← IGCSE Computer Science Paper 1

Destination address in a packet header

The address in a packet header of the device the packet must be delivered to. Routers read it to choose where to send the packet. It is usually an IP address.

Part of: IGCSE Computer Science 0478 — 2.1 Data Transmission Types and Methods — Module 9, IGCSE Computer Science 0478 — 3.4 Network Hardware — Module 26, ACARA Digital Technologies Years 7–8 — Wired and Wireless Networks — Module 42

Understanding Destination address in a packet header in IGCSE Computer Science Paper 1

What it does: A router checks the destination address in the header and uses it to decide the packet's next hop. The destination address is normally an IP address, and it identifies the receiving device.

Common mistakes: The router needs the device the packet must be delivered to. That is the destination address, packet.destination, not the source (originator) address. 10.0.0.4 is the sender's address from line 1. The destination address must be the address of the receiving device, such as 10.0.0.9.

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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 destination address in a packet header identify?

The device that sent the packetThe device the packet must be delivered to

Matches

header.destination = IP address of sender

↓

The destination address is the address of the device receiving the packet, not the sender. The sender's address is the originator's address.

Applying

Meanings

Term

destination address

Meaning

The address in a packet header of the device the packet must be delivered to. Routers read it to choose where to send the packet. It is usually an IP address.

Practice

packet.header.destination <- "192.168.1.20"

Testing

Gaps

packet. = "203.0.113.8"

Check Answer

Spots

1dest ← packet.source

2router reads dest

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Quick Questions

1. What does the destination address in a packet header identify?

A. The device that sent the packet

B. The device the packet must be delivered to ✓

The destination address says where the packet is going. The originator's address is the one that says where it came from.

2. Which part of a packet holds the destination address?

A. The header ✓

B. The payload

The destination address is stored in the header with other control information. The payload holds the actual data being sent.

3. Which part of a router's job uses the destination address?

A. Reading it to choose where to send the packet next ✓

B. Changing it to the router's own address

Routers read the destination address to decide which way to forward each packet. They do not replace it, because the packet still needs to reach the destination device.

4. What type of address is usually used as the destination address?

A. A packet number

B. An IP address ✓

The destination address is usually an IP address. A packet number is a different header item that shows the packet's order.

5. Which statement is correct?

A. Every packet in a message carries the destination address in its header ✓

B. Only the first packet in a message carries the destination address

Packets can take different routes, so every packet needs the destination address in its own header so routers know where to send it.

Match the Pairs

destination address → The address in a packet header of the device the packet must be delivered to

destination: 192.168.1.20 → A header field giving the IP address of the receiving device

Routers read the destination address in the header to choose where to send the packet next → router reads header.destination

The destination address is an IP address identifying the device the packet must reach → header.destination = IP address of receiver

header.destination = IP address of sender → The destination address is the address of the device receiving the packet, not the sender. The sender's address is the originator's address.

destination address is stored in the payload → The destination address is stored in the packet header, not the payload. The payload holds the data being sent.

Key Term

destination address

The address in a packet header of the device the packet must be delivered to. Routers read it to choose where to send the packet. It is usually an IP address.

Fill in the Blanks

1. packet.destination = "203.0.113.8"

Hint: Type the header field that stores the address of the device the packet must be delivered to.

2. OUTPUT packet.destination

Hint: Type the header field a router reads to choose where to send the packet.

3. IF packet.destination = myIP THEN accept(packet) ENDIF

Hint: Type the function name that takes in the packet when its destination address matches this device.

4. IF packet.destination = myIP THEN accept(packet) ENDIF

Hint: Type the comparison operator that checks whether the destination address matches this device's IP address.

5. forward(packet, packet.destination)

Hint: Type the function name that makes a router send the packet on towards the address in its header.

Spot the Error

Question 1

A dest ← packet.source

B router reads dest

C router sends packet onward

Line A — The router needs the device the packet must be delivered to. That is the destination address, packet.destination, not the source (originator) address.

Question 2

A packet.source ← 10.0.0.4

B packet.destination ← 10.0.0.4

C SEND packet TO router

Line B — 10.0.0.4 is the sender's address from line 1. The destination address must be the address of the receiving device, such as 10.0.0.9.

Question 3

A router receives packet

B dest ← packet.payload

C route ← choose path to dest

Line B — The payload is the data being carried. The router must read the destination address from the header, packet.destination, to choose where to send the packet.

Question 4

A dest ← packet.destination

B next ← lookup(dest)

C SEND packet TO packet.source

Line C — The router uses the destination address to look up the next hop and sends the packet there. Sending it to the source would return it to the sender.

Question 5

A IF packet.destination = myIP

B THEN keep packet

C ELSE discard packet

Line C — If the destination address is not this device's IP address, the packet is meant for another device. It must be forwarded towards that device, not discarded.

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