← IGCSE Computer Science Paper 1

Originator's address in a packet header

The IP address of the device that sent the packet, held in the header so the receiver knows who sent it and can reply or ask for a resend. It differs from the destination address.

Part of: IGCSE Computer Science 0478 — 2.1 Data Transmission Types and Methods — Module 10

Understanding Originator's address in a packet header in IGCSE Computer Science Paper 1

What it does: The reply or resend request is sent to the originator's address, because that is the sender's address. Both addresses sit in the header, and they play different roles: one for who sent the packet, one for where it is going. Routers use the destination address to deliver the packet. The originator's address shows where it came from.

Common mistakes: A reply must go back to the device that sent the packet, so it uses the originator's address: reply.dest ← packet.origin. Using the destination address would send the reply to the receiver itself. The originator's address is the address of the device that sends the packet, so it should be header.origin ← sender_ip. The receiver's address belongs in the destination field.

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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
What does the originator's address in a packet header identify?
The device the packet is going toThe device that sent the packet
Matches
reply_to(header.destination)
↓
The reply is sent to the destination address, which is the receiver's own address. A reply must go to the originator's address.
Applying
Meanings
Term
originator's address
Meaning
The IP address of the device that sent the packet, held in the header so the receiver knows who sent it and can reply or ask for a resend. It differs from the destination address.
Practice
header.originator ← "192.168.1.5"
Testing
Gaps
header. ← 10.0.0.5
Check Answer
Spots
2packet.dest ← 10.0.0.8
3reply.dest ← packet.dest

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Quick Questions

1. What does the originator's address in a packet header identify?
A. The device the packet is going to
B. The device that sent the packet ✓
The originator's address is the IP address of the sender. The destination address is the one that identifies the receiver.
2. Which statement is correct?
A. The originator's address tells the receiver who sent the packet ✓
B. The originator's address tells the router where to deliver the packet
Routers use the destination address to deliver the packet. The originator's address shows where it came from.
3. A packet arrives with a missing or damaged packet. How can the receiver use the originator's address?
A. To change the packet number
B. To ask the sender to resend the packet ✓
The receiver reads the originator's address in the header so it knows which device to contact to request a resend.
4. Laptop A sends a packet to Server B. Which IP address is the originator's address?
A. The IP address of Laptop A ✓
B. The IP address of Server B
Laptop A sent the packet, so its IP address is the originator's address. Server B is the destination.
5. Server B wants to reply to a packet from Laptop A. Which address does it use as the destination of its reply?
A. The originator's address from the received packet ✓
B. Its own IP address
The reply must go back to the sender, so the originator's address becomes the destination of the reply.

Match the Pairs

originator address: 192.168.1.20 → The IP address of the device that sent the packet, stored in the header
destination address: 203.0.113.9 → The IP address of the device the packet is being sent to, which is different from the originator's address
The receiver uses the originator's address to reply or to ask for a lost packet to be sent again → send_reply_to(header.originator)
A header holds two different addresses: the sender's (originator) and the receiver's (destination) → header = [originator, destination]
reply_to(header.destination) → The reply is sent to the destination address, which is the receiver's own address. A reply must go to the originator's address.
originator = address of the receiving device → The originator's address is the address of the device that sent the packet, not the device that receives it.

Key Term

originator's address
The IP address of the device that sent the packet, held in the header so the receiver knows who sent it and can reply or ask for a resend. It differs from the destination address.

Fill in the Blanks

1. // Device 10.0.0.5 sends the packet header.destination ← 10.0.0.9 header.originator ← 10.0.0.5
Hint: Type the header field name for the address of the device that sent the packet.
2. // Reply to the device that sent the packet SEND reply TO header.originator
Hint: Type the header field that holds the sender's address, which the receiver uses to reply.
3. // Packet 4 is missing, ask for a resend resendTo ← header.originator
Hint: Type the header field that tells the receiver which device to ask for the missing packet.
4. // Reply goes back to the sender reply.destination ← header.originator
Hint: Type the name of the part of the packet that stores the addresses.
5. // 10.0.0.9 is the receiving device header.originator ← 10.0.0.5 header.destination ← 10.0.0.9
Hint: Type the field name for the address of the device the packet is going to, not the sender.

Spot the Error

Question 1
A packet.origin ← 192.168.1.5
B packet.dest ← 10.0.0.8
C reply.dest ← packet.dest
Line C — A reply must go back to the device that sent the packet, so it uses the originator's address: reply.dest ← packet.origin. Using the destination address would send the reply to the receiver itself.
Question 2
A header.origin ← receiver_ip
B header.dest ← receiver_ip
C SEND(packet)
Line A — The originator's address is the address of the device that sends the packet, so it should be header.origin ← sender_ip. The receiver's address belongs in the destination field.
Question 3
A sender ← packet.dest
B OUTPUT 'Packet from ', sender
C reply.dest ← sender
Line A — The sender is found from the originator's address in the header: sender ← packet.origin. The destination address only tells you where the packet is going.
Question 4
A IF packet.corrupt = TRUE THEN
B request.dest ← packet.number
C SEND(request)
D ENDIF
Line B — A resend request must be addressed to the device that sent the packet, so it needs the originator's address: request.dest ← packet.origin. The packet number is not an address.
Question 5
A my_ip ← 192.168.1.5
B packet.origin ← 10.0.0.8
C packet.dest ← 10.0.0.8
D SEND(packet)
Line B — The originator's address is the sender's own IP address, so it should be packet.origin ← my_ip. 10.0.0.8 is the destination address, which has its own field.

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