linklayer

Link layer, or the data link layer, is the OSI model’s second layer. It enables node-to-node data transfer over a physical link by encapsulating network-layer packets into frames, providing hardware addressing (MAC addresses), and performing error detection with a frame check sequence. It also coordinates access to shared media and may implement simple flow control.

In IEEE 802 networks the data link layer is typically divided into the Logical Link Control (LLC) sublayer, which multiplexes higher-level protocols, and the Media Access Control (MAC) sublayer, which handles addressing and access rules. Most devices implement the MAC directly.

Functions include frame delimitation, addressing, error detection, and medium access control. Examples of link-layer technologies are Ethernet (wired), Wi‑Fi (802.11), Bluetooth, PPP and HDLC. Switches and bridges operate at this layer, forwarding frames based on MAC addresses; VLANs (802.1Q) segment networks at the link layer.

Security and management at the link layer include authentication and access-control mechanisms such as 802.1X, as well as link-layer encryption for wireless networks. The link layer interfaces with the physical layer and with higher layers, and it often relies on ARP to map IP addresses to MAC addresses. Its design underpins local-area networks and the way the Internet connects across network boundaries.