Read the structure of a data packet
Identify the packet header, payload and trailer and explain what addressing and sequence information do.
This sequence follows Cambridge IGCSE Computer Science 0478, 2026–2028 syllabus Version 6. It covers subject content sections 2.1–2.3 only. The separately published 2029–2031 syllabus is not mapped here. Practice is original, self-checked and does not award an official Cambridge grade. Full timed papers and human subject-teacher review remain outstanding.
Before you begin
- No programming language is required. Be comfortable with place value and multiplication; each lesson introduces the computer-science notation it uses.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Why might one long message be split into several packets?
What you will learn
- Name the three parts of a packet
- Identify originator address, destination address and packet number in a header
- Explain why packets carry control information as well as data
A packet separates control from content
A transmitted packet contains a header, payload and trailer. The payload is the data being sent. The header contains routing and ordering information, including the originator's address, destination address and packet number.
Apply and check the rule
The trailer can contain information used to check the packet. The syllabus requires the three-part structure; the exact fields depend on the protocol. Do not describe the payload as the header or assume every protocol uses identical fields.
Pause and explain
Where would you find a packet's originator address?
Worked example
A message is split into packet 3 of 8. Which packet details identify its order and destination?
Show the worked solution
- The packet number identifies its position in the message.
- The destination address identifies the intended receiving device.
- The payload contains the message data; the trailer follows it.
Answer The header contains packet number 3 and the destination address; the payload contains part of the message.
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Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Name the three parts of a packet and state the purpose of the payload.
Give me a hint
Separate the control fields from the data being sent.
Compare my reasoning
- A packet has a header, payload and trailer.
- The payload contains the data or part of the message being transmitted.
Header, payload and trailer; the payload carries the message data.
Look for these in your work
- All three parts named
- Payload purpose stated
A receiver has packet 7, whose header says it came from device A and is addressed to device B. Identify the sender, destination and sequence information.
Give me a hint
Use the header fields for source, destination and order.
Compare my reasoning
- Originator: device A.
- Destination: device B.
- Packet number: 7, indicating its order in the message.
Device A sent it to device B; packet 7 gives its sequence position.
Look for these in your work
- Originator correct
- Destination correct
- Packet number interpreted
A packet has an accurate destination address but its payload is corrupted. Explain why routing information alone cannot recover the original message.
Give me a hint
Distinguish delivery from data integrity.
Compare my reasoning
- The address helps route the packet to the right device.
- It does not guarantee the payload's bits arrived unchanged.
- The receiver needs an error-detection process to detect corruption and possibly request a resend.
The header can support delivery, but it does not prove the payload is intact. An error check is needed to detect corruption; a resend may then be requested.
Look for these in your work
- Address role stated
- Integrity distinguished
- Error handling suggested
Common mistakes
- Calling the payload the destination address
- Assuming the packet number is the same as a network address
What can you explain now?
Which packet component contains the data being transmitted?
Compare with the explanation
Payload
The payload carries the message content; the header carries packet-control information.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Which packet fields in the header help a receiver place a packet in the right message and order?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.