The function of a NIC is to connect a host device to the network medium. A NIC is a printed circuit board that fits into the expansion slot on the motherboard or peripheral device of a computer. The NIC is also referred to as a network adapter. On laptop or notebook computers a NIC is the size of a credit card. 

NICs are considered Layer 2 devices because each NIC carries a unique code called a MAC address. This address is used to control data communication for the host on the network. More will be learned about the MAC address later. As the name implies, the network interface card controls host access to the medium.

In some cases the type of connector on the NIC does not match the type of media that needs to be connected to it. A good example is a Cisco 2500 router. On the router an AUI connector is seen. That AUI connector needs to connect to a UTP Cat 5 Ethernet cable. To do this a transmitter/receiver, also known as a transceiver, is used. A transceiver converts one type of signal or connector to another. For example, a transceiver can connect a 15-pin AUI interface to an RJ-45 jack. It is considered a Layer 1 device because it only works with bits, and not with any address information or higher-level protocols.

In diagrams, NICs have no standardized symbol. It is implied that, when networking devices are attached to network media, there is a NIC or NIC-like device present. Wherever a dot is seen on a topology map, it represents either a NIC interface or port, which acts like a NIC.

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