Microsoft SQL Server licensing is designed to be a flexible yet simplified process to help businesses quickly implement database solution it into their environments. The licensing models available for Microsoft SQL Server provide customers with two basic streams for licensing:
Licensing based on the hardware provided in the server infrastructure, referred to as Per Core Licensing
Licensing based on the number of users or devices concurrently accessing the database, plus an additional server license for each server. This is referred to as Server + Per User/Device Client Access License (CAL)
Be aware that there is no way to transition/upgrade from Per Core Licensing to Server+CAL or vice versa without aquiring separate new licenses.
Per Core Licensing
Under the Per Core licensing model, each server running Microsoft SQL Server software or any of its components (such as Reporting Services or Integration Services) must be assigned an appropriate number of Microsoft SQL Server core licenses. The number of core licenses needed depends on the number of physical processor cores (even if each core has multiple processing threads) or the number of virtual processor cores (in the virtual case, each processing thread is considered a separate core) depending on if a physical or virtual server is used. A minimum of four core licenses are required for each server. Unlike the Server+CAL licensing model, the Per Core model allows access for an unlimited number of users or devices to connect from either inside or outside an organization’s firewall. With the Per Core model, customers do not need to purchase additional client access licenses (CALs) to access the Microsoft SQL Server software.
Server + Per User/Device CAL
When licensing the Microsoft SQL Server software under the Server+CAL model, customers purchase a server license for each server, and a client access license (CAL) for each device (Device CAL) and/or user (User CAL) accessing Microsoft SQL Server or any of its components. A CAL is not software; it is a license granting users and devices access to the SQL Server software.
Under the Server+CAL licensing model, each server running Microsoft SQL Server software or any of its components must have a Microsoft SQL Server server license assigned. Each server license allows customers to run any number of Microsoft SQL Server instances in a single server, either physical or virtual. To access a licensed Microsoft SQL Server, each user or device must have a Microsoft SQL Server CAL that is the same version or newer than the Microsoft SQL Server software version being accessed. For example, to access a server running Microsoft SQL Server 2014 software, a user needs a Microsoft SQL Server 2014 or 2016 CAL. While being version-specific, each Microsoft SQL Server 2016 CAL provides access to any number of current and/or prior version licensed Microsoft SQL Server instances in a customer’s organization, current or previous product edition, including legacy SQL Business Intelligence, SQL Server Enterprise, SQL Server Workgroup and SQL Server for Small Business edition servers.