A legacy network adapter : Hyper - V

A legacy network adapter works without installing a virtual machine driver because the driver is already available on most operating systems. The legacy network adapter emulates a physical network adapter, multiport DEC 21140 10/100TX 100 MB. A legacy network adapter also supports network-based installations because it includes the ability to boot to the Pre-Boot Execution Environment (PXE). The legacy network adapter is not supported in the 64-bit edition of Windows Server 2003 or the Windows XP Professional x64 Edition."

 

 

then you can see as below

 

Hyper – V : Installing a Legacy Network Adapter

Installing a Legacy Network Adapter.
(Legacy Adapters aren't supported in Windows Server 2003 64-bit or Windows XP 64-bit)
1. Make sure your Virtual Machine is shutdown.
2. Right-click the Virtual Machine in Hyper-v select Settings.
3. Select Add Hardware.
4. On the Add Hardware page select Legacy Adapter.
5. Then click Add.