When we talked with customers about their datacenters, we found that virtualization was not fully living up to its potential.  Customers readily acknowledged the benefits of machine virtualization, but they wanted even more IT agility.  In particular, customers wanted the ability to easily migrate VMs across the datacenter or even across cloud sites.  Unfortunately, too many pieces of the IT puzzle rely on a server’s IP address; moving a VM is one thing, but giving it a different IP address is another thing completely.  To deliver on the promises of virtualization, you need to virtualize both the machine and the network.  To fully realize the agility and flexibility benefits of network virtualization, you need to centrally coordinate lots of moving parts.  What you need is Software Defined Networking (SDN).

See the entire post at http://blogs.technet.com/b/windowsserver/archive/2012/08/22/software-defined-networking-enabled-in-windows-server-2012-and-system-center-2012-sp1-virtual-machine-manager.aspx.