Data Link Layer, MAC Addresses & Ethernet

Introduction

“The Physical Layer moves signals. The Data Link Layer turns those signals into local communication.”

In the previous article, we saw how bits become physical signals and travel through copper, fiber, or wireless media. But a signal reaching a device is only the beginning. A network still needs to organize those bits, identify devices, detect errors, and decide where the data should go.

This is where the Data Link Layer becomes important. It sits directly above the Physical Layer and focuses on communication across the local network — using frames, MAC addresses, Ethernet, switches, and error detection.

Frames Give Data a Structure

“Bits become useful when the network knows which bits belong to the same transmission.”

The Physical Layer deals with a stream of signals. The Data Link Layer gives that stream structure by organizing data into units called frames.

An Ethernet frame contains information such as the destination MAC address, source MAC address, the data being carried, and an error-checking value. A simplified view is: Destination MAC → Source MAC → Data → Error Check.

Think of a frame like a local delivery package. The physical network provides the road, while the package contains the information needed to handle that particular delivery.

The layer above may provide an IP packet, but the Data Link Layer wraps that packet inside a frame so it can move across the local network. At the receiving end, the device examines the frame, removes the Data Link information, and passes the packet upward.

This process connects the logical world of IP packets to the physical world of signals.

MAC Addresses Give Devices a Local Identity

“IP helps find the network; MAC helps deliver the frame locally.”

If frames provide structure, they also need a way to identify their sender and destination. This is where the Media Access Control (MAC) address comes in.

A MAC address identifies a network interface at the Data Link Layer. It is commonly represented using hexadecimal notation, such as 00:1A:2B:3C:4D:5E. When a device sends an Ethernet frame across a local network, the frame contains both a source MAC address and a destination MAC address. For example: Laptop → Switch → Printer.

This is different from an IP address. An IP address operates at the Network Layer and helps devices communicate across different networks. A MAC address is concerned with delivery across the local network link.

A useful way to remember the distinction is:  IP asks which network the data should reach; MAC identifies the local interface that should receive the frame.

As a packet travels beyond the local network, it can cross many networks. At each local link, however, it is carried inside a frame with the appropriate Data Link Layer addressing. The IP packet continues its larger journey while the frame around it can change.

Ethernet & Switches Make Local Networks Practical

“A network becomes efficient when devices know where to send frames.”

One of the most widely used technologies at the Data Link Layer is  Ethernet . It defines common rules for communication across local wired networks, including how frames are structured and how devices communicate over Ethernet links.

This standardization allows equipment from different manufacturers to work together using the same fundamental rules.

Now consider how devices are connected. An old-style  hub  does not make intelligent forwarding decisions. When it receives a signal, it repeats that signal to its other ports, meaning connected devices share the same communication environment.

This creates a shared  collision domain . If multiple devices transmit simultaneously on a shared medium, their signals can interfere with one another.

A  switch  changed this model. It learns which MAC addresses are associated with which ports. When a frame arrives, it examines the destination MAC address and uses its MAC address table to decide where the frame should be forwarded.

For example:  Computer A → Switch → Computer B . If the switch knows where Computer B is connected, it can forward the frame toward that port instead of sending it everywhere.

Modern switched Ethernet is therefore much more efficient than shared-medium networks. Each switch port can represent a separate  collision domain , while modern full-duplex Ethernet normally eliminates collisions on those links.

But switches do not automatically separate  broadcast domains . A broadcast frame can still be forwarded to multiple ports, meaning devices on the same Layer 2 network can remain part of the same broadcast domain.

Ethernet Also Needs to Detect Errors

“Fast communication is useful only when the receiver can recognize corrupted data.”

The Physical Layer deals with real signals moving through real environments. Noise, interference, damaged cables, and other physical conditions can cause transmitted data to become corrupted.

The Data Link Layer therefore needs a way to detect when a frame did not arrive correctly. Ethernet uses  Cyclic Redundancy Check (CRC)  for this purpose.

When the sender prepares a frame, it calculates a value based on the frame's contents and includes that value with the transmission. When the receiver gets the frame, it performs the calculation again and compares the result.

If the values do not match, the receiver knows that the frame was likely corrupted during transmission.

CRC is an  error-detection mechanism , not an error-repair mechanism. It does not reconstruct damaged data. Instead, it helps the receiver recognize the problem and discard the corrupted frame, while appropriate higher-level mechanisms can handle recovery.

This is another example of why networking is divided into layers. The Physical Layer carries the signal. The Data Link Layer organizes it into frames and checks the frame for errors. The Network Layer can then focus on moving packets between networks.

Conclusion

“The Data Link Layer turns a physical connection into organized local communication.”

Frames give structure to data. MAC addresses provide local identification. Ethernet establishes common communication rules, while switches make forwarding more efficient by learning where devices are connected. CRC helps detect corrupted frames.

Together, these mechanisms make the local network work. The bigger picture is important too: when a packet travels across the Internet, it moves through multiple networks and routers, but at every local link it still needs a Data Link Layer mechanism to carry it forward.

The  frame belongs to the local journey; the packet belongs to the larger journey . Understanding that distinction connects the Data Link Layer with everything we have explored so far — from physical signals and network models to IP addressing and packet delivery.

And once the frame has done its job locally, the next question naturally becomes bigger:  how does the Network Layer take that packet from one network and route it toward another?