A standalone computer or a computer that is not attached to any network, lives in its own world and carries out its tasks with its own inbuilt resources. But as soon as it becomes a workstation, then it needs an interface to help establish a connection with the network because without this, the workstation or node will not be able to share the network resources.
You can also say that, a Network Interface Unit (NIU) is a basically a device that is attached to each of the workstations and the server, and helps workstation establish the all important connection with the network.
Each Network Interface Unit (NIU) attached to a workstation has a unique number to identify it which is known as the node address.
The Network Interface Unit (NIU) is also called as Terminal Access Point (TAP) or Network Interface Card (NIC).
Different manufacturers have different names for the interface.
The Network Interface Card (NIC) assigns a unique physical address to each of the NIC card and this physical address also known as MAC address.