A link layer moves data across one local link or local network segment. Its unit of transmission is commonly called a frame.

A frame wraps a higher-layer packet with link-local control information. On Ethernet-like networks that includes source and destination MAC addresses, a protocol-type field and an integrity check.

MAC addresses identify link-layer interfaces for local delivery. They are not Internet-wide routes: a router normally removes the incoming link-layer frame and creates a new one for the next link while the IP packet continues onward.

For example, when host A sends an IP packet to its router, the Ethernet frame is addressed to the router's MAC address even though the IP packet's destination may be a remote server.

This distinction is fundamental: IP addresses guide end-to-end forwarding across networks; link-layer addresses guide delivery over the current link.