What is Ethernet?
Learn what Ethernet is, its frame format, MAC addressing, and how switches enable full-duplex LAN communication.
Expected Interview Answer
Ethernet is the family of wired networking standards (IEEE 802.3) that defines how devices on a local area network physically connect and frame data at the Physical and Data Link layers, using MAC addresses to deliver frames between devices on the same segment.
Ethernet specifies both the physical medium (twisted-pair copper cables like Cat5e/Cat6, or fiber) and the frame format used to carry data: an Ethernet frame includes source and destination MAC addresses, an EtherType field identifying the higher-layer protocol (typically IP), the payload, and a frame check sequence for error detection. Early Ethernet ran over shared coaxial cable or through hubs in half-duplex mode, requiring CSMA/CD to handle collisions, but modern Ethernet runs through switches in full-duplex mode, giving each device a dedicated collision-free path and letting speeds scale from 10 Mbps in the original standard up to 10, 25, 100, or even 400 Gbps in current data-center variants. Because Ethernet operates at Layers 1-2, it is protocol-agnostic above that: IP, ARP, and other protocols all ride inside Ethernet frames as the EtherType dictates, which is why Ethernet has remained the dominant LAN technology for decades even as everything running over it has evolved.
- Defines both physical medium and frame format for LANs
- Uses MAC addresses for local, Layer 2 frame delivery
- Scales from 10 Mbps to hundreds of Gbps across generations
- Carries higher-layer protocols like IP transparently via EtherType
AI Mentor Explanation
Ethernet is like the standardized set of rules a cricket ground uses for how the ball must physically be delivered and how a scorer must log it — the pitch and bowling crease are the physical medium, and the mandatory scorecard fields (bowler, batter, result) are the frame format every delivery must follow. Just as every ball bowled at any ground worldwide follows the same crease rules regardless of who is playing, every Ethernet frame follows the same structure regardless of what data (IP, ARP) it carries inside. Old grounds with a single shared practice net (like a hub) forced bowlers to wait their turn, while modern grounds with individual bowling lanes (like switches) let everyone bowl simultaneously.
Step-by-Step Explanation
Step 1
Physical medium
Ethernet defines the cabling (twisted-pair copper or fiber) devices connect through.
Step 2
Frame format
Data is wrapped in an Ethernet frame with source/destination MAC addresses, EtherType, payload, and FCS.
Step 3
Local delivery
A switch reads the destination MAC address and forwards the frame only to the correct port.
Step 4
Higher-layer protocols ride on top
IP, ARP, and other protocols are carried as the payload inside Ethernet frames, identified by EtherType.
What Interviewer Expects
- Correct definition: IEEE 802.3 family defining LAN wiring and frame format
- Knows Ethernet operates at Physical and Data Link layers
- Understands the Ethernet frame structure (MAC addresses, EtherType, FCS)
- Aware of the evolution from shared/half-duplex hubs to switched full-duplex
Common Mistakes
- Confusing Ethernet (a Layer 1/2 standard) with the internet itself
- Thinking Ethernet only refers to the cable, not also the frame format
- Not knowing EtherType is what identifies the higher-layer protocol inside a frame
- Assuming all Ethernet is half-duplex hub-based rather than switched full-duplex
Best Answer (HR Friendly)
“Ethernet is the standard technology behind most wired networks — it defines both the cables and connectors you plug in and the exact format the data is packaged in so devices understand each other. It has scaled from the original slow standard decades ago to today's multi-gigabit speeds while keeping the same basic frame format, which is a big reason it has remained the backbone of local networks for so long.”
Code Example
# Capture raw Ethernet frames on an interface, showing source/dest MAC and EtherType
sudo tcpdump -e -n -i eth0 -c 5
# Example output line:
# 00:1a:2b:3c:4d:5e > ff:ff:ff:ff:ff:ff, ethertype ARP (0x0806), length 42:
# arp who-has 192.168.1.1 tell 192.168.1.20
# Check the negotiated Ethernet link speed
ethtool eth0 | grep SpeedFollow-up Questions
- What fields make up an Ethernet frame?
- How does a switch differ from a hub in how it handles Ethernet frames?
- What is EtherType and what values does it commonly carry?
- How has Ethernet speed evolved from 10 Mbps to modern data-center standards?
MCQ Practice
1. At which OSI layers does Ethernet primarily operate?
Ethernet defines the Physical-layer medium and Data Link-layer framing, including MAC addressing.
2. What field in an Ethernet frame identifies the higher-layer protocol carried inside?
EtherType indicates which protocol (e.g., IPv4, ARP) the frame payload belongs to.
3. What device change moved Ethernet from half-duplex to switched full-duplex operation?
Switches give each device a dedicated collision-free path, enabling full-duplex operation, unlike shared hubs.
Flash Cards
What is Ethernet? — The IEEE 802.3 family of standards defining LAN wiring and frame format at Layers 1-2.
What identifies the payload protocol in a frame? — The EtherType field (e.g., IPv4, ARP).
Hub vs switch for Ethernet? — Hubs share bandwidth in half-duplex; switches give dedicated full-duplex paths per port.
What addresses do Ethernet frames use? — Source and destination MAC addresses.