With most products, you must assign a license to a server for each instance of the server software that you run on that server. You may run that instance in a physical or virtual operating system environment on that server. By exception to the licensing model, some products provide broader use rights. For example, a Windows Server 2008 Enterprise license permits a running instance in up to 4 virtual operating system environments at a time on the licensed server. With some products, such as Windows Server 2008 Datacenter, you may run any number of instances at the same time as long as you license each physical processor on the licensed server.
Except as outlined in the product use rights, all Server Operating System products require a CAL for each user or device that accesses the server software. There are two types of CALs: Device CALs and User CALs. You may license in either type but that selection must be determined at the time the license is acquired or Software Assurance is renewed.
CALs are version specific. They must be the same version or later than the server software being accessed. CALs permit access to servers licensed by the same entity. They do not permit access to another entity's licensed servers.
You may deploy network architectures that use hardware or software to reduce the number of devices or users that directly access the software on a server. This is referred to as multiplexing or pooling. However, this does not reduce the number of CALs required to access or use the server software. A CAL is required for each device or user that is connected to the multiplexing or pooling software or hardware front end.
An External Connector (EC) license is an alternative to CALs for each server that external users will access. External users are users who are not employees or onsite contractors. An EC license assigned to a server permits access by any number of external users, as long as that access is for the benefit of the licensee and not the external user. Each physical server that external users access requires only one EC license regardless of the number of instances running. The right to run instances of the server software is licensed separately; the EC, like the CAL, simply permits access. EC licenses, like CALs, are version and functionality specific. They must be the same version or later than the server software being accessed. The decision on whether to acquire CALs or an EC for external users is primarily a financial one.