About this interactive
What you're seeing: a fill-in-the-blank activity covering each field of an Ethernet frame — its size, key values, and purpose. Why it matters: the Ethernet frame is the workhorse of Layer 2; knowing its exact structure helps you read packet captures, understand switch behavior, and troubleshoot LAN issues. How to use it: complete each blank, then click Check Answers to see which fields you know cold and which ones need more review.
What you'll learn
- Identify the five main fields of an Ethernet frame and the purpose each field serves
- Recall the byte length and key values for the Destination MAC, Source MAC, EtherType, Payload, and FCS fields
- Distinguish between EtherType values for IPv4 (0x0800), ARP (0x0806) and IPv6 (0x86DD) and explain what each signals to the receiver
- State the minimum and maximum Ethernet payload sizes and explain why the minimum exists
- Account for the bytes that surround the frame's five main fields — the 7-byte preamble, the Start Frame Delimiter, an 802.1Q VLAN tag, and the minimum interframe gap
- Distinguish an IEEE 802.3 length field from an Ethernet II EtherType by the value the two bytes after the Source MAC carry
Key terms
- Ethernet Frame
- The Layer 2 data unit used on Ethernet networks, containing a preamble, source and destination MAC addresses, an EtherType field identifying the Layer 3 protocol, a payload (typically an IP packet), and a 4-byte FCS for error detection. Switches use the MAC addresses in Ethernet frames to make forwarding decisions.
- MAC Address
- A 48-bit hardware address assigned to a network interface and written as six hexadecimal octets, such as 00:1A:2B:3C:4D:5E. It identifies a device uniquely on a local network segment, and switches forward frames by looking it up in their MAC address table.
- EtherType
- A two-octet field in an Ethernet frame header that identifies which upper-layer protocol the payload carries — 0x0800 for IPv4, 0x0806 for ARP, 0x86DD for IPv6. The receiving device reads it to decide which protocol stack should process the frame.
- Frame Check Sequence
FCS
- A cyclic redundancy check value appended to the end of a data link frame that allows the receiver to detect transmission errors. The sending device calculates the FCS from the frame contents, and the receiving device recalculates it; a mismatch means the frame was corrupted and should be discarded.
- Broadcast Address
- The last address in an IP subnet range, formed when all host bits are set to one, used to send a single packet to every device on that network simultaneously.
- Cyclic Redundancy Check
CRC
- An error-detection algorithm applied to network frames where the sender appends a checksum computed from the frame contents and the receiver recalculates the checksum to verify data integrity. A CRC mismatch indicates the frame was corrupted in transit and should be discarded.
- Maximum Transmission Unit
MTU
- The largest size of a packet or frame that can be transmitted over a network link without fragmentation, typically 1500 bytes for Ethernet. When packets exceed the MTU, they must be fragmented at Layer 3 or the sender must reduce the packet size via Path MTU Discovery.
Topics
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