A network card, network adapter or NIC (network interface card) is a piece of computer hardware designed to allow computers to communicate over a computer network. It is an OSI model layer 2 item because it has a MAC address.

Every network card has a unique 48-bit serial number called a MAC address, which is written to ROM carried on the card. Every computer on a network must have a card with a unique MAC address. The IEEE is responsible for assigning MAC addresses to the vendors of network interface cards. No two cards ever manufactured should share the same address.

Whereas network cards used to be expansion cards to plug into a computer bus, most newer computers have a network interface built into the motherboard. A separate network card is not required unless multiple interfaces are needed or some other type of network is used.

The card implements the electronic circuitry required to communicate using a specific physical layer and data link layer standard such as ethernet or token ring. This provides a base for a full network protocol stack, allowing communication among small groups of computers on the same LAN and large-scale network communications through routable protocols, such as IP.

There are four techniques used for transfer of data, the NIC may use one or more of these techniques.

  • Polling is where the microprocessor examines the staus of the peripheral under program control.
  • Programmed I/O is where the microprocessor alerts the designated peripheral by applying its address to the system's address bus.
  • Interrupt-driven I/O is where the peripheral alerts the microprocessor that it's ready to transfer data.
  • DMA is where the intelligent peripheral assumes control of the system bus to access memory directly. This removes load from the CPU but requires a separate processor on the card.

A network card typically has a twisted pair, BNC, or AUI socket where the network cable is connected, and a few LEDs to inform the user of whether the network is active, and whether or not there is data being transmitted on it. The Network Cards are typically available in 10/100/1000 Mbits/s(Mbps). This means they can support a transfer rate of 10 or 100 or 1000 Megabit per second.