Virtual Machines communicate over one or more simulated wired networks (VMs do not simulate WiFi). In the real world a wired Ethernet nework (whether at work or at home) is created from devices called switches. Ethernet adapters in a PC are connected to a switch by an Ethernet cable, and switches can be connected to each other to build a larger network. Hyper-V describes its virtual network in terms of virtual switches, but the terms “switch” or “network” are interchangeable.
Hyper-V creates one initial virtual switch/network called Default, and if you are using Windows Home Edition that is all you get. On Pro or Enterprise you can install the full Hyper-V function with a Manager tool and create your own virtual switches. All Hyper-V virtual switches are identical, so the reason for having more than one is to separate which VMs talk to which other VMs, or to control how IP addresses are assigned.
Each VM can have one or more virtual network adapter cards, each of which is connected to one switch. VMs attached to the same switch share an IP subnet and can communicate directly to each other.
The Windows host OS on your physical laptop or desktop computer can itself get one or more virtual network adapter cards connected to Hyper-V virtual switches, thus allowing it to talk to the VMs.
A virtual switch can also be associated with a single physical Ethernet adapter in the physical computer. When used this way, the adapter itself is not a communications endpoint. It has no IP address itself. Rather, it is a raw low level Ethernet connection between a virtual switch and some external physical network or device. This allows VMs to appear on a real network as if they were real computers.
In Powershell commands, there are independent operations to create a virtual switch, attach a VM to a virtual switch, create a virtual network adapter on the host system attached to a virtual switch, and attach a physical Ethernet adapter to a virtual switch. However, the Hyper-V Manager is user friendly and allows you to do several things at once. For example, you can create an External virtual switch associated with an Ethernet adapter that the host system is already using. In one operation, the Hyper-V Manager creates a new virtual switch, and a virtual Ethernet adapter on the host system, transfers all the existing IP configuration on the physical Ethernet adapter to the new host virtual adapter, and not attaches the physical adapter (now stripped of all its IP configuration) to the virtual switch. This is a request that users make frequently enough that it is convenient to be able to do it all in one request. However, the Hyper-V Manager definition of “types” of virtual switches as External, Internal, and Private are not actual mutually exclusive choices but rather names it gives to the most popular three ways of configuring the switch, host virtual adapter, and physical Ethernet adapter options.
Hyper-V automatically creates a single virtual switch named Default.
This provides a self configuring virtual network with all the options needed for most casual users.
There is no configuration or administration of Default.
When the host system boots up, Default gets a “factory reset” and chooses a random subnet range of addresses.
There is a virtual Ethernet adapter on the host Windows system attached to Default and assigned the first IP address in the subnet range.
Default uses DHCP to assign new IP addresses to each virtual adapter on a VM that is attached to it. It also sets the host virtual adapter address as the gateway and DNS server address for VMs.
The host Windows system provides a gateway service using the NAT protocol. When a client program on a VM connects to an Internet service, it passes the request to the host Windows system. The NAT function changes the request so it appears to be coming from a program on the host physical computer, and then sends the data out in the same way it would send data for a local application program. It will use wired, or WiFi, or VPN for this data in exactly the same way as it sends data from Firefox or Outlook.
When a VM gets an IP address from DHCP, the host Windows system writes the VM hostname (with a dummy domain suffix of *.mshome.net) and IP address in the file C:\Windows\System32\drivers\etc\hosts.ics. This is treated as an extension of the normal “hosts” file on every Windows or Linux system. When Windows is looking for a computer name, it checks the hosts files first before going to a DNS server. Therefore, if there is a VM named “docker” then the host can communicate to it using the dummy name of “docker.mshome.net”.
The VMs also get a dummy DNS server from the host. When they ask for the IP address associated with a name, then host OS looks the name up using its own standard API. This checks the hosts files first, then makes a DNS request if the name is not in the file. Therefore, the VMs get the same DNS server the host Windows system provides and can find each other using vmname.mshome.net.
Default provides so many useful services that you will probably want to connect it to all your VMs. However, there are a few things it does not do.
You cannot assign static IP addresses on Default.
External clients cannot access VM services through Default.
You cannot connect Default to a physical adapter card.
Default only supports local VMs running on your physical host computer.
Therefore, a technically sophisticated user with more complex network requirements may want to create additional virtual switches with other characteristics and additional virtual adapters for specific VMs to attach them to these custom networks.
A virtual switch is a concept for Hyper-V configuration.
Data does not move from the VM to some Hyper-V “switch” memory, get processed by some simulated “switch” firmware, and then get forwarded to another VM. Rather when a virtual Ethernet adapter is ready to transmit data to the network, Hyper-V determines what other virtual adapters are connected to the same virtual switch and moves the data from the memory of one VM directly to the memory of the other VM.
In a real network physical switches are connected to each other and forward data from switch to switch to switch. This is the one thing you cannot do with a Hyper-V virtual switch. By analogy, each virtual switch is a dead end street. You can send data outside through a gateway NAT function provided by the Host or by a VM with gateway/router/firewall software, or you can create a connection to an external real network through a single physical Ethernet adapter connected to a real network. You cannot “bridge” one virtual switch to another, even using “bridge” software on a VM connected to both switches.
The Microsoft Hyper-V Managers utility give three names to specific configurations and if you use the GUI you have to choose one of the names. The Powershell commands, however, allow you to add or remove physical LAN adapters and a virtual host LAN adapter to existing switches without regard to these limited names. In reality there are four possibilities and two ways to use the fourth option:
Hyper-V “Private” - A virtual switch that is neither connected to a LAN adapter or to the host operating system is “private”. You connect VMs to it and they can talk to each other. Since this would leave the VMs unable to update themselves if it were the only network they have, you typically connect at least one of the VMs to another switch that is connected out and have that VM serve as a NAT gateway or router for the other VMs.
Hyper-V “Internal” - A virtual switch that is connected to the Windows 10 host (through a Hyper-V virtual LAN) can provide the VMs with some type of protected client access to the real network and internet if the Windows 10 host will provide the router function. Any Windows 10 system can provide a NAT gateway using the Powershell New-NetNAT command, but Windows 10 doesn’t have a DHCP service. So either you have to configure static addresses for each VM, or you need one of the VMs to provide DHCP to all the other VMs (and make sure that VM is running all the time).
Hyper-V “External” (special case) - If you only connect a spare unused physical LAN adapter on your host computer to the virtual switch and you don’t connect the host to that switch, then you can physically connect the VMs directly to the real network without going through the host OS. Each VM can have its own network address just like real computers on the physical network. The host can talk to them, but through the external network as if they were other real computers and not internal VMs. Typically this requires the host to have two LAN adapters. Of course, many people connect their laptop to the WiFi and they have a spare wired Ethernet port they aren’t using, but if that is not the case you can get an adapter card for a desktop or a USB Network Adapter for a laptop.
Hyper-V “External” Bridge - This is the more common configuration where you connect both a physical host LAN adapter and a virtual host LAN adapter to the same switch. Normally you ask the Hyper-V Manager to create both types of connections at the same time. After a brief break in network connectivity, the host OS still has a wired connection to the same network with the same network address as before. Now, however, it can share that LAN adapter and cable with the VMs connected to the switch. They also appear to be on the physical network, get the same type of network address that the host had, can talk to other computers on the network, and can share files and provide services to other computers.
Hyper-V “External” (physical private VM network) - The last type is something like the previous special case because you associate the virtual switch with a spare LAN adapter, but in this case you do not connect that adapter to the public network. You can connect it to a cable that runs between two computers, or to a private switch not connected to the internet. You may do most of your work on a laptop, but laptops are not powerful machines. You can use this to connect your laptop privately to a more powerful desktop workstation. Run Hyper-V on both the laptop and the desktop and the laptop host system you normally use where all your tools and files reside is not only able to talk to its own VMs but also to the VMs created by the connected desktop. Again you have to solve the problem of creating a gateway or router on either the laptop or the desktop so the VMs can install software and get updates from the internet.
If you start out with a physical Network Adapter that is configured to the host OS, and you then associate it with a Hyper-V virtual switch, the configuration of the Network Adapter splits in half. On the host Windows 10 system, you start with one Network adapter and end up with two.
One network adapter on the host system represents the physical device that is now being shared with the VMs. If you look at it in Device Manager, it still has all the parameters associated with firmware (except for the Ethernet hardware address). If you want to allow Jumbo Frames, this is the device that manages those options.
A new Virtual Ethernet Adapter has been created in the Host Windows 10 system which gets the Ethernet Hardware (MAC) Address from the adapter and also all the higher level network drivers, especially the IPv4 and IPv6 configuration options. This is because the Host Windows 10 system must have its own addresses, both at the Ethernet level and at the Internet level. Other VMs will also be connected to the switch, and each will have its virtual Network Adapter with its own Ethernet hardware address and its own IPv4 and IPv6 configurations separate from the host.
Name InterfaceDescription ifIndex Status MacAddress Ethernet Marvell AQtion 10Gbit Network Adapter 15 Up 70-85-C2-C7-BF-6E vEthernet (Bridge) Hyper-V Virtual Ethernet Adapter #3 13 Up 70-85-C2-C7-BF-6E |
Here a Marvell AQtion 10Gbit adapter card has been shared with a Hyper-V virtual switch named “Bridge”. It has generated two Network Adapter entries. The first represents the card itself, while the second contains all the addresses that the host uses to send and receive data. Each VM attached to the switch will have its own distinct addresses. Data from the host and the VMs will go through the one adapter card to whatever physical switch the adapter is connected to.
As mentioned above, you can spend $15 to buy a simple 8 port switch or $20 to buy a “smart switch” that supports VLANs. Corporations like Yale use VLANs to partition the campus network up into groups of devices for security or traffic management. Home users may want a VLAN to separate their Internet of Trash devices from work devices, but you can do that by physically separating the two, especially since the IOT is wireless and the work devices can connect to each other wired.
So despite what the letters “VLAN” stand for, in Hyper-V Virtual Machine networks the most interesting use of VLAN configuration is to give a Virtual Machine a physical network port (it is an anti-virtual trick turning virtual stuff into real hardware).
You spend $20 for 8 ports or $40 for a 16 port smart switch. You create a Bridge virtual switch on your host computer and connect the adapter you assign to it to one of the ports on the smart switch. If you have more than one host running VMs, you may want to connect each of them to a port on the switch. If you want to use this to access the internet you need another port to provide a connection to the gateway or to a firewall through which you can get to the gateway.
Now in a quick oversimplified version of the configuration, you read the manual on the switch, connect a browser and sign into the web based administration tool that runs in the switch, and assign a VLAN ID number to a set of empty ports. The number is arbitrary, so lets assume you assign individual VLAN IDs 11, 12, …, 16 to six currently empty ports on the switch. Ports can be assigned to multiple VLAN IDs if they are “tagged”, and you do this to the ports where you just connected the Bridge physical adapter from the host computers and you make them part of all six VLAN IDs (in this example, and more if you get a bigger switch).
Now in the Hyper-V Manager you can display the configuration of each Virtual Machines and click on the configuration of the virtual LAN adapter connected to this Bridge switch.
You can click the check box for Enable virtual LAN identifier and then in the text box below it type in one the the IDs you assigned to the smart switch ports (11 to 16 in this example).
If one VM is assigned to VLAN ID 14 and one port on the smart switch is assigned VLAN ID 14, then this physical port on the smart switch has become essentially a physical representation of what would have been the LAN adapter jack on the back of the physical computer if the VM has been magically turned into a real physical box. You can plug in a wire to that port and it is directly connected to this virtual adapter on this VM.
Creating a physical LAN port in the real world for something that is otherwise virtual may be a useful trick for specialized types of network connections. More generally, VLAN capability allows a single network adapter to behave as multiple LAN adapters connected to different devices or networks, which can save money and PCIe slots in more complicated networking situations.