Why are MAC addresses needed?

MAC addresses are needed to deliver data between devices within a single local network (LAN) at the data link layer of the OSI model.

More simply, and to the point: a MAC address determines which specific network device a frame is intended for within a single network.

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#Main reasons why MAC addresses are needed

#1. Addressing at the data link layer

A MAC address:

• uniquely identifies a network interface
• is used in every Ethernet / Wi-Fi frame
• allows specifying the sender and recipient

Without MAC addresses:

• frames could not be delivered by address
• the network would turn into "blind broadcasting"

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#2. How switches work

Switches use MAC addresses to:

• remember which MAC is behind which port
• send frames only to the right port
• avoid flooding data across the whole network unnecessarily

This makes the network:

• faster
• more efficient
• less congested

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#3. Delivering data within a single network

MAC addresses work:

• only within a single local network
• up to the first router

When the data goes further:

• the old frame is discarded
• a new frame is created with different MAC addresses

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#4. Linking IP and the physical network

An IP address:

• is logical
• is used for routing between networks

A MAC address:

• is physical (data link)
• is needed to actually deliver data over a wire or through the air

The ARP protocol exists precisely to:

• map an IP address to a MAC address

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#5. Supporting different delivery types

MAC addresses make it possible to implement:

• unicast: to one device
• multicast: to a group of devices
• broadcast: to all devices on the network

Without MAC addresses these modes would be impossible.

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#What's important to remember for an interview

• A MAC address:
  • is used at the data link layer (L2)
  • is needed for delivering frames
• An IP address:
  • is used at the network layer (L3)
  • is needed for routing

They do not replace each other, they complement each other.