windows Server 2016 Essentials edition, CALs are not required.

Want to learn more about Core-Based Licensing? We’ve got the perfect video and blog for you! Check it out: Windows Server 2016 Core Based Licensing Explained


Microsoft Windows Server Operating System (OS) Licenses
When a customer purchases a Windows Server OS license (Windows Server 2016 Datacenter edition for example), they receive a license that allows them to install the operating system onto the server. However, the Windows Server OS license does not give them the legal right for users or devices to connect to the server… that is where CALs come in!

Microsoft Windows Server Client Access Licenses (CALs)
A Windows Server CAL is a license that allows customers to access Windows Server. CALs are used in conjunction with Microsoft Windows Server OS licenses to allow Users and/or Devices to access and utilize the services of that server OS.

If your customers are wanting to run Windows Server 2016 on their new HPE Gen10 server, they would acquire a Windows Server 2016 OS license in order to run the Windows Server software on the physical server AND they would need to acquire CALs for the Users or Devices who will access the server.

This two-component license models provides a low entry price and a way to pay for capacity used: the more devices or users accessing the server software, the greater the license fees or price. This model therefore offers license affordability to organizations of all sizes.


What are the different types of CALs?
HPE offers several different types of Windows Server CALs.

  • User CALs allow a single user access to Windows Server from an unlimited number of devices
  • Device CALs allow an unlimited number of users to access Windows Server from a single device

Does the organization have roaming employees who need to access the corporate network from several devices? In this case, User CALs make the most economical and administrative sense.

Does the organization have multiple users for one device, such as shift workers using a kiosk-style workstation? In this case, Device CALs make the most economical and administrative sense.

There is a third type of CAL that we must talk about, Remote Desktop Service (RDS) CALs. RDS CALS are required for users or devices that want to utilize Remote Desktop Service functionality on Windows Server. Regular User or Device CALs are required in addition to the RDS User or RDS Device CALs.
CAL types.jpg

More Fun Facts about CALs!
Windows Server CALs can be purchased from Hewlett Packard Enterprise, or an authorized partner, at any time and do not require a new HPE server purchase. HPE currently offers User CALs and Device CALs in packs of 1, 5, 10, and 50 CALs. RDS User and RDS Device CALs are available in packs of 5 CALs.

Windows Server CALs must be the same version as—or higher than—the server software they are accessing. In other words, users or devices accessing a server running Windows Server 2016 must have Windows Server 2016 CALs. Additionally, Windows Server 2016 CALs may be used to access Windows Server 2012 (but Windows Server 2012 CALs cannot be used to access Windows Server 2016).

It is imperative that businesses stay up to date with their CAL compliance requirements so they don’t fall behind or find themselves in error!

Adding new users? More User CALs may be required!

Adding new devices? More Device CALs may be required!

If you are a reseller, now is a great time to check in with your clients to ensure their CAL needs are filled! If you're a customer, now is a great time to check your server environments to ensure you're in compliance!

Have questions about HPE OEM Microsoft products/solutions, Windows Server 2016, or HPE Servers? Join the Coffee Coaching community to keep up with the latest HPE OEM Microsoft news and interact with HPE and Microsoft experts.

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About the Author


Willa manages the HPE | Microsoft Coffee Coaching program. Follow along to learn more about the latest HPE OEM Microsoft product releases and how the HPE Microsoft partnership can benefit partners and customers.

Mohammad Anwar

Dear Willa,

It's a good and simple explanation on MS licensing. Thanks

I have one query, though. In my environment I have mixed versions of Windows as below:

Windows Domain infrastructure - Windows Server 2008 R2

Few Installations of Windows Server 2012 on which we have applications / DB for users to access (limited users).

Now my queries:

Should I purchase new Windows Server 2016 CALs for already purchased (old) Windows Server 2008 R2 CALs?

Or, can I have the mixed CALs in my environment, that is I will simply purchase Windows Server 2016 CALs for my new requirement and continue to have old CALs as well?

Please note that, we are upgrading our existing Windows environment which is on Windows Server 2008 R2 and 2012 R2.

We are talking to vendors, and everybody coming up different answers. Your help will be greatly appreciated.



Mohammad Anwar Zahoor

Jorge Aznarez

Dear sirs,


I have a question, if CAL´s for Windows Server 2016 Standard are not installable why to buy them?

I just buyed 20 User Cals for WINDOWS 2016 SERVER Standard and i don´t know what to do with them.

Can anybody explain what should i do with the User Cals i bought?


Thank you


Jorge Aznarez  


Hello Jorge,

A CAL gives users or devices the right to access and use services provided by a Windows Server. Windows Server CALs do not come with a “product key” (as Windows Server Operating System licenses do) and do not need to be registered anywhere in the OS. However, every user—or device—that access Windows Server needs a CAL to be compliant with Microsoft licensing regulations.


You can learn more about CALs in our Windows Server FAQ:


Hello Mohammad, 
CALs are ‘versioned’ in that they permit access to their CAL generation Windows Servers – or older generations of Windows Server within your domain.
Example: A Windows Server 2016 1 User CAL grants access to Windows Server 2016 / 2012 R2 / 2012 / 2008 R2 / 2008 – A Windows Server 2008 R2 User CAL for contrast grants a user access to WS2008 R2 or older Windows Server OS generations.


The critical question in the scenario you are asking about is: Do any of the users currently covered with Windows Server 2008 R2 level CALs require access to / are using Windows Server 2016 or Windows Server 2012 R2 services/roles? If the answer is yes, those users need to be covered by a Windows Server 2016 (or looking ahead Windows Server 2019) CAL.

CALs do not have a product key and are not registered in the OS so mixed-generation CAL environments are generally possible – always assuming all users that access a newer generation Windows Server OS are covered with an appropriate (same OS gen or newer) CAL.

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