A network switch adds the source MAC address and the incoming port number of each frame to its switch (MAC) table. In slightly more detail: when a frame arrives on a switch port, the switch inspects the Ethernet header and learns “the device with this source MAC is reachable via this port.” It records that mapping (MAC → port) in its CAM/MAC address table, and may optionally include extra metadata like VLAN ID and a timestamp for aging, depending on the switch model and configuration.