In these days Microsoft published an overview on Windows 2012’s Editions and Licensing.
From an editions perspective we find two “main” editions, Standard and Datacenter, differentiated only by virtualization rights: two virtual instances for Standard and unlimited for Datacenter.
The VMs are entitled to run on up to two processors for both editions.
All the features available in the Datacenter edition are also available in the Standard edition, including high availability features like failover clustering, that make the choice between Windows Server 2012′ editions based only on companies’ virtualization strategy.
Windows 2012 Essential and Foundation don’t have any virtualization right.
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