Last updated · reviewed for accuracy
How switches learn
A switch is born knowing nothing. It learns by eavesdropping: every arriving frame teaches it “that source MAC lives on that port.” Frames to unknown destinations get flooded everywhere; the reply teaches the switch where the host actually is, and from then on delivery is precise.
This self-filling table is why switches need zero configuration, and why the first packet to a quiet host behaves differently from the second. You’ll see both in a capture.