When a new virtual machine is created in Hyper-V it plugs in a synthetic network adapter. This is done because all supported operating systems have integration component drivers with support for that network adapter. If you attempt to load an operating system that is not a supported, you will not have integration component drivers and therefore no network connectivity by default.
To get network connectivity back, you need to modify the default hardware configuration. Open the configuration settings for the virtual machine, remove the existing Network Adapter, then use the add hardware option and add a Legacy Network Adapter to the virtual machine.
When the VM powers on the motherboard will see a new network adapter that emulates an Intel 21140-based PCI Fast Ethernet Adapter. Most older operating systems have built in support for this adapter or a driver is available.
Note that the legacy network adapter does not perform as well as the synthetic adapter.
