What's a host?

This whole course starts from nothing. Every word gets explained before we use it, starting with the everyday ones.

Passing a note in class

Imagine you want to tell a friend on the other side of the classroom something. You write it on a scrap of paper, fold it, write their name on the outside, and pass it along the row. Each person in between looks at the name and passes it on, until your friend gets it, opens it and reads it.

That’s everything a computer network does. The rest of this course is just the details.

in one sentence Sending something over a network is like passing a named note across a classroom.

Data and messages

Data is any information a computer keeps or sends: words, photos, songs, videos, the moves in a game. Inside the computer, all of it is stored as numbers.

A message is some data sent from one place to another, like your note.

in one sentence Data is information; a message is some data on its way somewhere.

What’s a network?

A network is a group of things joined together so they can send each other messages. The classroom is a network: people who write notes, and a way of passing them along.

Computers join up in two ways:

• with a cable: a wire with a plug on each end. Electricity travels along it, carrying the message. The usual network cable is called an Ethernet cable. Ethernet is just the name of the most common kind of wired network; the plug looks like a chunky phone plug.
• with Wi-Fi: the same idea, but using radio waves, invisible signals through the air, like a walkie-talkie. No cable needed.

Two laptops joined by one cable are a network. So is the whole internet: millions of networks all joined together, all over the world.

in one sentence A network is things joined by cables or Wi-Fi so they can send each other messages.

Hosts: the ones writing and reading

In the classroom, some people write and read notes: you and your friend. Others just pass them along.

On a network, the things that send and receive their own messages are called hosts. Your laptop is a host. So is a phone, a games console, a smart TV, a printer, and the big computers far away that send you videos and web pages.

The boxes in between that only pass messages along are called network devices. You’ll meet them in chapter 2.

in one sentence A host is anything that sends or receives its own messages, like a laptop or a phone.

Addresses: who is it for?

On the classroom note, you wrote your friend’s name on the outside so everyone knew who it was for. On an envelope you write an address. An address is just the thing that says who, or where, a message is going to.

Every host on a network needs one too, or nobody would know who a message is for.

in one sentence An address says who a message is for, like the name on the outside of a note.

The network card and its MAC address

The part of a host that actually sends and receives messages is its network card. It’s the bit the cable plugs into, or the little radio that does Wi-Fi. (People also call it a NIC, short for network interface card: interface means the place where two things meet, here the computer and the network.) A laptop usually has two: one for Wi-Fi, one for a cable.

Every network card is given a name at the factory, and it keeps it for life. That name is its MAC address. (MAC stands for Media Access Control, a technical way of saying the card’s own address on the network. It’s nothing to do with Apple Mac computers: every network card has one.) It looks like this:

AA:AA:AA:11:11:11
3C:22:FB:9A:04:7E

Six pairs of characters, with colons between them. They use hexadecimal (hex for short), a way of counting with sixteen digits instead of ten: 0 to 9, then A to F, so A means ten and F means fifteen. Don’t worry about the maths: just know it’s a name made of numbers and letters. The first half usually says which company made the card; the second half is that card’s own number.

in one sentence Every network card has a MAC address: a name it got at the factory, like AA:AA:AA:11:11:11.

Packets: lots of small notes

What if your note is really long, like a whole story? You might tear it into numbered postcards and send them one at a time. Then other people can pass their notes in between, and if one postcard gets lost, you only rewrite that one.

Computers do exactly this. They chop a big message into small pieces called packets. Every packet has a to address and a from address written on it, like an envelope.

in one sentence Big messages are chopped into small packets, and each packet carries a to and a from address.