Network Interface Card

A Network Interface Card (NIC) is used to connect a computer to an Ethernet network. This was originally a full-sized interface card installed within the computer. Today, this is often integrated as a part of the computer, but sometimes still called a NIC. The NIC usually also combines the protocol control firmware and Ethernet Controller needed to support the Medium Access Control (MAC) link protocol used by Ethernet.

Modern day NICs often use two queues:

One queue of frames corresponds forms the transmit (Tx) queue for the physical interface The Host protocol stack completes a Tx descriptor data structure for each packet it sends and adds this to the queue (specifying the length of data, location in memory, etc). The host software then writes a register in the interface to ask for the queued frames to be sent. The interface then takes over. It typically performs a DMA transfer of the data, and then serialises this to form a physical layer frame by adding the information needed to create a physical frame.

A second queue holds received frames after processing by the NIC. The NIC processes each received frame, checking calidity and addresses and stores received frames to be passed to the host. The frame of data is DMA'ed to the host memory, and an interrupt call is made to the host to inform it that frames have been received (see note on promiscuous mode below).

Some interfaces provide much more processing, and can be programmable.

The network interface (shown in the figure below) connects to the media. This may use either an external transceiver (as shown) or an internal integrated transceiver mounted as a part of the network interface.

There is also a page showing examples of various types of networking equipment, include NICs for Ethernet.

Ethernet Source Address

Each network interface card is assigned an Ethernet source address by the manufacturer of the network interface card (this is normally stored in a PROM on the network interface card).

All source addresses are globally unique, and are assigned in blocks of 16 (or 8) million address to the Ethernet interface manufacturers, according to a flat addressing structure. This ensures that no two Ethernet network interface will ever have the same source address.

Promiscous Mode

The promiscuous mode in Ethernet suspends the normal receive filtering, where a NIC normally only forwards frames with a destination addresses that matches a pre-configured set of address, including the broadcast address, any registered multicast addresses and card's own Source Address,

In Pormiscuous mode all frames are forwarded. This mode is required for some packet capture applications. It is also needed for a system to use a NIC in a bridging mode.

See also




List of Source Addresses

Transceivers and AUI Drop Cables

VLAN and Priority Tags

Gorry Fairhurst - Date: 18/03/2020