A Client Access License (CAL) is a license granting access to certain Microsoft server software. CALs are used in conjunction with Microsoft Server software licenses to allow Users and Devices to access and utilize the services of that server software. For instance, a company looking to utilize Microsoft Windows Server would acquire a Microsoft Windows Server license in order to install and run the Windows Server software on the physical server itself. In order to provide the rights for Users or Devices to access the Windows Server software running on the server, CALs would need to be acquired for those Users or Devices in order to do so.

 Different types of CALs: There are three different options available for acquiring CALs depending on the needs of your company and the server software you are acquiring CALs for:
  1. User CAL – A User CAL allows a single unique physical user to access Microsoft server software from many devices, such as a work computer, a home computer, a laptop, an Internet kiosk, or a personal digital assistant (PDA), without having to acquire CALs for each device.
    • NOTE: We license by physical user, not log-in name. Take a look at the John Smith example for more on this.
  2. Device CAL - A Device CAL allows any number of physical users to access Microsoft server software through a single device.
  3. Processor License - A Processor License includes access for an unlimited number of users to connect from either inside the local area network (LAN) or wide area network (WAN) or outside the firewall. You do not need to purchase additional server licenses, CALs, or Internet Connector Licenses when you acquire Processor Licenses. If utilizing Processor licenses, one processor license must be purchased for each physical processor.
    • Processor licenses are only available for some Microsoft server products, such as: Application Center, BizTalk Server, Commerce Server, Content Management Server, Host Integration Server, Identity Integration Server, ISA Server, SQL Server, and Operations Management Server. There is a chart of server products showing which ones offer Per Processor licensing on the Microsoft CAL Guide page.
    • NOTE: Windows Server, Exchange Server, and Small Business Server do NOT offer Per Processor options
Version numbers matter: Just like the Server products that CALs are associated with, they have version numbers. For instance, for Windows Server 2003, there is a Windows Server 2003 license and there are Windows Server 2003 CALs. The version number of the CALs being used to access the server software must be the same or higher than the version of the Server software running. As an example, once Windows Server 2008 is released, if you were to purchase a license of Windows Server 2008 to replace your Windows Server 2003 (or if you had Software Assurance on your Windows Server license and received the 2008 version through your upgrade protection) and install that in your company, your Windows Server 2003 CALs would no longer have rights to access the server running Windows Server 2008, since the 2008 version is newer than the 2003 version. As such, you would need Windows Server 2008 CALs to access the Windows Server 2008 server software. Again, if you have Software Assurance for your Windows Server 2003 CALs when Windows Server 2008 is released, then you would receive rights to Windows Server 2008 CALs through the upgrade protection included in the Software Assurance. Note: You can purchase newer CALs to access older server versions. For instance, a Windows Server 2008 CAL can be used to access a Windows 2003 server since the CAL version is newer than the Server version.
Different Microsoft server products: If you are running multiple Microsoft server products, remember that there are CALs associated with each and you would need to acquire the appropriate CALs for each. For instance, if you are running Microsoft Windows Server and Microsoft Exchange Server, you would need both a Microsoft Windows Server CAL and a Microsoft Exchange Server CAL to access these servers since Microsoft Exchange uses Active Directory (Windows Server technology) for authentication; therefore, you are accessing both the Microsoft Windows Server and the Microsoft Exchange server when checking your email.
Multiple servers in domain: Microsoft server CALs can be used to access multiple servers of the same kind throughout your domain. For instance, if you have a Windows Server 2003 Device CAL for a workstation, that Windows Server 2003 CAL gives that workstation the rights to access any Windows Server 2003 throughout the domain, not just a single Windows Server 2003.

 Here is some more information on CALs that you may be interested in:
Thank you and have a wonderful day,
Eric Ligman
Microsoft US Senior Manager
Small Business Community Engagement
This posting is provided "AS IS" with no warranties, and confers no rights