The corridor inside the building (frames, switches and VLANs)

Your building, its doorman (the router), and the flats behind him. This lesson walks one floor further in: the corridors between the flats, and the attendant who decides which corridor a letter goes down.

Before you start. From Inside your building (your router's life) you have the building: the router is a doorman who hands every device a flat number (its IP address on the home network), and the wire inside delivers by a different address, a doorplate burned into each device’s hardware, called a MAC address. You also have ARP, the shout “who has flat 1?” that everyone hears and only the owner answers. From How a parcel finds any house on earth you have depots handing parcels between buildings. This lesson stays inside one building, where the street address no longer helps and only the doorplate does.