How does data travel on the internet
Suppose you send a letter to a friend who lives far away. The letter does not fly to your friend's house in one go. It passes through your local post office, a big sorting center, and your friend's local post office, one after another. Data on the internet likewise reaches its destination by passing through intermediate stops one at a time.
It Cannot Get There in One Go
My computer and my friend's computer each have their own unique address, the number address we just saw.
But the two are not joined directly by a single wire. There is no way to tie every computer in the world to every other with its own cable.
So data cannot go straight there; it needs places to pass through along the way.
Tap between the start and the end, and the path that looked like one straight line reveals three intermediate stops that were hidden in it.
The path that looked straight actually had several stops on it. Just as a letter passes through post offices, data passes through intermediate stops.
These stops are called routers. So what do they do with the data they receive?
A Stop Receives and Passes It On
What one stop does is simple: it takes in the data that arrives and passes it on to the next stop.
It is like a postal worker who looks only at the address on the envelope and sends it on to the next post office. A stop does not open the letter inside; it just passes on what it receives.
This simple action is the heart of it.
Tap the middle stop once and it receives the packet from the left; tap it again and it passes the packet on to the next stop on the right. (stop = router) Those two things are all a stop does.
When you tapped the stop, the packet moved on to the next one. Receive, pass on: that is all a stop does.
But with only one stop you cannot get far, because your friend is not one stop away but many.
So many stops are needed.
Passing Through Many Stops
One stop passes it on, the next stop receives it and passes it on again, and that one passes it on to the next. Repeated over and over, the packet moves from stop to stop, getting one step closer to its destination each time.
It is a bit like crossing a stream on stepping stones. The gap between two stones is short, but step on enough of them and you are across.
Each press of send moves the packet one stop further. After passing the four intermediate stops in turn, it reaches your friend.
The packet stepped through the stops one by one and finally arrived. There may seem to be a lot of stops in between, but that is nothing to worry about, since each one only has to pass the packet on.
That simple job, chained together, is what links faraway places. Now let us look more closely at what happens when a stop passes a packet on.
Holding It, Then Sending It Again
A stop does not bounce an incoming packet straight back out. It first takes the packet in, holds it for a moment, and then passes it on.
So receiving and sending are separate actions. Just as you have to hold a letter before you can drop it in the next mailbox, a stop has to receive a packet before it can pass it on.
While a stop is holding packets, they can pile up and have to wait in line, which is one of the reasons the internet sometimes slows down.
Press receive and the stop holds the packet for a moment; press send and it passes the packet on to the next stop.
Receive, then send: the two actions form a pair. Repeat them at every stop and the packet is carried all the way.
But while the packet waits at a stop, the stop has one more thing to decide: which stop should it go to next? Choosing a path when there are several branches is something we will meet a little later.
Summing Up
In one line: data cannot reach its destination in one go, so it passes through several intermediate stops called routers.
Each stop receives the packet and passes it on, repeating receive and send at every stop. Getting one step closer each time, it can reach any place, however far.
Each press of the button reveals one more key point. (it cannot go in one go → a stop passes it on → through many stops → receive, then send again) Once all four have appeared, the review is done.
Now you can see how data moves from stop to stop until it reaches somewhere far away. How a stop chooses the next stop at a fork is called routing, and that story comes a little later.
First there is something else to see. Even sending one letter involves separate people: one writes it, one addresses the envelope, one carries it. The next lesson is about how a network splits its work into layers like this.