Client access license

A client access license ("CAL") is a commercial software license that allow clients to connect to server software and use their services. Most commercial client software are licensed so that payment is required for each installation. However, some server software, which are accessed by multiple devices or users concurrently, are licensed so that payment is required for each concurrent use. Thus each concurrent use is authorized with a client access license. For example, an instance of Windows Server 2003 for which ten CALs is purchased allows 10 concurrent connections.

Commercial software is licensed to end users or businesses: in a legally binding agreement between the proprietor of the software (the "licensor") and the end user or business (the "licensee"), the licensor gives permission to the licensee to use the software under certain limitations, which are set forth in the license agreement. In the case of Microsoft Corporation, the consumer retail or "off-the-shelf" products generally use very similar licences, allowing the licensee to use the software on one computer, subject to the usual terms and conditions. For businesses however, Microsoft offers several types of licensing schemes for a range of their products, which are designed to be cost effective, flexible, or both.

Commercial server software, such as Windows Server 2003 and SQL Server 2005 require licenses that are more expensive than those which are purchased for desktop software like Windows Vista. All clients that connect to these server products must have a license to connect in order to use their services. These special purpose licenses come in the form of a CAL.

A CAL legally permits client computers to connect to commercial server software. They usually come in the form of a certificate of authenticity (CoA) and a license key, which is sometimes attached to the certificate itself. The various editions of most of Microsoft's server software usually include a small number of CALs, and this allows the software to be used by either a few users or a few computers, depending on the CAL licensing mode. If more connections to the server are needed, then additional CALs must be purchased.

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