Purpose of a MAC address

A unique address that identifies a device's NIC, so data on a local network is delivered to the correct device.

Part of: IGCSE Computer Science 0478 — 3.4 Network Hardware — Module 10

Understanding Purpose of a MAC address in IGCSE Computer Science Paper 1

What it does: A unique MAC address such as this one identifies exactly one NIC, which is how devices are told apart. A list of approved MAC addresses lets the network recognise known devices and decide whether to let them connect. The MAC address is used to deliver data to the right device on the local network. It does not change speed. The MAC address is the unique identifier of the NIC. A web page title has nothing to do with network hardware. MAC addresses are for identification and delivery on the local network. They do not store files.

Common mistakes: A device should accept data only when the destination MAC address matches its own MAC address. It should be if dest == mac:, not !=. On a local network the frame is delivered using the destination device's MAC address, so f.dest must hold a MAC address such as 'AA:BB:CC:11:22:33', not an IP address.

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This module's key terms are also in a printable crossword (Modules 325–341) and word search (Modules 325–354).

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Learning

Duo

What does a MAC address identify?

The website the user is visitingThe network interface card (NIC) of a device

Matches

Two laptops on one LAN share the MAC address 3C-52-A1-0B-7E-19

↓

MAC addresses must be unique. If two devices shared one, the network could not tell which NIC should receive the data.

Applying

Meanings

Term

MAC address

Meaning

A unique address that identifies a device's NIC, so data on a local network is delivered to the correct device.

Practice

destinationMAC ← "00:1A:2B:3C:4D:5E"

Testing

Gaps

A address identifies a device's NIC

Check Answer

Spots

2dest = '00:1A:2B:3C:4D:5E'

3if dest != mac:

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Quick Questions

1. What does a MAC address identify?

A. The network interface card (NIC) of a device ✓

B. The website the user is visiting

A MAC address identifies the device's NIC, so the network knows exactly which device it is talking to.

2. Which statement describes the purpose of a MAC address?

A. It makes sure data on a local network is delivered to the correct device ✓

B. It makes the internet connection faster

The MAC address is used to deliver data to the right device on the local network. It does not change speed.

3. Two devices on the same network can have the same MAC address.

A. True

B. False ✓

A MAC address is unique to each NIC. If two devices shared one, the network could not tell which device should receive the data.

4. A switch on a LAN receives data for MAC address 00:1A:2B:3C:4D:5E. What does it do with the data?

A. Sends it to every device on the network

B. Sends it to the device with that MAC address ✓

The MAC address lets the data be delivered to the one correct device on the local network.

5. A router recognises a laptop it has connected to before because of its MAC address. Why can it do this?

A. The MAC address is unique, so it always identifies the same device ✓

B. The MAC address changes each time to show who is connecting

Because a MAC address is unique and fixed, a network can recognise known devices, for example in a list of allowed devices.

Match the Pairs

MAC address → A unique address that identifies a device's NIC so it can be told apart from every other device

Destination MAC address → Tells the local network which device's NIC should receive the data

Every NIC has its own unique address, so no two devices on a network are confused with each other → MAC: 3C-52-A1-0B-7E-19

A network can recognise a known device by its MAC address, for example to allow only approved devices to join → Allowed MACs: 3C-52-A1-0B-7E-19

Two laptops on one LAN share the MAC address 3C-52-A1-0B-7E-19 → MAC addresses must be unique. If two devices shared one, the network could not tell which NIC should receive the data.

The MAC address is used to deliver data across the internet to a website → The MAC address is used for delivery on the local network, not across the internet. IP addresses are used for routing between networks.

Key Term

MAC address

A unique address that identifies a device's NIC, so data on a local network is delivered to the correct device.

Fill in the Blanks

1. A MAC address identifies a device's NIC

Hint: Type the three-letter abbreviation for the hardware address that identifies a NIC.

2. Each NIC has a unique MAC address

Hint: Type the word meaning no two devices share the same address.

3. IF destMAC = myMAC THEN accept()

Hint: Type the pseudocode keyword that introduces what happens when the condition is true.

4. MAC addresses deliver data on a LAN

Hint: Type the abbreviation for the type of network that covers a small area, where MAC addresses are used for delivery.

5. IF senderMAC IN knownMACs THEN allow

Hint: Type the two-letter keyword that checks whether a value is found in a list of known devices.

Spot the Error

Question 1

A mac = '00:1A:2B:3C:4D:5E'

B dest = '00:1A:2B:3C:4D:5E'

C if dest != mac:

D accept_data()

Line C — A device should accept data only when the destination MAC address matches its own MAC address. It should be if dest == mac:, not !=.

Question 2

A f = Frame()

B f.dest = '192.168.0.12'

C f.data = 'Hello'

D send(f)

Line B — On a local network the frame is delivered using the destination device's MAC address, so f.dest must hold a MAC address such as 'AA:BB:CC:11:22:33', not an IP address.

Question 3

A pc = 'A1:B2:C3:D4:E5:F6'

B phone = 'A1:B2:C3:D4:E5:F6'

C if pc != phone:

D print('Both unique')

Line B — Every NIC has a unique MAC address. The phone has the same MAC as the pc, so the two devices could not be told apart. The phone needs a different MAC, such as ending in F7.

Question 4

A dest = frame.dest_mac

B for nic in lan:

C if nic.mac == dest:

D nic.discard(frame)

Line D — When a NIC's MAC address matches the destination MAC, the data is delivered to that device. It should call nic.receive(frame), not discard it.

Question 5

A dest_mac = frame.dest_mac

B if dest_mac = my_mac:

C keep(frame)

D else:

E ignore(frame)

Line B — A single = assigns a value instead of comparing. To check whether the frame's MAC address matches the device's own MAC address, use ==.

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