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← CourseSwitching, ARP, MAC, and VLAN intro~5 min read

Switches, MAC/ARP, and VLANs

Switches forward frames by MAC address

A switch connects many devices on one LAN. It learns which MAC address lives on which port and forwards frames only where they need to go. If it doesn't know the destination yet, it floods the frame to every port in the same VLAN.

ARP maps IP to MAC

To deliver locally, a device needs the destination's MAC. It broadcasts 'who has 192.168.1.50?' (ARP); the owner replies with its MAC, and the switch learns the path. This is why local delivery is a two-step: resolve the MAC, then forward the frame.

VLANs split one switch into many

A VLAN puts each access port into a virtual network. Ports in different VLANs can't talk through the switch - instant segmentation with no new cabling. A trunk carries multiple VLANs between switches; a VLAN left off the trunk simply can't cross it.

Concept cards

MAC address

A hardware address identifying a network interface on the local segment.

A device's fingerprint on the LAN.

ARP

Address Resolution Protocol - finds the MAC for a known local IP.

Shouting a name across the room to find who answers.

VLAN

A virtual LAN that isolates a group of switch ports into their own broadcast domain.

Dividing one open office into separate locked rooms.

Trunk

A switch-to-switch link that carries several tagged VLANs at once.

A multi-lane highway between buildings, one lane per department.

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