Hubs, switches and routers

The apartment building’s mail room

Two computers can share one cable. But plug in three, or thirty, and you need a box in the middle that everyone’s cable plugs into, to pass messages between them.

Think of a big apartment building, and three different ways the post could get from flat to flat:

• The loudspeaker. Whoever has a letter reads it out over a loudspeaker in every flat. Everyone hears everything, and only one person can use the loudspeaker at a time.
• The mail room clerk. A clerk who knows which flat everyone lives in takes each letter straight to the right door.
• The post office. For a letter to another town, the clerk hands it to the post office, which knows the way to every town.

Those are the three boxes in this lesson: the hub, the switch and the router.

in one sentence A hub shouts everything to everyone, a switch delivers to the right door, and a router sends things to other networks.

Ports

A port on one of these boxes is a socket where a cable plugs in, like the numbered mail slots in a mail room. Ports are numbered: port 1, port 2, and so on. (This is a different thing from the port numbers in lesson 1.2: same word, different meaning. Networking does that sometimes, sorry.)

in one sentence A port is a numbered socket on a box, where a cable plugs in.

The hub: the loudspeaker

A hub doesn’t read anything. Whatever signal comes in on one port, it repeats straight out of every other port. That makes it a layer 1 box: it only deals with the signal itself.

Because everyone shares, only one host can talk at a time. If two talk at once, their signals crash into each other and both are ruined. That’s called a collision, and both have to try again.

Hubs are very old: you’d only find one in a museum now. But they’re the easiest way to see why switches were invented.

in one sentence A hub repeats every message to everyone, so only one host can talk at a time.

The switch: the mail room clerk

A switch reads the MAC address on every frame (remember from lesson 1.2: a frame is a message in its layer 2 envelope, with a to and from MAC address on it). That makes it a layer 2 box.

It keeps a list called the MAC address table: which MAC address lives on which port. Nobody types it in; the switch fills it in by itself. Every frame says who it’s from, so when one arrives, the switch notes that MAC address is on this port.

Then it sends each frame only out of the port where the to address lives. Sending a message on like this is called forwarding it. If the switch doesn’t know where the to address is yet, it sends a copy out of every port, just this once. That’s called flooding.

in one sentence A switch learns which MAC address is on which port, and sends each frame only where it needs to go.

The router: the post office

A switch joins hosts on one network. To reach a host on a different network, like your home reaching a website, you need a router.

A router reads a different address: the IP address. You’ll learn all about IP addresses in lesson 3.1; for now, think of it as a postal address with a town in it. The first part of an IP address says which network (which town) a host is in. That makes the router a layer 3 box.

A router has a foot in each network it joins, and keeps a routing table: a list of networks and which way to send messages for each one, like the post office knowing which van goes to which town.

in one sentence A router joins different networks, and reads the IP address to pick the way to the right one.

To keep the pictures readable, the MAC addresses below are shortened to one letter: PC A’s MAC is just A. (A PC, personal computer, is any laptop or desktop computer.)