Server 2012 introduced Hyper-V Network Virtualisation (HNV), using a protocol called Network Virtualization for Generic Routing Encapsulation (NVGRE).
This encapsulates a virtual data packet within an outer packet, enabling virtual subnets that are independent of physical subnets.
The same physical network can accommodate overlapping virtual subnets without conflict. This is a huge feature for the hybrid cloud, since every tenant can extend its on-premise network to a public or hosted cloud without having to re-plan its network addresses.
New in Server 2012 R2 is the Inbox HNV Gateway, which handles site-to-site VPN, NAT (Network Address Translation) and forwarding. The new gateway integrates with System Center Virtual Machine Manager for easier management.
Another new feature, called Dynamic IP Address Learning, which means HNV is aware of both dynamic and manually assigned IP addresses in a virtual network, and both inbound and outbound support for NIC Teaming, which means efficient use of multiple network cards where available.