Purpose of a MAC address

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Quick Questions

Shade in the A or B circle for each question.

1. What does a MAC address identify?

A The network interface card (NIC) of a device

B The website the user is visiting

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

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

A True

B False

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

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

Match the Pairs

Draw a line connecting each item on the left to its match on the right.

Left column		Right column
1MAC address		AA unique address that identifies a device's NIC so it can be told apart from every other device
2Destination MAC address		BTells the local network which device's NIC should receive the data
3Every NIC has its own unique address, so no two devices on a network are confused with each other		CMAC addresses must be unique. If two devices shared one, the network could not tell which NIC should receive the data.
4A network can recognise a known device by its MAC address, for example to allow only approved devices to join		DThe MAC address is used for delivery on the local network, not across the internet. IP addresses are used for routing between networks.
5Two laptops on one LAN share the MAC address 3C-52-A1-0B-7E-19		EAllowed MACs: 3C-52-A1-0B-7E-19
6The MAC address is used to deliver data across the internet to a website		FMAC: 3C-52-A1-0B-7E-19

Definitions

MAC address

Fill in the Blanks

Complete each line of code by filling in the missing parts.

A ___ address identifies a device's NIC

Each NIC has a ______ MAC address

IF destMAC = myMAC ____ accept()

MAC addresses deliver data on a ___

IF senderMAC __ knownMACs THEN allow

Spot the Error

Shade in the letter of the line that contains an 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()

Question 2

A f = Frame()

B f.dest = '192.168.0.12'

C f.data = 'Hello'

D send(f)

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')

Question 4

A dest = frame.dest_mac

B for nic in lan:

C if nic.mac == dest:

D nic.discard(frame)

Question 5

A dest_mac = frame.dest_mac

B if dest_mac = my_mac:

C keep(frame)

D else:

E ignore(frame)

Answer Key

Purpose of a MAC address

Section A: Quick Questions

A — The network interface card (NIC) of a device

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

B — False

B — Sends it to the device with that MAC address

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

Section B: 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.

Section C: Definitions

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

Section D: Fill in the Blanks

A MAC address identifies a device's NIC

Each NIC has a unique MAC address

IF destMAC = myMAC THEN accept()

MAC addresses deliver data on a LAN

IF senderMAC IN knownMACs THEN allow

Section E: Spot the Error

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

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.

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.

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.

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 ==.