What's an IP address?

Your name and where you live

You have a name, and it goes wherever you go. You also have a home address, and that changes when you move house. If a friend in another country wants to post you a letter, your name alone isn’t enough: the post office needs the address, with the country, town and street, to know where to take it.

Every host already has a MAC address (from lesson 1.1): that’s its name. It never changes, and it’s only used to get a frame to the next stop on the same network (layer 2).

So hosts get a second address, an IP address: that’s their home address. It depends on which network they’re plugged into, and it’s used to get a packet across the world, through lots of networks and routers (layer 3). IP stands for Internet Protocol: the agreed rules (a protocol) for addressing and delivering packets across the internet.

in one sentence A MAC address is a host’s name; an IP address is where it lives right now.

What one looks like

The IP addresses in this course are IPv4 addresses (Internet Protocol version 4). They look like this:

192.168.1.10

Four numbers with dots between them. Each number can be anything from 0 to 255, so 192.168.1.300 isn’t a real address. Each of the four numbers is called an octet (oct means eight, like an octopus’s eight arms), because inside the computer each one is made of 8 bits. Four octets make 32 bits in all.

in one sentence An IPv4 address is four numbers from 0 to 255 with dots between, like 192.168.1.10.

Numbers as light switches

Remember that a bit is a 1 or a 0, like a light switch that’s on or off. Imagine a row of 8 light switches, and each switch is worth a different amount: 128, 64, 32, 16, 8, 4, 2 and 1. To make a number, switch on the ones that add up to it. That’s called binary: counting with only 1s and 0s.

That’s why 255 is the biggest: it’s all eight switches on.

in one sentence Each octet is 8 bits, and binary turns them into a number by adding up the switches that are on.

The street and the house number

A home address has a street (shared by everyone who lives on it) and a house number (different for each house). An IP address is split the same way:

• the network part is the street: every host on the same network has the same one.
• the host part is the house number: different for every host on that network.

For 192.168.1.10 on a home network, 192.168.1 is the network (every device at home starts with it) and 10 is this host. Exactly where the line between the two goes is the next lesson.

in one sentence The front of an IP address says which network, the end says which host on it.

Private and public addresses

Think of a hotel. Inside, the rooms have numbers, like room 214, and every hotel in the world has a room 214. But to post a letter to the hotel, you use its street address, which is unique. The front desk takes the letter and brings it up to the right room.

IP addresses work the same way. Some are kept for use inside homes, schools and companies, and are never used out on the internet. They’re called private addresses (the room numbers):

Every other address is public: a real, unique address out on the internet (the street address). Millions of homes all use 192.168.1.something inside, and that’s fine, because the home router acts like the front desk: it swaps the private address for the home’s one public address on the way out, and back again on the way in. That swap is called NAT, network address translation (translation means changing one thing into another).

in one sentence Private addresses are used inside a home or school; public addresses are unique on the internet; the router swaps between them.

And IPv6?

There are only about 4 billion IPv4 addresses, and that isn’t enough for every phone, laptop and smart light bulb on Earth. So there’s a newer version, IPv6 (Internet Protocol version 6), with much longer addresses that look like 2001:db8::10. They work the same way, with far more addresses. This course sticks to IPv4, because it’s still what you’ll see first.

in one sentence IPv6 is the newer version with far more addresses; the ideas are the same.