← IGCSE Computer Science Paper 1

Why a device needs both a MAC address and an IP address

The IP address shows where a device is on the network so routers can route data to it. The MAC address identifies its physical NIC for local delivery. Both are used to deliver data.

Part of: IGCSE Computer Science 0478 — 3.4 Network Hardware — Module 24

Understanding Why a device needs both a MAC address and an IP address in IGCSE Computer Science Paper 1

What it does: The router reads the destination IP address in the packet to choose the route towards the correct network. On the local network, the MAC address picks out the exact NIC that should receive the data. The MAC address is tied to the NIC and is given to it at manufacture. Routers read the IP address to work out where to send the packet. The manufacturer code is part of the MAC address and is not used for routing. The MAC address identifies the physical NIC, so it is used for local delivery to the correct device. A MAC address identifies the NIC but gives no location, so the IP address is needed for routing. MAC addresses are set at manufacture and do not change each time.

Common mistakes: Routers route data across networks using the IP address, which shows where the device is. The MAC address is only used for local delivery. Line 2 should be routeAcross(destinationIP). A MAC address is given to the NIC at manufacture and stays the same on any network. Only the IP address changes when a device joins a new network. Line 4 should say the MAC stays the same.

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Learning
Duo
Which address do routers mainly use to send data towards a device on another network?
The MAC addressThe IP address
Matches
A device only needs an IP address, so the MAC address is not used to deliver data.
↓
Both addresses are used. The IP address shows where the device is on the network, and the MAC address identifies its physical NIC for local delivery.
Applying
Meanings
Term
IP address and MAC address
Meaning
The IP address shows where a device is on the network so routers can route data to it. The MAC address identifies its physical NIC for local delivery. Both are used to deliver data.
Practice
IP address = where the device is, used by routers
Testing
Gaps
route ← FindRoute(packet.)
Check Answer
Spots
1message ← 'Hello'
2routeAcross(destinationMAC)

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Quick Questions

1. Which address do routers mainly use to send data towards a device on another network?
A. The MAC address
B. The IP address ✓
The IP address says where a device is on the network, so routers use it to route data across networks.
2. Which address identifies the physical network interface card (NIC) of a device?
A. The MAC address ✓
B. The IP address
The MAC address is tied to the NIC and is given to it at manufacture.
3. Which statement is correct?
A. Both addresses are used to deliver data ✓
B. Only the IP address is needed to deliver data
The IP address helps data reach the right network and device, and the MAC address is used for local delivery to the physical NIC. Both work together.
4. A message is sent from a laptop to a server on a different network. What does the IP address tell the routers?
A. Which company made the NIC
B. Where the device is on the network ✓
Routers read the IP address to work out where to send the packet. The manufacturer code is part of the MAC address and is not used for routing.
5. In the last step of delivery on the local network, which address points to the exact NIC that receives the data?
A. The MAC address ✓
B. The IP address
The MAC address identifies the physical NIC, so it is used for local delivery to the correct device.

Match the Pairs

IP address: 192.168.1.20 → Shows where the device is on the network, so routers can work out the route to it across networks
MAC address: 3C:52:82:1A:B4:F0 → Identifies the physical NIC of the device, so data can be delivered to the right device on the local network
The IP address is used for routing data across different networks until it reaches the right network → Destination IP: 82.14.7.9
The MAC address is used for the final local delivery to the NIC of the receiving device → Destination MAC: 5E:A1:09:7C:22:D8
A device only needs an IP address, so the MAC address is not used to deliver data. → Both addresses are used. The IP address shows where the device is on the network, and the MAC address identifies its physical NIC for local delivery.
The MAC address tells routers where a device is on the network, so it is used to route data across networks. → This mixes up the two addresses. The IP address shows where the device is on the network for routing. The MAC address only identifies the physical NIC.

Key Term

IP address and MAC address
The IP address shows where a device is on the network so routers can route data to it. The MAC address identifies its physical NIC for local delivery. Both are used to deliver data.

Fill in the Blanks

1. route ← FindRoute(packet.destIP)
Hint: Type the packet field holding the address that shows where the device is on the network, which a router reads to route data between networks.
2. DeliverLocally(packet.destMAC)
Hint: Type the packet field holding the address that identifies the physical NIC, used for the final local delivery.
3. packet.destIP ← "192.168.1.20" packet.destMAC ← "3C-52-82-1A-9F-D4" SEND(packet)
Hint: Type the command name that sends the packet once both of its addresses have been stored.
4. IF packet.destIP <> myIP THEN FORWARD(packet) ENDIF
Hint: Type the keyword that starts the action when the IP address shows the packet is meant for another device.
5. delivered ← routedByIP AND matchedByMAC
Hint: Type the logical operator that means both the IP address step and the MAC address step must happen.

Spot the Error

Question 1
A message ← 'Hello'
B routeAcross(destinationMAC)
C deliverLocal(destinationMAC)
Line B — Routers route data across networks using the IP address, which shows where the device is. The MAC address is only used for local delivery. Line 2 should be routeAcross(destinationIP).
Question 2
A IP ← network gives address
B MAC ← set at manufacture
C IP can change on new network
D MAC changes on a new network
Line D — A MAC address is given to the NIC at manufacture and stays the same on any network. Only the IP address changes when a device joins a new network. Line 4 should say the MAC stays the same.
Question 3
A mac ← '3C:52:82:1A:9F:04'
B PRINT mac
C ip ← '192.168.1.320'
D PRINT ip
Line C — Each part of an IPv4 address is an 8-bit number from 0 to 255, so 320 is invalid. The line should be ip ← '192.168.1.20' or another valid value.
Question 4
A // Ava sends a message to Ben
B src IP: 192.168.1.5
C dest IP: 192.168.1.9
D dest MAC: 192.168.1.9
Line D — A MAC address is written as hexadecimal pairs such as 5E:11:A0:7B:22:C9. 192.168.1.9 is an IPv4 address, so it was put in the wrong field.
Question 5
A packet arrives at router
B router reads destination IP
C router picks route by MAC
D NIC checks destination MAC
Line C — The router chooses the route using the destination IP address, which shows where the device is on the network. The MAC address is only used for the local delivery. Line 3 should say the route is picked using the IP.

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