A switch can learn a host's Media Access Control (MAC) address by inspecting the source MAC address of a frame arriving on one of its ports.

### Why this is right A switch associates the source MAC address in an arriving frame with the port on which that frame arrived. It stores that association in its MAC table, which helps it forward later frames toward the learned destination. ### The concept A MAC table is a switch's record of learned MAC addresses and their associated ports. The switch learns from incoming traffic rather than needing an administrator to enter every host address manually. The material gives a default learned-address lifetime of 5 minutes, or 300 seconds. If an address is not refreshed before it ages out, the switch can remove the entry and learn it again when traffic arrives. Learning a source address is different from forwarding based on a destination address: the source teaches the switch where a host is, while the destination determines where the current frame should go. ### Terms - **MAC**: Media Access Control. The addressing system used for Ethernet delivery on a local link. - **Switch**: A device that forwards frames within a local network. - **MAC table**: A switch record that maps learned MAC addresses to ports. - **Frame**: A data unit carried over a link such as Ethernet. - **Port**: A switch connection through which frames enter or leave. ### Remember A switch learns from the source MAC and forwards according to the destination MAC.