Network Switch

A network switch is a Layer 2 device that learns which MAC address sits behind each of its ports and forwards each Ethernet frame only to the correct port, instead of repeating it everywhere as an old hub did.

Why it matters

Switches give every connected device its own collision-free link and full bandwidth, and they are the backbone of offices and data centers. Managed switches add VLANs, port mirroring, link aggregation and monitoring. A switch normally does not route between subnets; a router or Layer 3 switch does that.

Example

Port 1  aa:bb:cc:00:00:01  laptop
Port 2  aa:bb:cc:00:00:02  printer
Port 3  aa:bb:cc:00:00:03  uplink to router

A frame addressed to the printer's MAC leaves only on port 2. An unknown or broadcast address is flooded to all ports except the one it arrived on.

Common mistakes

• Plugging both ends of one cable into the same switch, creating a loop and a broadcast storm when spanning tree is off.
• Buying an unmanaged switch when you need VLANs or visibility.
• Expecting a switch to share one Internet address among devices. That needs a router.

Related

• MAC address
• VLAN
• Ethernet
• Router

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