Hubs, Switches and Access Points

In a drawer at Kite Studio sits an old 8-port box labelled "hub", left from the studio's first year. The rack now holds a 24-port switch, and a Wi-Fi access point hangs from the ceiling. All three connect devices inside one LAN, but they differ in one decisive way: how much of each frame they actually read.

The rule

A hub shouts to all, a switch speaks to one, an access point bridges air to wire.

Hub — reads nothing. Whatever arrives on one port is repeated, as raw signal, out of every other port. Every device hears every frame and must ignore the ones not meant for it. Only one device can transmit at a time without their signals colliding, so everyone shares the bandwidth. Hubs are obsolete, but they explain why switches exist.

Switch — reads the MAC addresses in each frame. It keeps a MAC address table that maps each MAC to the port where it was last seen, learned simply by noting the source MAC of every frame that arrives. When a frame comes in, it looks up the destination MAC:

• known → send it out of that one port only;
• unknown, or the broadcast address ff:ff:ff:ff:ff:ff → flood it out of every port except the one it came in on.

Each port gets its own full-speed, two-way link, so many pairs of devices can talk at once.

Access point (AP) — the doorway between Wi-Fi and the wired LAN. It turns radio frames from laptops and phones into Ethernet frames for the switch, and back again. Wireless clients on one AP share the same radio channel, a little like devices on a hub, which is why a crowded meeting room feels slow.

The shortcut

For any frame, ask: "Does this device know where the destination lives?" A hub never knows, so it repeats everywhere. A switch knows once it has seen that MAC as a source, so it sends to one port. An AP passes frames between radio and cable and lets the switch decide the rest.

The trap is thinking flooding means the switch is broken. Flooding unknown and broadcast frames is normal; it is exactly how the table fills in.

Drills

Answer fast — trust the rule.

Remember

• A hub shouts to all, a switch speaks to one, an access point bridges air to wire.
• A switch learns from source MACs and forwards by destination MAC.
• Unknown and broadcast frames are flooded — that is normal learning, not a fault.