An Ethernet switch forwards frames between local ports without examining application data.

The switch learns which MAC addresses are reachable through which ports by observing the source address of arriving frames. It stores these associations in a forwarding table.

When a frame arrives:

• if the destination MAC is known, the switch forwards it only to the learned port;
• if the destination is unknown, it floods the frame to other ports in the same broadcast domain;
• broadcasts are flooded by design.

Suppose a frame from MAC A arrives on port 1. The switch learns A → port 1. Later, a frame for A arriving on port 4 can be sent directly to port 1.

Learning makes Ethernet forwarding adaptive without requiring every host to know the physical topology.