What fields does an Ethernet frame consist of?

An Ethernet frame consists of a set of strictly defined fields, each of which performs its own task at the data link layer.

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#Fields of an Ethernet frame

#1. Destination MAC address

• 6 bytes
• the MAC address of the device the frame is intended for
• can be:
  • unicast
  • multicast
  • broadcast (FF:FF:FF:FF:FF:FF)

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#2. Source MAC address

• 6 bytes
• the MAC address of the device that sent the frame

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#3. Protocol type (EtherType)

• 2 bytes
• indicates which protocol is inside the frame

Common values:

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

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#4. Payload / Data

• from 46 to 1500 bytes
• contains higher-layer data (usually an IP packet)

If there is less than 46 bytes of data:

• padding is added

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#5. Checksum (FCS, Frame Check Sequence)

• 4 bytes
• used to detect errors
• calculated by the sender and checked by the receiver

If the FCS does not match:

• the frame is discarded

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#Size of an Ethernet frame

Without VLAN:

• minimum: 64 bytes
• maximum: 1518 bytes

With a VLAN tag (802.1Q):

• up to 1522 bytes

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#VLAN tag (if used)

If the network has VLANs, this is added between the Source MAC and EtherType:

• 802.1Q VLAN tag: 4 bytes
  • VLAN ID
  • priority
  • service information

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#Quick summary (easy to remember)

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#Short version for an interview

An Ethernet frame consists of the destination MAC address, the source MAC address, the EtherType field, the payload, and the FCS checksum; when VLAN is used, a VLAN tag is added.