For those of you with a large number of servers, Microsoft has made some changes in its licensing in the past year to accommodate virtualization. You can license by server farm, instead of by server. Microsoft customers are able to reassign licenses freely across servers within that server farm, but only for certain server applications. It does not apply to software licenses for the Windows Server operating system, Client Access Licenses (CALs), User Subscription Licenses (USLs), Device Subscription Licenses (DSLs), Add-on Subscription Licenses (Add-on SLs), or Management Licenses (MLs). You must purchase applications through the volume licensing program as well; those purchased through retail outlets are not covered under this same rule.
What exactly is a server farm? Definition: A server farm consists of up to two data centers with each physically located in the following areas:
Each data center may be part of only one server farm. You may reassign a data center from one server farm to another, but not on a short-term basis (that is, not within 90 days of the last assignment). You must meet the qualifications of a server farm in order to reassign freely, as long as you are not running the same application at the same time on two separate servers. Otherwise, you must maintain the standard 90-day waiting period to reassign applications.