← IGCSE Computer Science Paper 1

Packet header

The first part of a packet. It holds control information: the destination address, the originator's address and the packet number, used to deliver and reassemble the packet.

Part of: IGCSE Computer Science 0478 — 2.1 Data Transmission Types and Methods — Module 8, ACARA Digital Technologies Years 7–8 — Wired and Wireless Networks — Module 41, ACARA Digital Technologies Years 9–10 — Networked Systems and Access Control — Module 37, WA Computer Science ATAR Year 11 — Network Communications — Module 65

Understanding Packet header in IGCSE Computer Science Paper 1

What it does: The originator's address is control information stored in the header, so the receiver knows the sender. The header holds the three control details used to deliver and reassemble the packet. The payload carries the actual data. The header is the first part of a packet. The trailer is the part at the end. Packets can arrive out of order, so the packet number lets the receiving device reassemble them correctly. Routers read the destination address in the header to decide where to forward each packet.

Common mistakes: The header holds control information such as the destination address. The file content is the payload, not part of the header. The packet number is stored in the packet header, not the trailer. It should be packet.header.number = 3.

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This module's key terms are also in a printable crossword (Modules 110–120) and word search (Modules 86–120).

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Learning
Duo
Where is the packet header found in a packet?
At the start of the packet, before the payloadAt the end of the packet, after the payload
Matches
packet = [payload, header, trailer]
↓
The header is in the wrong place. It is the first part of a packet, so it must come before the payload.
Applying
Meanings
Term
packet header
Meaning
The first part of a packet. It holds control information: the destination address, the originator's address and the packet number, used to deliver and reassemble the packet.
Practice
PACKET HEADER
Testing
Gaps
packet = + payload + trailer
Check Answer
Spots
2header = control information
3header holds the file content

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Quick Questions

1. Where is the packet header found in a packet?
A. At the start of the packet, before the payload ✓
B. At the end of the packet, after the payload
The header is the first part of a packet. The trailer is the part at the end.
2. Which of these is stored in a packet header?
A. The destination address ✓
B. The file's actual contents
The header holds control information such as the destination address. The data being sent is in the payload.
3. What is the packet number in the header used for?
A. To put the packets back in the right order ✓
B. To encrypt the data in the packet
Packets can arrive out of order, so the packet number lets the receiving device reassemble them correctly.
4. Why does the header contain the destination address?
A. So the receiver knows how fast the packet travelled
B. So the router knows where to send the packet ✓
Routers read the destination address in the header to decide where to forward each packet.
5. What does the originator's address in the header show?
A. The IP address of the router that will receive it next
B. The IP address of the device that sent the packet ✓
The originator's address identifies the sender, so the receiver knows where the packet came from.

Match the Pairs

header: dest=192.168.1.20 → The destination address field, which tells routers where the packet must be delivered
header: packet_no=4 → The packet number field, which lets the receiving device put packets back in the correct order
The header holds the originator's address so the receiver knows which device sent the packet → header: from=10.0.0.5
A packet header contains control information (destination, originator and packet number), not the data itself → header = [dest, from, packet_no]
packet = [payload, header, trailer] → The header is in the wrong place. It is the first part of a packet, so it must come before the payload.
header = [email text, dest, from] → The header does not hold the data being sent. The message content belongs in the payload, and the header should include the packet number.

Key Term

packet header
The first part of a packet. It holds control information: the destination address, the originator's address and the packet number, used to deliver and reassemble the packet.

Fill in the Blanks

1. packet = header + payload + trailer
Hint: Type the name of the first part of a packet, which holds the control information.
2. header = [destination, originator, number]
Hint: Type the header item that tells the router where the packet is going.
3. header = [destination, originator, number]
Hint: Type the header item that gives the address of the device that sent the packet.
4. header = [destination, originator, number]
Hint: Type the header item that shows where this packet fits in the sequence.
5. SORT packets BY packet.header.number
Hint: Type the header field the receiving device uses to put the packets back in the correct order.

Spot the Error

Question 1
A header = first part of packet
B header = control information
C header holds the file content
Line C — The header holds control information such as the destination address. The file content is the payload, not part of the header.
Question 2
A packet.header.dest = 'B'
B packet.header.origin = 'A'
C packet.trailer.number = 3
Line C — The packet number is stored in the packet header, not the trailer. It should be packet.header.number = 3.
Question 3
A packet 4 arrives
B READ payload for packet number
C place packet 4 in order
Line B — The packet number is part of the header, so the computer must read the header. The payload only holds the data.
Question 4
A header = LAST part of packet
B payload = the data itself
C trailer = LAST part of packet
Line A — The header is the FIRST part of a packet. The trailer is the last part.
Question 5
A router reads the packet header
B it finds the dest address
C it sends packet to the origin
Line C — The router uses the destination address in the header to send the packet onward. It should send the packet to the destination, not back to the origin.

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