What is an IP address

Every House Has an Address

Last lesson we saw that each packet carries a tag with a number and an address. That address is the address of the receiving computer.
Think about sending a letter. You write the recipient's address, and that is how the mail carrier finds the house.
On the internet, too, every computer needs an address of its own.

Tap the houses one by one to put up address signs. Once all four have a sign, every computer has exactly one address.

The more signs went up, the clearer it became which house was which. Without addresses, the houses were just identical boxes.
With an address, you can finally point and say "that one, right there." So what does such an address look like?

An Address Made of Numbers

People's addresses are written in words, but a computer's address is written in numbers, because computers are good at handling numbers.
The common kind of IP address is four slots of numbers separated by dots. It looks like 203.0.113.5, and each slot holds a number from 0 to 255. The dots work as fences between the slots.
These days a newer, longer kind of address with more slots is used alongside it, but the idea of pointing to a computer with numbers is the same.

Tap the slots of the address one by one and the number in each appears in turn. After all four, you can see how four slots of numbers separated by dots make up one address.

We looked at the four slots in order. Each holds a number, and the dots divide them. Written this way, both people and computers can read it.
A different address has different numbers in its slots. One bundle of numbers like this points to one computer on the internet.

Finding the Way by Address

A packet has the destination address written on it. The internet reads that address and sends the packet to the matching computer.
It is like a mail carrier reading the address on an envelope and putting the letter in that house's mailbox. As long as the address is right, it finds exactly that house among countless others.

Read the destination address written on the packet, then tap the house with the same address to deliver it. Tap a different house and you are told the address does not match.

Tapping the house with the destination address delivered the packet exactly. Tapping another house failed, because the address did not match.
Read the address, find the same address, send it there: that is how the internet works.
There is one condition, though. No two computers may share the same address.

An Address Must Be One of a Kind

What if two houses in the same neighborhood used the same address? The mail carrier would not know which house to deliver to.
IP addresses are the same. If two computers have the same address, the packet does not know where to go.
So an address that is visible on the internet must be one of a kind. Only then does a packet arrive exactly, without confusion.

Press send to the clashing address, then send to the unique address. Where two houses share an address, the packet wanders, not knowing where to go; where the address is unique, it arrives at exactly one house.

Sent to the clashing address, the packet wandered between two houses. Sent to the unique address, it went straight to one.
The only difference was whether the address was one of a kind. That is why, on the real internet, designated organizations hand out addresses so that no two ever clash.
How the devices behind a home router share one address is the subject of a later lesson.

Summing Up

In one line: every computer has a unique address, the IP address, usually written as four slots of numbers separated by dots.
A packet reads the destination address and finds the right computer. The address must be one of a kind so nothing gets confused: like a home address, there is only one of it in the world.

Tap the four key points in order to review. (an address for every house → an address made of numbers → finding the way by address → must be one of a kind) When all four are lit, the review is done.

Now you know every computer has an address. Numbers like 203.0.113.5 are hard for people to remember, so we usually use names made of words instead; the help desk that turns those names into number addresses comes in a later lesson.
Before that, one question. Now that the address is known, does the packet fly straight to it in one go? The next lesson follows the stops a packet passes through on the way to its destination.