Packet Postman
Every message travels in pieces.
Your mission
Dani is sending you a message across the Internet, but the Internet won't carry it as one big block. Watch it split into packets, race through different routers and arrive in a jumble. Then lose some packets on purpose, rescue them with TCP, and deliver six tricky messages yourself.
Explore
Send Dani's message, look at what happens on the map, then tap a packet to look inside.
Dani wants to send you a message across the Internet. Press Send it and watch what happens on the way. Then tap a packet to look inside it.
0 of 5 discoveries made
Experiment
Change the message and the packet size, lose packets on purpose, cut cables and switch TCP on and off.
Tap the scissors on a cable to cut it, and the red plug to repair it. Routers send packets around broken cables.
- 1A packet is lost on the way.
- 2The receiver notices that a piece is missing.
- 3TCP asks the sender to send that packet again.
- 4The sender retransmits it, and the receiver rebuilds the message in the right order.
Post office experiments
Try each one. They tick themselves off.
0 of 5 experiments done
Challenge
Six deliveries: rebuild messages, rescue lost packets, find a route and pick the right protocol.
Postman's notes
- Messages travel as numbered packets
- Packets can take different routes
- They can arrive in any order
- TCP spots missing packets and asks for them again
- HTTP/HTTPS: web pages · TCP/IP: Internet communication · FTP: file transfer · SMTP: email
Finish all 6 deliveries to finish this stage.
Understand
Here is what the post office was showing you, in the words you need for the exam.
Computers need rules too
Two people can only talk if they follow the same rules, like speaking the same language and taking turns. Computers are the same. A protocol is a set of rules governing communication: how data is formatted, sent, received and checked. Different jobs use different protocols.
| Protocol | Job | Used for |
|---|---|---|
| HTTP / HTTPS | Web communication | Loading web pages (HTTPS is the secure version) |
| TCP/IP | Internet communication | Reliable communication across the Internet |
| FTP | File transfer | Uploading and downloading files between devices |
| SMTP | Sending email | Sending electronic mail |
Packets
The Internet doesn't send a message as one giant block. The data is divided into smaller units called packets. Each packet carries a small piece of the data, and a label with where it's going and which piece it is.
For example, "HELLO DANI!" became four numbered packets: HEL, LO␣, DAN and I! (␣ marks a space).
Packet switching
Packets travel independently. Routers can send packets of the same message along different routes to the same destination, so they may arrive in a different order. The receiver uses the packet numbers to reconstruct the original data.
This is called packet switching. It makes the Internet more reliable and efficient: if one route is broken or busy, packets go another way.
Networks are not perfect: how TCP helps
Sometimes a packet is lost during transmission. TCP (Transmission Control Protocol) makes delivery reliable:
- 1Packet 3 is lost on the way.
- 2The receiver notices that some data is missing.
- 3TCP asks for the missing packet to be sent again.
- 4The sender retransmits packet 3, and the receiver reassembles the data in the correct order.
So TCP detects missing data, requests retransmission and reassembles packets in the correct order. Without it, the message would arrive with a gap and nobody would notice.
Exam tip
When asked how a message crosses the Internet, use the key words in order: the data is split into packets, packets may take different routes (packet switching), they may arrive out of order, and they are reassembled using their numbers. If a packet is lost, TCP detects it and asks for it to be retransmitted. For protocol questions, match the job: web pages → HTTP/HTTPS, email → SMTP, files → FTP.