Ethernet Interview questions

1. What is Ethernet?

Ethernet is a medium which is used by devices like computers, switches, routers, hubs etc. If two devices wishes to send data among each other using ethernet technology, both the devices should support ethernet. For example, there are network interface cards which support ethernet, switches and routers which support ethernet network cards. Ethernet comes at the layer 2 of the OSI Model and Layer 1 of the TCP/IP Model.

2. What is an ethernet frame?

An ethernet frame is used for encapsulating data from the higher layers of the TCP/IP Model. Assume that a PC1 pings PC2. The ping , which is an ICMP packet along with the IP Header is encapsulated in the ethernet frame. The frame is used for carrying the ping packet and ultimately deliver it to PC2.

3. What are the components of an ethernet frame?

Ethernet frames are of different types. Typically they consist of the source mac-address, destination mac-address, data from the upper layer, type of data like IP, IPX etc, preamble (for clocking) and a FCS value for identifying the validity of the frame (Checks if the frame is invalid, corrupt, etc)

4. How is an ethernet frame constructed?

An ethernet frame is constructed by the network card. When data on the operating system, comes down to the network card, the frame would be constructed to send the data out on the network.

5. How is a frame received at the destination?

The preamble field is looked into first inside a frame. This is purely for synchronization. This is followed by the destination looking into whether the frame is intended for itself or not, by checking the destination mac-address. Once the mac-address is verified, the FCS value is calculated. If a mismatch is found, the frame is discarded. Else, the data type of the frame is looked into and the data is sent to the appropriate layers of the operating system
(IP Layer).

1. What is Ethernet?

Ethernet is the most widely used LAN (Local Area Network) technology, defined by the IEEE 802.3 standard. It uses frames to transmit data over copper or fiber.

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2. At which OSI layers does Ethernet operate?

• Layer 1 (Physical Layer) → Cables, signaling.

• Layer 2 (Data Link Layer) → Framing, MAC addressing, error detection.

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3. What are Ethernet frame components?

1. Preamble (7 bytes) + SFD (1 byte) → Synchronization.

2. Destination MAC (6 bytes).

3. Source MAC (6 bytes).

4. EtherType/Length (2 bytes).

5. Payload/Data (46–1500 bytes).

6. FCS (4 bytes, CRC for error detection).

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4. What is the maximum transmission unit (MTU) in Ethernet?

• Standard Ethernet MTU = 1500 bytes.

• Jumbo Frames (optional) = up to 9000 bytes.

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5. What are MAC addresses in Ethernet?

• Unique 48-bit hardware address (e.g., 00:1A:2B:3C:4D:5E).

• Used for device identification at Layer 2.

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6. What is the difference between half-duplex and full-duplex Ethernet?

• Half-duplex → Data flows one way at a time (older hubs).

• Full-duplex → Data flows both ways simultaneously (modern switches).

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7. What is CSMA/CD?

• Carrier Sense Multiple Access with Collision Detection.

• Used in half-duplex Ethernet to avoid collisions.

• Modern full-duplex switched Ethernet does not need CSMA/CD.

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8. What are common Ethernet speeds?

• 10 Mbps → 10BASE-T

• 100 Mbps → Fast Ethernet (100BASE-TX)

• 1 Gbps → Gigabit Ethernet (1000BASE-T)

• 10 Gbps, 40 Gbps, 100 Gbps → High-speed Ethernet (fiber/copper)

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9. What is the difference between a hub, switch, and router in Ethernet?

• Hub → Broadcasts frames to all ports (Layer 1).

• Switch → Forwards based on MAC address (Layer 2).

• Router → Forwards based on IP address (Layer 3).

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10. What is VLAN in Ethernet?

• Virtual LAN allows logical segmentation of networks within the same physical switch.

• Defined in IEEE 802.1Q (adds a 4-byte VLAN tag in Ethernet frame).

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11. What is auto-negotiation in Ethernet?

A process where two Ethernet devices agree on the best speed and duplex settings automatically.

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12. What are some common Ethernet cable categories?

• Cat5e → Up to 1 Gbps.

• Cat6 → Up to 10 Gbps (short distance).

• Cat6a/Cat7 → Better shielding, supports higher speeds/longer distance.

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Quick Recap for Interviews:

• Ethernet = IEEE 802.3 (Layer 1 & 2).

• Frame size: 64–1518 bytes (without jumbo).

• Uses MAC addressing.

• Full-duplex replaced CSMA/CD.

• VLANs via 802.1Q.

1. What is Ethernet and what is its role in networking?

Ethernet is a family of wired computer networking technologies commonly used in Local Area Networks (LANs), Metropolitan Area Networks (MANs), and Wide Area Networks (WANs). It operates primarily at Layer 2 (the data link layer) of the OSI model, defining the rules and standards for transmitting data frames over a physical medium. Its role is to enable communication between devices on a network.

2. What are the key components of an Ethernet frame?

An Ethernet frame is the basic unit of data transmission. The key components of an Ethernet II frame (the most common type) are:

• Preamble & Start Frame Delimiter (SFD): Synchronization fields used to signal the beginning of a new frame.

• Destination MAC Address: The 48-bit physical address of the receiving device.

• Source MAC Address: The 48-bit physical address of the sending device.

• EtherType: A 16-bit field that identifies the protocol being carried in the payload (e.g., 0x0800 for IPv4, 0x0806 for ARP).

• Payload (Data): The actual data being transmitted, typically a higher-level protocol packet like an IP packet. The payload can range from 46 to 1500 bytes.

• Frame Check Sequence (FCS): A 32-bit field used for error detection. The receiving device performs a Cyclic Redundancy Check (CRC) to ensure data integrity.

3. What is a MAC address and how is it different from an IP address?

A MAC (Media Access Control) address is a unique 48-bit physical hardware address assigned to a network interface controller (NIC) by the manufacturer. It is a Layer 2 address used for communication within a local network segment.

An IP (Internet Protocol) address is a logical address used for routing data between different networks (Layer 3). While a MAC address is fixed to the hardware, an IP address is assigned to a device on a network and can change (e.g., when a device moves to a new network).

4. Explain the difference between a half-duplex and full-duplex connection.

• Half-Duplex: In a half-duplex connection, devices can either send or receive data, but not simultaneously. The communication is two-way, but only one device can transmit at a time. This requires a mechanism like CSMA/CD to prevent data collisions.

• Full-Duplex: In a full-duplex connection, devices can send and receive data simultaneously. This is the standard for modern Ethernet networks using switches, as it eliminates the possibility of collisions.

5. What is CSMA/CD and why is it no longer widely used?

CSMA/CD stands for Carrier Sense Multiple Access with Collision Detection. It is a protocol used in half-duplex Ethernet to manage data access on a shared medium.

• Carrier Sense: A device "listens" to the network to see if it is idle before transmitting.

• Multiple Access: All devices on the network have equal access to the medium.

• Collision Detection: If two devices transmit at the same time, a collision occurs. Both devices detect this, stop transmitting, and wait a random amount of time before attempting to retransmit.

CSMA/CD is no longer widely used because modern Ethernet networks primarily use full-duplex communication with switches, which eliminates collisions entirely.

6. What is the difference between a network hub and a network switch?

• Hub: A hub is a Layer 1 (physical layer) device that functions like a repeater. It takes data from one port and broadcasts it to all other connected ports. This creates a single collision domain and a single broadcast domain.

• Switch: A switch is a Layer 2 (data link layer) device that learns the MAC addresses of connected devices. It forwards frames only to the specific port where the destination device is located. This creates a separate collision domain for each port and reduces network traffic.

7. What is Power over Ethernet (PoE)?

PoE is a technology that allows network cables to carry electrical power to powered devices. This eliminates the need for a separate power cable, simplifying installation for devices like IP phones, wireless access points, and security cameras in locations where power outlets are not readily available.