A Layer 2 switch forwards Ethernet frames based on MAC (media access control) addresses. Unlike a hub, which repeats every bit out every port, a switch sends each frame only where it needs to go. That gives every port its own collision domain and allows full-duplex operation, so collisions disappear on modern switched networks. Understanding exactly how a switch decides where a frame goes is the foundation for VLANs (virtual LANs), spanning tree and Layer 2 security features such as port security.
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