Windows Server 2025 White Paper

1. Introduction

This comprehensive white paper is designed to help IT professionals, system administrators, and procurement specialists understand the licensing model for Windows Server 2025. Proper licensing is essential for compliance, cost optimization, and ensuring your organization has the right capabilities to meet its business needs.

Windows Server 2025 is Microsoft’s cloud-ready operating system that enables organizations to securely run their workloads, implement hybrid cloud scenarios, and modernize applications to meet evolving business requirements. It brings advanced security and resiliency, hybrid cloud agility, and a future-ready, high-performance infrastructure.

Key Features of Windows Server 2025

Advanced Multi-layered Security: Windows Server 2025 includes Secured-core server technology, which provides comprehensive protection across hardware, firmware, and virtualization layers. This allows IT and security operations teams to apply advanced protection broadly throughout their environment.

Hybrid Capabilities with Azure: Organizations adopting a hybrid and multicloud approach can leverage cloud services with on-premises Windows Server 2025 by connecting with Azure Arc. Features like Hotpatching are available for on-premises Windows Server 2025 when connected to Azure Arc. File Server enhancements include SMB Compression, which improves application file transfer by compressing data while in transit over a network.

Flexible Application Platform: Windows Server 2025 offers significant scalability improvements, supporting up to 48TB of memory and 2,048 logical cores running on 64 physical sockets for demanding Tier1 applications. It also includes advancements to Windows containers, improving application compatibility, supporting HostProcess containers for node configuration, enabling IPv6 and dual-stack, and allowing consistent network policy implementation with Calico.

2. Windows Server 2025 Editions

Windows Server 2025 is available in three primary editions, each designed to meet specific organizational needs based on size, virtualization requirements, and datacenter complexity.

Datacenter Edition

Ideal for: Highly virtualized and software-defined datacenter environments

Key capabilities:

• Unlimited virtualization rights when licensed based on physical cores
• Software-defined networking and storage features
• Advanced security features
• Shielded virtual machines
• Storage Spaces Direct
• Storage Replica
• Software-defined networking stack

Standard Edition

Ideal for: Low-density or non-virtualized environments

Key capabilities:

• Limited virtualization rights (two OSEs or Hyper-V containers when licensed based on physical cores)
• Core Windows Server functionality
• Basic software-defined networking features

Essentials Edition

Ideal for: Small businesses with up to 25 users and 50 devices

Key capabilities:

• Simplified interface designed for small businesses
• Integration with Microsoft 365
• Remote Web Access
• Available through OEM channels only
• Limited to a single server with up to 10 cores and 1 VM on single-socket servers

Edition Comparison

Edition Selection Guidance

• Choose Datacenter Edition when:
• You need to run many virtual machines on a few physical servers
• You require advanced software-defined storage and networking features
• You need unlimited virtualization rights
• You have 13 or more OSEs or Hyper-V containers per server
• Choose Standard Edition when:
• You have limited virtualization needs (fewer than 13 OSEs per server)
• You don’t require advanced software-defined datacenter features
• You’re running physical servers or have low VM density
• Choose Essentials Edition when:
• You’re a small business with fewer than 25 users and 50 devices
• You need a simple, first-server solution
• You have minimal virtualization needs

3. Core-Based Licensing Model

Windows Server 2025 Datacenter and Standard editions use a core-based licensing model. This model offers two licensing options: licensing based on physical cores or licensing by virtual machine.

Physical Core Licensing

When licensing based on physical cores, you must license all physical cores in the server, subject to the following minimums:

• Minimum of 8 core licenses per physical processor
• Minimum of 16 core licenses per server

For example, if you have a server with 2 processors and 6 cores per processor (12 cores total), you still need to license 16 cores because of the minimum requirement.

Core licenses are sold in packs:

• 2-pack core licenses
• 16-pack core licenses (for convenience, equivalent to eight 2-packs)

Calculating Required Licenses

To determine how many core licenses you need:

1. Count the number of physical cores in each processor
2. Ensure you’re licensing at least 8 cores per processor
3. Ensure you’re licensing at least 16 cores per server
4. Calculate the total number of 2-packs or 16-packs needed

Example Scenarios:

Virtual Machine Licensing

The option to license Windows Server by virtual machine was introduced in October 2022 and is available only to customers with:

• Subscription licenses, or
• Licenses with active Software Assurance

When licensing by virtual machine:

• The number of core licenses required equals the number of virtual cores in the virtual OSE
• Subject to a minimum of 8 core licenses per virtual machine
• Subject to a minimum of 16 core licenses per customer

This licensing option provides more flexibility for environments with varying VM sizes and can be more cost-effective for certain scenarios, especially when VMs have fewer virtual cores.

4. Virtualization Rights

Virtualization rights vary by edition and licensing method. Understanding these rights is crucial for proper licensing and cost optimization.

“Stacking” Licenses for Additional OSEs

For Standard edition licensed based on physical cores, you can add virtualization rights by “stacking” licenses. For each additional set of two OSEs or two Hyper-V containers, you must assign an additional set of licenses equal to the number of physical cores on the server.

Example: Standard Edition “Stacking” on a 2-processor server with 8 cores per processor

Alternatively, customers with subscription licenses or licenses with active Software Assurance can license additional OSEs by virtual machine instead of stacking.

Container Licensing

Windows Server 2025 supports two types of containers:

1. Windows Server containers without Hyper-V isolation:

• Standard and Datacenter editions include rights to unlimited Windows Server containers without Hyper-V isolation
• These containers share the kernel of the host operating system

1. Windows Server containers with Hyper-V isolation:

• Each container runs in a highly optimized virtual machine
• Licensing follows the same rules as virtual OSEs
• Datacenter edition (physical core licensing): Unlimited containers
• Standard edition (physical core licensing): Two containers (plus additional with stacking)
• Both editions (virtual machine licensing): One container per licensed VM

5. Access Licensing

In addition to server licenses, Windows Server 2025 requires access licenses for users and devices that access the server. This is true whether you license Windows Server based on physical cores or by virtual machine.

Client Access Licenses (CALs) Overview

A Client Access License (CAL) is required for each user or device that accesses a Windows Server. CALs are categorized as:

1. Base CALs: Required for basic access to Windows Server
2. Additive CALs: Required for access to advanced functionality, in addition to Base CALs

User CALs vs. Device CALs

• User CAL: Allows one user to access Windows Server from any device
• Device CAL: Allows any user to access Windows Server from one device

Organizations can choose the most cost-effective option based on their usage patterns:

• Choose User CALs when users access the server from multiple devices
• Choose Device CALs when multiple users share devices (e.g., shift workers)

Organizations can use a mix of User and Device CALs as needed.

External Connector Licenses

For external users (users who are not employees, onsite contractors, or onsite agents), organizations have two options:

1. Purchase CALs for each external user/device, or
2. Purchase External Connector (EC) licenses for each server accessed by external users

External Connector licenses are assigned to servers and allow unlimited external user access to that server. Like CALs, External Connectors come in Base and Additive varieties.

Base and Additive CALs

Base CALs and ECs:

• Windows Server CAL
• Windows Server External Connector

Additive CALs and ECs (required in addition to Base CALs/ECs):

• Windows Server Remote Desktop Services CAL/EC
• Windows Server Active Directory Rights Management Services CAL/EC
• Microsoft Identity Manager CAL/EC

Access Licensing Requirements for Internal Users

Access Licensing Requirements for External Users

CAL Version Compatibility

CALs and ECs permit access to the same version or earlier versions of Windows Server:

CAL Exceptions and Exemptions

CALs and ECs are not required in the following scenarios:

1. Server-to-server communication: When one licensed server accesses another licensed server
2. Web workloads: When accessing server software running a web workload (content served within an Internet web solution on a publicly available website)
3. High-performance computing (HPC) workloads: When accessing server software used to run a cluster node for HPC applications
4. Management OSE: When accessing a physical OSE used solely for hosting and managing virtual OSEs

6. Software Assurance Benefits

Software Assurance (SA) is a comprehensive maintenance offering that provides additional benefits for Windows Server licenses. Organizations with active Software Assurance receive the following benefits:

7. Deployment Scenarios and Licensing Examples

Understanding how to apply Windows Server 2025 licensing to real-world scenarios helps ensure compliance and optimize costs. Here are several common deployment scenarios and their licensing requirements.

Physical Server Deployment

Scenario: A single physical server with 2 processors, 10 cores per processor, running Windows Server 2025 Standard as the host operating system.

Licensing Requirements:

• 20 core licenses (10 cores × 2 processors)
• Windows Server 2025 Standard edition
• Windows Server CALs for each user or device accessing the server

Virtualized Environment with Standard Edition

Scenario: A physical server with 2 processors, 8 cores per processor, hosting 6 virtual machines running Windows Server 2025.

Licensing Requirements with Physical Core Licensing:

• Base requirement: 16 core licenses of Windows Server 2025 Standard (covers 2 VMs)
• Additional requirement: 32 more core licenses (16 × 2) to cover the remaining 4 VMs
• Total: 48 core licenses (24 2-packs or 3 16-packs)
• Windows Server CALs for each user or device accessing the VMs

Licensing Requirements with Virtual Machine Licensing (requires subscription or SA):

• 8 core licenses per VM (minimum) × 6 VMs = 48 core licenses
• If VMs have more than 8 virtual cores, license for the actual number of virtual cores
• Windows Server CALs for each user or device accessing the VMs

Virtualized Environment with Datacenter Edition

Scenario: A physical server with 2 processors, 12 cores per processor, hosting 20 virtual machines running Windows Server 2025.

Licensing Requirements with Physical Core Licensing:

• 24 core licenses of Windows Server 2025 Datacenter (12 cores × 2 processors)
• Windows Server CALs for each user or device accessing the VMs

Licensing Requirements with Virtual Machine Licensing (requires subscription or SA):

• 8 core licenses per VM (minimum) × 20 VMs = 160 core licenses
• If VMs have more than 8 virtual cores, license for the actual number of virtual cores
• Windows Server CALs for each user or device accessing the VMs

Hybrid Cloud Scenario

Scenario: An organization with on-premises Windows Server 2025 deployment wants to extend workloads to Azure.

Licensing Requirements:

• On-premises: Standard or Datacenter edition core licenses as appropriate
• Azure: Can use Azure Hybrid Benefit with Software Assurance
• Each set of 16 core licenses with SA allows up to 16 virtual cores in Azure across 2 VMs
• Each additional set of 8 core licenses allows up to 8 virtual cores and 1 VM

Container Deployment

Scenario: A physical server with 2 processors, 8 cores per processor, running 5 Windows Server containers with Hyper-V isolation and 15 Windows Server containers without Hyper-V isolation.

Licensing Requirements with Standard Edition (Physical Core Licensing):

• Base requirement: 16 core licenses (covers 2 Hyper-V containers and unlimited non-Hyper-V containers)
• Additional requirement: 24 more core licenses to cover the remaining 3 Hyper-V containers
• Total: 40 core licenses (20 2-packs or 2 16-packs + 4 2-packs)

Licensing Requirements with Datacenter Edition (Physical Core Licensing):

• 16 core licenses of Windows Server 2025 Datacenter
• Covers unlimited containers of both types

Windows Server Pay-as-you-go Scenario

Scenario: An organization with Windows Server 2025 Standard needs temporary additional capacity for a seasonal workload.

Licensing Requirements:

• Base deployment: Standard edition core licenses as appropriate
• Connect to Azure Arc
• Enable pay-as-you-go for additional VMs as needed
• Pay $33.58 per CPU core / month or $0.046 / hour for the additional capacity

8. Windows Server Pay-as-you-go Option

Windows Server 2025 introduces a new cloud billing option that provides pay-as-you-go flexibility across on-premises and cloud environments.

Overview and Requirements

The pay-as-you-go option allows customers without unlimited virtualization to scale up with additional virtual machines to meet capacity bursts or temporary needs. Key requirements include:

• Windows Server 2025 only (not available for earlier versions)
• Azure Arc connection required
• Azure subscription for billing

Azure Arc Integration

To use the pay-as-you-go option:

1. Connect your Windows Server 2025 environment to Azure Arc
2. Configure Windows Server pay-as-you-go through the Azure portal
3. Deploy additional VMs as needed
4. Pay only for what you use

Pricing and Billing

• $33.58 per CPU core/month
• $0.046 / hour
• Billed through your Azure subscription
• Prices are subject to change and may vary by region

Use Cases and Benefits

• Seasonal workloads: Scale up during peak periods without purchasing permanent licenses
• Development and testing: Create temporary environments without long-term licensing commitments
• Disaster recovery: Quickly deploy additional capacity during recovery scenarios
• Project-based needs: Support short-term projects without capital expenditure
• Capacity planning: Avoid overprovisioning licenses for occasional peak usage

9. Volume Activation

Volume Activation is a set of technologies and tools designed to automate the product activation process for systems deployed under a Microsoft Commercial Licensing agreement. Windows Server 2025 includes several technologies to simplify the configuration, distribution, and management of volume software licenses.

Key Management Service (KMS)

KMS is a role service that allows organizations to activate systems within their network from a server where a KMS host has been installed.

Benefits:

• Activations occur on the local network, eliminating the need for individual computers to connect to Microsoft
• No dedicated system required; can be cohosted on a system providing other services
• Volume editions of Windows client and server operating systems automatically connect to the KMS service
• No action required from the user

Implementation Steps:

1. Install the Volume Activation Services server role
2. Configure the KMS host
3. Ensure client systems can reach the KMS host
4. Systems will automatically activate when they contact the KMS host

Active Directory-based Activation

Active Directory-based activation is a role service that allows you to use Active Directory Domain Services (AD DS) to store activation objects, further simplifying volume activation services.

Benefits:

• No additional host server needed
• Activation requests processed during computer startup
• Computers running Windows Server 2016 or later with a Generic Volume License Key (GVLK) activate automatically
• Systems stay activated as long as they remain domain members and maintain periodic contact with a domain controller

Implementation Steps:

1. Install the Volume Activation Services server role
2. Configure Active Directory-based activation
3. Join client systems to the domain
4. Systems will automatically activate when the licensing service starts
   

Volume Activation Services Server Role

The Volume Activation Services server role in Windows Server enables you to automate and simplify the issuance and management of Microsoft software volume licenses. This role allows you to:

• Install and configure KMS
• Enable Active Directory-based activation
• Manage activation for various scenarios and environments
• Track activation status across your organization

10. Licensing Programs and Channels

Windows Server 2025 is available through various licensing programs and channels designed to meet the unique needs of different organizations.

Enterprise Agreement (EA) Options

Enterprise Agreement (EA):

• For organizations with 500+ users/devices
• Three-year agreement with annual payments
• Software Assurance included
• Available for Windows Server Datacenter and Standard editions

Enterprise Agreement Subscription (EAS):

• Subscription-based alternative to EA
• Lower initial costs with annual payments
• Non-perpetual licenses (subscription only)
• Software Assurance benefits included

Server and Cloud Enrollment (SCE)

• An enrollment under the Microsoft Enterprise Agreement
• For organizations standardizing on key server and cloud technologies
• Requires installed base-wide commitment to one or more SCE components
• Provides best pricing and terms, plus other benefits
• Includes cloud-optimized licensing options and simplified license management

Microsoft Products and Services Agreement (MPSA)

• Transactional licensing program
• No minimum purchase requirements
• Software Assurance optional
• Available for Windows Server Datacenter and Standard editions
• Flexible purchasing options

Microsoft Customer Agreement (MCA)

• Modern purchasing agreement that simplifies and consolidates licensing
• Perpetual and subscription options
• Software Assurance optional
• Available for Windows Server Datacenter and Standard editions
• Managed through the Microsoft 365 admin center

Open Value Options

Open Value (OV):

• For organizations with 5+ users/devices
• Three-year agreement with annual payments
• Software Assurance included
• Available for Windows Server Datacenter and Standard editions

Open Value Subscription (OVS):

• Subscription-based alternative to OV
• Lower initial costs with annual payments
• Non-perpetual licenses (subscription only)
• Software Assurance benefits included

OEM Licensing

• Pre-installed licenses with hardware systems
• Available for Windows Server Datacenter, Standard, and Essentials editions
• Tied to the original hardware
• Software Assurance can be added within 90 days of purchase
• Limited downgrade rights compared to volume licensing

Full Packaged Product (FPP)

• Available through online retailers
• Retail box product
• Available for Windows Server Standard edition
• Limited availability and higher per-unit cost
• Best for very small deployments

Services Provider License Agreement (SPLA)

• For service providers offering hosted services to customers
• Monthly subscription model
• Report and pay only for what you use each month
• Available for Windows Server Datacenter and Standard editions

Channel Availability Summary

11. Downgrade Rights and License Transitions

Downgrade rights allow you to use earlier versions or lower editions of Windows Server instead of the version you’re licensed for. This provides flexibility when migrating to new versions or when you need to maintain compatibility with existing applications.

Downgrade Rights Explained

When you purchase a Windows Server 2025 license, you have the right to use:

• Earlier versions of the same edition
• Lower editions of the same version or earlier versions

The license terms of the version and edition acquired still govern the use of the software, even when using downgrade rights.

Down Edition Rights

License Version Compatibility

When using downgrade rights:

• You must have the appropriate media and keys for the version you want to use
• You must comply with the license terms of the version you purchased, not the version you’re running
• The virtualization rights of your purchased license apply, not those of the downgraded version

Transition Paths

Upgrading from Standard to Datacenter:

• With Software Assurance: Use Step-up licenses
• Without Software Assurance: Purchase new Datacenter licenses

Transitioning from physical core licensing to virtual machine licensing:

• Requires subscription licenses or licenses with active Software Assurance
• Can be more cost-effective for environments with few virtual cores per VM

Transitioning from on-premises to cloud:

• Use Azure Hybrid Benefit with Software Assurance
• Consider Windows Server pay-as-you-go for temporary capacity

12. Best Practices and Recommendations

Edition Selection Guidance

Choose Datacenter Edition When:

• You have high-density virtualization (13+ VMs per server)
• You need advanced software-defined datacenter features
• You require unlimited virtualization rights
• You use many Windows Server containers with Hyper-V isolation

Choose Standard Edition When:

• You have low-density virtualization (fewer than 13 VMs per server)
• You don’t need advanced software-defined datacenter features
• You’re running primarily physical workloads
• Budget constraints make Datacenter edition prohibitive

Choose Essentials Edition When:

• You’re a small business with fewer than 25 users
• You need a simple, first-server solution
• You have minimal virtualization needs

Optimizing License Costs

Physical Core Licensing Optimization:

• Consolidate workloads on fewer, more powerful servers
• Use Datacenter edition for servers with high VM density
• Use Standard edition for servers with low VM density
• Consider hardware with higher core-to-performance ratio

Virtual Machine Licensing Optimization:

• Allocate the minimum necessary virtual cores to each VM
• Group VMs with similar workloads to maximize density
• Use subscription or Software Assurance benefits for flexibility

CAL Optimization:

• Choose User CALs when users access from multiple devices
• Choose Device CALs when multiple users share devices
• Consider CAL Suites (Core CAL, Enterprise CAL) for cost savings
• Use External Connector for servers with many external users

Virtualization Strategy

High-Density Virtualization:

• Use Datacenter edition for unlimited virtualization rights
• Consolidate VMs on fewer physical servers
• Implement clustering for high availability

Mixed Virtualization:

• Use Datacenter edition for high-density hosts
• Use Standard edition for low-density hosts
• Consider Windows Server pay-as-you-go for temporary capacity

Container Strategy:

• Use Datacenter edition for extensive use of Hyper-V containers
• Use Standard edition for limited container usage
• Leverage unlimited Windows Server containers without Hyper-V isolation

CAL Management

• Maintain an accurate inventory of users and devices
• Regularly audit CAL compliance
• Consider CAL Suites for comprehensive access rights
• Use User CALs for mobile workforces
• Use Device CALs for shift workers or shared devices

License Compliance

• Document all server deployments and their licensing
• Track virtual machine migrations to ensure proper licensing
• Maintain Software Assurance for maximum flexibility
• Conduct regular licensing audits
• Keep records of downgrade deployments
• Ensure CALs match your server versions

13. Frequently Asked Questions

General Licensing Questions

Q: How are Windows Server Standard and Datacenter editions licensed?

A: Both editions use a core-based license model. You can license based on physical cores or by virtual machine (the latter requires subscription licenses or licenses with active Software Assurance). When licensing based on physical cores, you must license all physical cores on the server, subject to a minimum of 8 core licenses per physical processor and 16 core licenses per server. When licensing by virtual machine, you must license the number of virtual cores in each VM, subject to a minimum of 8 core licenses per VM.

Q: Do the minimum licensing requirements apply to all servers?

A: Yes. When licensing based on physical cores, regardless of the actual number of cores, you must license at least 8 cores per physical processor and at least 16 cores per server. When licensing by virtual machine, you must license at least 8 cores per VM.

Q: Do I have to license all cores on the server, even if they are disabled?

A: Yes, when licensing based on physical cores. All physical cores on all physical processors must be licensed, even if they have been disabled.

Q: Can I split my Windows Server core license packs across multiple servers?

A: Yes. Core licenses are sold in 2-packs and 16-packs, and each individual license may be assigned to a separate physical server.

Virtualization Questions

Q: What are the virtualization rights for Windows Server Standard and Datacenter editions?

A: When licensed based on physical cores, Datacenter edition provides rights to unlimited OSEs or Windows Server containers with Hyper-V isolation. Standard edition provides rights to two OSEs or two Windows Server containers with Hyper-V isolation. When licensed by virtual machine, both editions provide rights to one OSE or one Windows Server container with Hyper-V isolation per licensed VM.

Q: If I have a server licensed for Windows Server Standard, how can I increase my virtualization rights?

A: You have three options:

1. Purchase Step-Up licenses to Datacenter edition (requires Software Assurance)
2. Purchase additional Standard edition licenses to “stack” on the same server
3. License additional virtual machines individually (requires subscription or Software Assurance)
4. Use Windows Server pay-as-you-go through Azure Arc for temporary capacity

Q: How do I determine whether Standard or Datacenter edition is more economical?

A: For servers running 13 or more OSEs or Windows Server containers with Hyper-V isolation, Datacenter edition is typically more cost-effective. For servers running fewer than 13 OSEs or containers, Standard edition is usually more economical.

CAL Requirements

Q: Are CALs required for access to Windows Server Standard and Datacenter editions?

A: Yes, server software access requires either Client Access Licenses (CALs) or External Connectors (ECs), regardless of whether you license based on physical cores or by virtual machine.

Q: Which access licenses do I use to access Windows Server?

A: Windows Server CALs and External Connectors permit access to the same version (or earlier versions) of Windows Server. Each user or device accessing the server requires a Windows Server CAL, or you can purchase an External Connector for each server accessed by external users.

Q: Are there any exceptions to the CAL and External Connector requirements?

A: Yes, CALs and External Connectors are not required for:

• Access by another licensed server
• Access to server software running a web workload or HPC workload
• Access to a physical OSE used solely for hosting and managing virtual OSEs

Cloud Deployment Questions

Q: Can Windows Server licenses be moved between my on-premises servers and Azure?

A: Yes. Customers can deploy on-premises images in Azure using the Azure Hybrid Benefit.

Q: Can Windows Server images be used in Azure under the Azure Hybrid Benefit?

A: Yes. Under the Azure Hybrid Benefit, customers with Windows Server licenses covered with Software Assurance may upload and use their own Windows Server images on Azure and pay only for the cost of service utilization.

Q: Are CALs required for Windows Server instances running on Azure?

A: Base access licenses (CALs and External Connectors) are not required for use of Windows Server on Microsoft Azure. However, Remote Desktop Services (RDS) CALs are required when using corresponding functionality.

Q: Am I permitted to move Windows Server licenses to other cloud providers’ environments?

A: Yes. Windows Server subscription licenses or licenses with active Software Assurance allocated by virtual machine may be moved to Authorized Outsourcers’ clouds (on shared or dedicated servers). An Authorized Outsourcer is generally any cloud provider who is not a Listed Provider or using a Listed Provider for data center services.

14. Glossary of Terms

15. Additional Resources