Link layer

7.   Link layer

The link layer is the networking scope of the local network connection to which a host is attached. This regime is called the link in TCP/IP literature. It is the lowest component layer of the Internet protocols, as TCP/IP is designed to be hardware independent. As a result TCP/IP may be implemented on top of virtually any hardware networking technology.

The link layer is used to move packets between the Internet layer interfaces of two different hosts on the same link. The processes of transmitting and receiving packets on a given link can be controlled both in the software device driver for the network card, as well as on firmware or specialized chipsets. These perform data link functions such as adding a packet header to prepare it for transmission, then actually transmit the frame over a physical medium. The TCP/IP model includes specifications of translating the network addressing methods used in the Internet Protocol to data link addressing, such as Media Access Control (MAC), however all other aspects below that level are implicitly assumed to exist in the link layer, but are not explicitly defined.

This is also the layer where packets may be selected to be sent over a virtual private network or other networking tunnel. The TCP/IP model's link layer corresponds to the Open Systems Interconnection (OSI) model physical and data link layers, layers one and two of the OSI model.