Why Data Is Split into Packets
When you send a photo, stream a video or load a web page, the data does not travel as one big block.
Instead, it is broken into many small pieces called packets. Each packet travels across the network on its own. When all the packets reach the destination device, they are put back together to rebuild the original data.
↓ split into packets
Packets travel separately
↓ reassembled at the destination
File
Why go to all this trouble? Because small packets make a network much more efficient and reliable:
- Sharing the network: packets from many different users can be sent along the same connections, taking turns, so no single transfer blocks everyone else.
- Avoiding problems: if one route is busy or broken, packets can be sent another way.
- Fixing errors efficiently: if something goes wrong, usually only the damaged or missing packet needs to be sent again — not the whole file.
Think of Numbered Envelopes
Imagine posting a long story to a friend, one page per envelope. Each envelope has the address on the front, one page inside, and a seal on the back. You number every envelope, so even if they arrive on different days, your friend can put the pages back in the right order.
That is exactly how packets work, and we will use this picture throughout the guide.
Where This Fits in the Syllabus
Cambridge IGCSE Computer Science (0478, 2026–2028) section 2.1 asks you to:
- understand that data is broken down into packets to be transmitted
- describe the structure of a packet
- describe the process of packet switching
The syllabus names three items in the packet header: the destination address, the packet number and the originator's address. Learn those three exactly. Anything marked Extension below is useful background rather than named syllabus content.
Check Your Understanding 1
Give one reason why data is split into packets instead of being sent as one large block.
Check the answer
Any one of: packets from many users can share the same network connections; packets can be sent along different routes to avoid busy or broken links; if an error occurs, only the affected packet needs to be resent, not the whole file.
The Structure of a Packet Syllabus
Every packet has three parts: a header, a payload and a trailer.
| Part | What it contains | Why it is needed |
|---|---|---|
| Header | The destination address, the packet number and the originator's address | Tells the network where the packet is going, where it came from, and where it fits in the order |
| Payload | The actual data being sent — one piece of the original file or message | This is the content the receiver wants |
| Trailer | Data that marks the end of the packet, and error-checking data | Shows where the packet ends, and lets the receiver check whether the packet was damaged on the way |
The three header items in detail
| Header item | What it is | What it is used for |
|---|---|---|
| Destination address | The IP address of the device the packet is being sent to | Routers read it to decide where to send the packet next |
| Packet number | The packet's position in the sequence, e.g. packet 3 | Lets the receiver put the packets back in the correct order, and spot any that are missing |
| Originator's address | The IP address of the device that sent the packet | Lets the receiver reply, or ask for a missing or damaged packet to be sent again |
Back to the envelopes
| Envelope | Packet |
|---|---|
| Address on the front, return address and page number | Header |
| The page of the story inside | Payload |
| The seal, showing where the envelope ends and that it has not been damaged on the way | Trailer |
Addresses and the packet number are in the header. The trailer holds the end-of-packet marker and error-checking data.
Check Your Understanding 2
A packet arrives at a device. Which part of the packet would the device use to work out where it fits in the original file, and which item in that part?
Reveal the answer
The header — specifically the packet number.
Check Your Understanding 3
Why does the header include the originator's address, when the packet is being delivered somewhere else?
Reveal the answer
So the receiving device knows where the packet came from. It can then send a reply, or ask for a missing or damaged packet to be sent again.
Worked Example: One Message, Three Packets
A laptop sends the message "MEET AT THE LIBRARY AT 4PM" to a phone. The message is split into three packets. (Real packets carry far more data than this; it is shortened so you can see the idea. The addresses are example addresses.)
| Packet number (header) | Destination address (header) | Originator's address (header) | Payload |
|---|---|---|---|
| 1 | 203.0.113.25 | 198.51.100.7 | "MEET AT " |
| 2 | 203.0.113.25 | 198.51.100.7 | "THE LIBRARY " |
| 3 | 203.0.113.25 | 198.51.100.7 | "AT 4PM" |
Notice what stays the same and what changes. Every packet has the same destination and originator's addresses, because they all belong to the same message. The packet number and the payload are different in each one.
Now suppose they arrive in the order 2, 3, 1. (The next section shows how this can happen.) Without packet numbers, the phone would simply join the payloads in arrival order: "THE LIBRARY " + "AT 4PM" + "MEET AT " gives "THE LIBRARY AT 4PMMEET AT". Notice that "4PMMEET" has no space between the words. This is because we joined the exact packet contents in the wrong order. The receiving device must restore the correct sequence before the message can be read properly. With packet numbers, it puts them back in the order 1, 2, 3: "MEET AT " + "THE LIBRARY " + "AT 4PM" gives exactly "MEET AT THE LIBRARY AT 4PM". (The phone also needs to know that three packets were sent, so it knows when the last one has arrived — see the Extension note above about the total number of packets.)
Packet Switching — How Packets Travel Syllabus
Packet switching is the method of sending data where each packet travels across the network independently, possibly along a different route from the others, and the packets are reassembled at the destination.
The key devices along the way are routers. A router receives a packet, reads the destination address in its header, and sends it on towards that destination. The syllabus puts it simply: a router controls the route a packet takes.
| Step | What happens in packet switching |
|---|---|
| 1 | The data is broken down into packets. Each packet is given a header (including its packet number) and a trailer. |
| 2 | Each packet is sent onto the network separately. |
| 3 | At each router, the destination address is read and the router decides where to send the packet next, often choosing a route that is less busy at that moment. |
| 4 | Because conditions change, each packet could take a different route. |
| 5 | Packets may therefore arrive out of order. |
| 6 | Once the last packet has arrived, the packets are reordered using their packet numbers and the original data is rebuilt. |
Packet 1
Laptop → A → C → D → Phone
Packet 2
Laptop → A → B → D → Phone
Packet 3
Laptop → A → B → D → Phone
This is the same message as the worked example. Packet 1 was sent first, but its route through Router C was busier, so packets 2 and 3 overtook it. They arrive in the order 2, 3, 1 — exactly the arrival order used in the worked example above. Notice too that packets 2 and 3 followed the same route: packets may take different routes, not must.
Why do packets arrive out of order?
Different routes have different lengths and different amounts of traffic. A packet sent later along a quick, quiet route can overtake one sent earlier along a slow, busy route. It is like two envelopes posted on the same day: one goes through a busy sorting office and arrives a day later than the other.
That is not a fault — it is expected. The packet numbers make it harmless.
What if a packet goes missing?
Sometimes a packet is lost or damaged. Two separate things then happen:
- Detecting the problem: the packet numbers let the receiving device see that one is missing, and exactly which one (for example, a gap between packets 3 and 5).
- Getting it sent again: the packet number on its own does not cause a resend. The request is handled by an error-control method agreed between the two devices, such as ARQ. The originator's address tells the receiver where that request should go, so only the missing packet needs to be resent.
Part 3 of this series explains how errors are detected and how ARQ gets data resent.
Routers forward packets. Packets are reordered and reassembled at the destination device, once the last packet has arrived.
Check Your Understanding 4
Packets 1, 2 and 3 are sent in that order, but arrive in the order 1, 3, 2. Explain why this can happen.
Reveal the answer
Each packet can take a different route through the network. Packet 2's route may have been longer or busier, so packet 3 arrived first. The receiver uses the packet numbers to put them back in the order 1, 2, 3.
Advantages and Disadvantages of Packet Switching
Exam questions often ask you to evaluate packet switching. These points follow directly from how it works.
| Advantages | Disadvantages |
|---|---|
| Packets can be routed around busy or broken parts of the network | Packets can arrive out of order, so they must be reordered at the destination |
| Only lost or damaged packets need to be resent, not the whole file | Packets can be lost, and resending them causes delays |
| Many users can share the same network connections at the same time | Extra data (headers and trailers) is added to every packet |
| Busy routes can be avoided, which spreads the traffic across the network | The data cannot be used until every packet has arrived and been reordered, so one delayed or lost packet holds up the whole file |
Worked Scenarios
Read each scenario, decide what is happening, then reveal the explanation.
Scenario A — The broken link
Halfway through a large download, a cable between two routers is damaged. The download still finishes, just a little more slowly.
Reveal the analysis
Routers stop sending packets along the broken link and send them by other routes instead. Because every packet is routed independently, the transfer can continue. It is slower because the remaining routes are longer or busier.
Scenario B — The missing packet
A tablet receives packets 1, 2, 3, 5 and 6 of a six-packet file. Packet 4 never arrives.
Reveal the analysis
The packet numbers let the tablet detect that packet 4 is missing. (It knows the file has six packets — see the Extension note on the total number of packets.) Detecting the gap does not resend anything by itself: the error-control method in use, such as ARQ (Part 3), sends a request for packet 4 to the originator's address. Only that one packet is resent, and once it arrives the file can be reassembled.
Scenario C — Same address, different routes
A student notices that two packets from the same email, with the same destination address, passed through different routers.
Reveal the analysis
This is normal in packet switching. Each router decides where to send each packet at the moment it arrives, based on conditions such as how busy each route is. Those conditions can change between one packet and the next.
Exam Focus — Describe the Process in Order
These are original FutureLogic practice questions in the Cambridge style. Try each one before revealing the model answer. The marks shown are only a guide: a model answer shows the kind of points examiners look for, but it cannot guarantee marks. Real mark schemes decide, and they accept a range of correct points.
Question 1 (3 marks)
Give three pieces of information stored in a packet header.
Reveal the model answer
Syllabus link: 2.1.1(b) — describe the structure of a packet.
The destination address; the packet number; the originator's address.
Why this is a strong answer: it gives the three items named in the syllabus. "The data" would be wrong here, because the data is the payload, not part of the header.
Question 2 (4 marks)
Describe the process of packet switching.
Reveal the model answer
Syllabus link: 2.1.1(c) — describe the process of packet switching.
The data is broken down into packets, each with a header containing a packet number. Each packet is sent separately, and routers control the route each packet takes, so packets may take different routes. As a result, the packets may arrive out of order. Once the last packet has arrived, the packets are reordered using their packet numbers.
Why this is a strong answer: it covers the process in order — splitting, independent routing by routers, out-of-order arrival and reordering — using the syllabus's own terms.
Question 3 (2 marks)
Explain why a packet number is included in each packet.
Reveal the model answer
Syllabus link: 2.1.1(b) and (c) — packet structure and reordering.
Packets can take different routes and may arrive out of order. The packet number allows the receiving device to put the packets back into the correct order, and to see if any packet is missing.
Why this is a strong answer: it explains the cause (out-of-order arrival) and the purpose (reordering and spotting missing packets).
Common Cambridge Exam Mistakes
The payload is the actual data. The header holds the destination address, packet number and originator's address.
Each packet is routed independently and could take a different route.
Packets may take different routes. Some will follow the same one.
Routers forward packets. The destination device reorders and reassembles them.
Name both: the destination address and the originator's address.
The packet number is what allows out-of-order packets to be put back in the correct sequence.
Quick Quiz
- What are the three parts of a packet?
- Name the three header items listed in the syllabus.
- What does the payload contain?
- What does the trailer contain?
- Which device controls the route a packet takes?
- Which header item does a router read to decide where to send a packet?
- Why might packets arrive out of order?
- When are packets reordered, and where?
Quick Quiz Answers
Reveal all answers
- Header, payload and trailer.
- Destination address, packet number, originator's address.
- The actual data being sent — part of the original file or message.
- Data marking the end of the packet, and error-checking data.
- A router.
- The destination address.
- Each packet can take a different route, and some routes are longer or busier than others.
- Once the last packet has arrived, at the destination device, using the packet numbers.
Final Summary
Packets
Data is split into small packets that travel separately.
Header
Destination address, packet number, originator's address.
Payload & Trailer
Payload = the data. Trailer = end marker and error-checking data.
Routers
Read the destination address and control the route each packet takes.
Out of Order
Different routes mean packets may arrive in a different order.
Reordering
Once the last packet arrives, packet numbers restore the correct order.