Understanding Virtual Networking in Microsoft Hyper-V


Networking in Hyper-V is sort of interesting for those who are used to physical networking or are used to the way networking is implemented in VMware. With Hyper-V, the guest operating systems never have direct access to the hardware, and the Hyper-V management interfaces control the traffic moving through the physical and virtual interfaces.

Hyper-V has a Virtual Network Manager. The Virtual Network Manager is responsible for creating and controlling virtual switches. There is no limit on the number of virtual switches you can create – depending on the type of virtual network you are working with. For example, external virtual networks are basically on physical NICs, so you can only have the number of external networks that is the same as the number of physical NICs.

The concept of a virtual network is an important one to understand. Consider a virtual network to be like a switch, but instead of a physical switch, it’s a virtual switch. All the virtual machines that connect to the same virtual network switch are connected to the same switch. Each virtual switch is logically isolated from all other virtual switches. If you want hosts that are connected to one virtual switch to communicate with hosts that are on another virtual switch, you can create a virtual router, VPN server, firewall or similar device to those you would have on a physical network. It’s a good idea to create different virtual switches if you have scenarios where you need to have network isolation similar to what you would have on a physical network, where the different segments are separated by firewalls or some other access control device.

Virtual Network Types

There are three types of virtual networks that you can connect to in Hyper-V:

  • Private Virtual Network
  • Internal Virtual Network
  • External Virtual Network

A Private Virtual Network is a virtual switch to which only virtual machines can connect. Guest operating systems that are connected to the same Private Virtual Network can communicate with each other, but they cannot communicate with the Host operating system and the Host operating system cannot connect to the VMs on the Private Virtual Network. Private Virtual Networks are great if you need total isolation from all other Virtual Networks, but they can sometimes be problematic if you need to copy files to the virtual machines, since there is no connectivity to any physical network or to the Host operating system. In this scenario, you can create a virtual machine that functions as a firewall or a router, and connect one virtual network adapter to the Private Virtual Network and another virtual network adapter to the physical network (through an External Virtual Network, which we’ll talk about in just a bit).

An Internal Virtual Network is similar to a Private Virtual Network in that it isn’t bound to any physical NIC. The Internal Virtual Network is an isolated virtual switch like the Private Virtual Network, but in the case of the Internal Virtual Network, the Host operating system has access to the guest virtual machines through the Internal Virtual Network virtual switch. However, there is no DHCP-like functionality with this virtual switch (as there is with some instantiations of VMware Workstation), so if you want to communicate with virtual machines that are connected to the Internal Virtual Network switch, you are going to need to assign an IP address to the virtual NIC that’s associated with that Internal Virtual Network for the Host operating system that is valid on the virtual network to which you are trying to connect.

An External Virtual Network is different from the other Virtual Networks because this type of virtual network is associated with physical network adapters. You can have one External Virtual Network for each physical NIC that’s installed on the Hyper-V server. The External Virtual Network switch will actually appear to take the place of the physical NIC on the Hyper-V server – so that if you were to look at the configuration of the former NIC, it would appear to not have any IP addressing assigned to it. Instead, a virtual NIC is added to the Network Connections window and that virtual NIC is connected to the External Virtual Network switch, and it is that virtual NIC that has the IP addressing assigned to it that allows it to communicate with the physical network.

There are some important things you should know about Virtual Networks and Virtual NICs before you get started working with them in Hyper-V:

  • If you connect to the Hyper-V server over the network using RDP, the network connection that you used before will be gone, and you will probably need to visit the Hyper-V server and configure the new virtual External NIC at the server in order to regain connectivity
  • You can not create an External Virtual Network based on a wireless NIC (which is a big drawback, in my opinion, as you are able to do this with VMware)
  • You can tell the name of the Virtual NIC associated with the virtual switch because the NIC and switch will have the same name; the only difference is that the virtual switch won’t have any IP addressing information bound to it and the NIC will
  • Because of the connectivity issue I mentioned above, it’s a good idea to have at least two physical NICs on your Hyper-V server. That way, you can use one NIC as a dedicated management interface for the Host operating system, and assign an External Virtual Network to the other NIC.

Virtual NICs

Hyper-V enables support for two types of virtual NICs:

  • Legacy NICs
  • High Speed NICs

Each virtual machine can have up to 12 virtual NICs attached to it;more specifically, each can have up to 8 high speed NICs and up to 4 legacy NICs.

A legacy NIC emulates a physical NIC (DEC 21140 NIC) and will work right away without the need to install any additional software since most operating systems will support this NIC right out of the box. The legacy NIC also supports PXE for network based operating system installation.

You can use the legacy NIC to get you up and running so that you have network connectivity before you install the Hyper-V integration services. However, in some cases you will have to install integration services, such as in the case of Windows XP and Windows Server 2003 64-bit operating systems, which don’t include support for the legacy NIC emulation of the DEC virtual NIC.

The high speed NIC is just that – a high speed NIC, and as you might guess, it will give you the best performance. However, in order to make use of the high speed NIC, you will need to install the Hyper-V integration services after the guest operating system is installed.

Virtual NICs support VLAN tagging with VLAN ID numbers assigned to them and Virtual Networks can be assigned VLAN IDs.

Some things that you should keep in mind regarding VLAN configuration and virtual networks and NICs:

  • The physical NIC on the Hyper-V server needs to support VLAN tagging and the VLAN feature needs to be enabled on the physical NIC.
  • However, you should not set a VLAN ID on the physical NIC. Instead, configure it on the virtual NIC that’s associated with that External Network switch or to the virtual machines themselves.
  • The virtual machine will use the VLAN ID that is assigned to its virtual NIC. The VM isn’t aware of the ID that’s assigned to the virtual switch.

Hyper-V Networking Recommendations

The following represent a collection of recommendations and best practices so that you can get the most out of your Hyper-V networking environment:

  • As mentioned earlier, you should have at least two physical NICs on your Hyper-V server, so that there’s one for management of the Host operating system and one that you can assign to an External Virtual Network
  • You should use Private Virtual Networks when you want complete isolation from both the Host operating system and the physical network
  • You should use the Internal Virtual Network when you need to transfer files between the Host operating system and the virtual machines
  • You can dynamically unplug and plug virtual machines into different virtual networks. You do not have to shut down the virtual machine in order to change its network connection
  • If you want to segment your virtual networks, you can create a virtual switch for each network (that is to say, create a Virtual Network for each network) and then connect them using a TMG firewall or Windows RRAS virtual machines
  • Remember to assign the Host operating system’s Internal Virtual NIC a valid IP address so that it can communicate with VMs on the same Internal Virtual Network
  • Some virtual machines will require large amounts of bandwidth to communicate with the physical network. In this scenario, you should configure a dedicated External Virtual Network for these NICs and you should not connect any other virtual machines to them
  • If you want to put a Hyper-V server on the edge of your network, make sure that the Host operating system’s virtual NIC on the untrusted network does not have valid IP addressing information. This will prevent intruders from connecting to the Host operating system over the Internet


Hyper-V networking provides you with a way to customize your virtual network environment. You can use the various Virtual Networks to control the level of connectivity the virtual machines have with one another, as well as connectivity to the Host operating system and the physical network. However, before you start working with Hyper-V virtual networks, you need to have an understanding of how they work and some of the key features and limitations imposed by these networks. In a future article, we’ll go through a walk-through on configuring and managing virtual networks and see how they work with a virtual server that has four physical NICs. See you then! –Deb.

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