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In VMM 2012 SP1, Microsoft introduced the VM Networking, in support of Windows 2012.

On the scenario that has multiple VM’s from different organizational divisions or customers, network secure isolation is highly required and  working with(VLANs), can present significant disadvantages in this scenario, on the other way, Windows Server 2012 Hyper-V Network Virtualization provides policy-based, software-controlled network virtualization that reduces the management overhead that is faced by enterprises providing better flexibility and scalability for managing virtual machines to achieve higher resource utilization.

The key functionalities, benefits, and capabilities of Hyper-V Network Virtualization in Windows Server 2012:

• Enables flexible workload placement – Network isolation and IP address re-use without VLANs
• Enables easier moves for workloads to a shared IaaS cloud
• Enables live migration across subnets
• Enables easier management of decoupled server and network administration
• Simplifies the network and improves server/network resource utilization
• Is compatible with existing infrastructure and emerging technology
• Provides for interoperability and ecosystem readiness
• Uses Windows PowerShell and WMI.

Nigel Cain from Microsoft started an series of blog post to explain in detail the VM Network concepts with detailed steps by steps. Have a look here

http://blogs.technet.com/b/scvmm/archive/2013/01/08/virtual-networking-in-vmm-2012-sp1.aspx