What is a packet

It Cannot Go All at Once

Suppose you have a big piece of data: a photo, a video, a large file. You would like to send it whole, all at once.
But the road the data travels on is narrow, and a big lump will not fit. As one lump, it cannot even set off.

Press send whole and the big lump of data tries to enter the narrow road and gets stuck at the entrance. The road is too narrow for it to go in one piece.

Trying to push it through whole, it got stuck at the entrance. The lump was simply too big for the narrow road.
The real internet works the same way. There is a fixed size that can be sent at one time, and anything bigger cannot go as it is.
So there is only one way: cut it into small pieces.

Cutting It into Small Pieces

The big lump is divided into several small pieces, each sized to slip onto the narrow road.
These small pieces are called packets. It is a bit like splitting a long letter across several envelopes.
Now the pieces can set off.

Each press on the lump of data breaks off one more small packet. Once it is split into four pieces, every piece fits the narrow road.

The big lump has become several pieces, and each can now go onto the narrow road one at a time.
But a problem appears. The pieces all look the same, so there is no way to tell which one comes where in the order, or where each should go.
So each piece needs a mark.

Every Piece Gets a Tag

Each piece gets a tag with two things written on it. One is a number showing which piece it is in the order; the other is the address the piece has to reach.
With a number and an address, a piece can find its place even if it travels alone. Real tags also carry the sender's address, so the receiver can send a reply back.

Press each packet to attach a tag with its number and destination address. Once all four pieces are tagged, they are ready to go.

Now every piece carries a number and an address, and the pieces set off. They do not have to travel side by side on the same road, though.
If one road is blocked, a piece takes another. So the pieces can arrive in a jumbled order.

Putting Them Back Together on Arrival

All the pieces have arrived, but the order is mixed up. Number 3 may have come first and number 1 last.
There is no need to worry, because every piece has its number written on it. Line them up in number order and the scattered pieces become the original data again.

Press the scattered packets in number order (1, 2, 3, 4) and the pieces join one after another until the original data is back. Press the wrong one and it tells you which number comes next.

Joined in number order, the original data came back: the photo that had been split is one picture again.
The receiver never even notices that the pieces came separately; it simply sees the finished photo. This is how packets work.

Summing Up

In one line: big data cannot be sent all at once, so it is cut into small packets, and each piece gets a tag with its number and address.
The pieces may travel separately, so they can go around a blocked road, and on arrival they are joined in number order. Split, tag, reassemble: that is a packet.

Each press of the button reveals one more key point. (split → tag (number + address) → send separately → join in number order) Once all four have appeared, the review is done.

Now you know what a packet is. Even big data, cut into pieces, can travel far along a narrow road.
But what does the address on the tag actually look like? Among countless computers, how does it point to exactly one?
The next lesson is about that address, the one every computer is given.