What is an Ethernet frame?

An Ethernet frame is the unit of data transmission on Ethernet networks at the data link layer (L2). Data is transmitted between devices within a single local network precisely in the form of Ethernet frames.

Briefly: An Ethernet frame is the format Ethernet uses to package and transmit data.

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#Why an Ethernet frame is needed

The physical layer transmits only bits. An Ethernet frame is needed to:

• mark the start and end of the data
• specify the recipient's and sender's MAC address
• identify which protocol is inside
• check whether the data is corrupted

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#Structure of an Ethernet frame

A classic Ethernet II frame consists of the following fields:

#1. Destination MAC address

• 6 bytes
• indicates who the frame is intended for
• can be unicast, multicast, or broadcast

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#2. Source MAC address

• 6 bytes
• the MAC address of the sending device

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#3. Type (EtherType)

• 2 bytes
• shows which protocol is inside the frame

Examples:

• 0x0800: IPv4
• 0x0806: ARP
• 0x86DD: IPv6

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#4. Payload

• from 46 to 1500 bytes
• usually contains an IP packet
• if there is less than 46 bytes of data, padding is added

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#5. Checksum (FCS)

• 4 bytes
• used to detect errors
• if the check fails, the frame is discarded

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#Size of an Ethernet frame

• Minimum: 64 bytes
• Maximum: 1518 bytes (without a VLAN tag)

With a VLAN tag:

• up to 1522 bytes

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#Where an Ethernet frame is used

Ethernet frames:

• are used on wired Ethernet networks
• are also used as the basis for Wi-Fi (with a format conversion)
• exist only within a single local network

When passing through a router:

• the Ethernet frame is discarded
• a new frame is created for the next network segment

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#What matters for an interview

• an Ethernet frame is L2
• an L3 packet is contained inside the frame
• switches work specifically with Ethernet frames
• routers do not forward frames, they rebuild them

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#Short version for an interview

An Ethernet frame is the data link layer's data format, used on Ethernet networks, containing MAC addresses, service fields, and a payload for transmitting data within a local network.