How does the internet send a message from Perth to London?

You press send

Imagine you live in Perth and you send a message to your cousin in London, on the other side of the world. You press send, and a moment later it pops up on their screen. It feels like magic, but it is really a very fast relay race.

The internet is a giant network of computers all linked together: phones, laptops, game consoles and huge computers called servers. They are joined by cables, radio signals and special computers that act like traffic controllers. Your message has to find its way across all of them.

Chopped into packets

Your device does not send your message in one big piece. It chops it into small chunks called packets. A big photo or video can be split into thousands of them.

Each packet is a bit like a postcard. Part of it holds a piece of your message. The other part, called the header, is like the address written on the front. It says where the packet is going, where it came from, and a number showing where it fits in the message.

Here is the clever part. The packets do not all have to travel the same way. Some may take one path and some another, so they can arrive jumbled up. The computer at the other end uses the numbers to put them back in the right order. Because each packet finds its own way, the internet can steer around a broken or crowded link without stopping.

Every device has an address

To deliver a packet, the internet needs an address. Every device that connects to the internet needs an IP address. It is written as numbers, such as 192.0.32.7.

Numbers are hard for people to remember, so websites also have names, like icann.org. A system called the Domain Name System, or DNS, works like a giant phone book. It looks up the name you type and finds the matching number.

There are so many devices now that the older kind of IP address has almost run out. A newer kind, called IPv6, has room for about 340 trillion trillion trillion addresses. That should last a while!

Hop, hop, hop

Your packets first go to a router. The Wi-Fi box in your home is one. A router is a computer that reads the address on each packet and sends it one step closer to where it is going.

No single router knows the whole route to London. Each one simply passes the packet to the next router that seems best, a bit like passing a note along a line of friends. Your packets travel from your home into your internet company's network, then on through bigger and busier routers, hop after hop.

Under the sea

Australia is an island, so how do packets reach London? Around 99 per cent of Australia's international internet and phone traffic travels through cables lying on the sea floor.

These cables are usually no wider than a garden hose. Inside are optical fibres, thin threads of glass that carry information as flashes of light, wrapped in thick plastic to protect them. Some lie as deep as 8,000 metres under the ocean.

Here is one path your packets could take. A cable called INDIGO-West runs about 4,600 kilometres from Perth to Singapore. From Singapore, cables such as SeaMeWe-5 run past Sri Lanka, up the Red Sea, across Egypt and through the Mediterranean Sea to the south of France. From there, more cables and networks carry the packets on to London. Other packets might go a completely different way.

All of this happens in a fraction of a second. Then your cousin's phone puts the packets back in order, and your message appears.

📚 Words to know

Packet

A small chunk of a message, with a header that says where it is going and where it fits.

IP address

A number that identifies a device on the internet, so packets can find it.

Router

A computer that reads the address on each packet and sends it one step closer to where it is going.

Optical fibre

A thin thread of glass that carries information as flashes of light.

Network

A group of computers and other devices linked together so they can share information.

🧪 Try this: be the internet

1. Write a short message of about 12 words on a strip of paper. Cut it into four pieces, cutting between words.
2. On each piece write a header: who it is to, who it is from, and its number (1 of 4, 2 of 4, and so on). Now each piece is a packet.
3. Ask family members or classmates to stand around the room as routers. Routers can only pass a packet to someone next to them. Hand the packets to different routers and let them pass them along until they reach the person the message is for.
4. The receiver puts the packets back in order using the numbers. Now ask one router to sit down, as if their cable broke. Send another message. Can the packets find a different way?