How MAC Addresses Are Used for Communication Between Devices

MAC addresses enable devices to communicate within a local network (LAN). This guide explains how a device discovers another device's MAC address using ARP and how a Layer 2 switch learns MAC addresses to forward Ethernet frames efficiently.

Example LAN Topology

In this example, three computers are connected to the same Layer 2 switch and share the same subnet: 192.168.1.0/24.

LAN Topology Diagram

                                     Computer-3
                               MAC: ac:43:30:00:00:03
                               IP:  192.168.1.3/24
                                       |
                                       |
Computer-1 ------------------------ L2 Switch ---------------- Computer-2
MAC: ac:43:30:00:00:01                                  MAC: ac:43:30:00:00:02
IP:  192.168.1.1/24                                     IP:  192.168.1.2/24

Key idea: IP addresses identify destinations at Layer 3, but Ethernet delivery on a LAN requires a destination MAC address at Layer 2.

Step-by-Step: Computer-1 Communicates With Computer-2

Step 1 — Computer-1 Knows the Destination IP, Not the Destination MAC

Computer-1 wants to send data to 192.168.1.2 (Computer-2). However, to deliver an Ethernet frame on the LAN, it must know Computer-2's MAC address.

Step 2 — ARP Request (Broadcast)

Computer-1 sends an ARP request asking: "Who has IP 192.168.1.2? Tell 192.168.1.1." Because Computer-1 doesn't know the destination MAC yet, it uses the Ethernet broadcast address ff:ff:ff:ff:ff:ff. The switch floods this to all ports.

ARP Request Frame

Ethernet Header:
  Destination MAC: ff:ff:ff:ff:ff:ff  (broadcast)
  Source MAC:      ac:43:30:00:00:01

ARP Payload:
  Who has 192.168.1.2? Tell 192.168.1.1

Step 3 — ARP Reply (Unicast)

Computer-2 receives the request, sees the IP matches, and replies directly to Computer-1 with its MAC address. This reply is unicast.

ARP Reply

IP:  192.168.1.2
MAC: ac:43:30:00:00:02

Step 4 — The L2 Switch Learns MAC Addresses

While forwarding frames, the switch learns which MAC addresses live on which ports by reading the source MAC of incoming frames.

Learned MAC Address

Switch Port

ac:43:30:00:00:01Port 1

ac:43:30:00:00:02Port 2

ac:43:30:00:00:03Port 3

Step 5 — Data Frames Sent Using Destination MAC

Computer-1 now sends data using Computer-2's MAC in the Ethernet header. The switch looks up the destination MAC and forwards the frame only to the correct port — no longer a broadcast.

Data Frame (Unicast)

Ethernet Header:
  Destination MAC: ac:43:30:00:00:02
  Source MAC:      ac:43:30:00:00:01

IP Header:
  Destination IP:  192.168.1.2
  Source IP:       192.168.1.1

This is why switches are efficient: once MAC-to-port mappings are learned, traffic becomes unicast instead of broadcast.

What If the Destination Is Not on the Same Subnet?

If Computer-1 needs to reach a remote IP (e.g. 192.168.2.10), it cannot reach it directly at Layer 2. Instead:

Computer-1 sends the packet to its default gateway (router)
The destination MAC in the Ethernet frame becomes the router's MAC address
The router forwards the packet toward the remote network

Rule of thumb: On a LAN, Ethernet uses MAC addresses. Across networks, routing uses IP addresses.

Summary

MAC addresses are used for delivery within a local network (LAN)
ARP resolves a destination IP address to a destination MAC address
ARP requests are broadcast; ARP replies are usually unicast
Layer 2 switches learn MAC addresses and forward frames based on a MAC table
For remote networks, devices send traffic to the default gateway (router)

Try it on your own network:

Use our MAC Vendor Lookup tool to identify the vendor/manufacturer from a MAC address prefix.

FAQ

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How MAC Addresses Work — ARP, Switch Tables, Broadcast Domains