The exact maximum frame size of Ethernet, including its overhead, is 1518 bytes.
Understanding the Ethernet Frame Size
Ethernet frames carry data across a network. The total size of an Ethernet frame is the sum of its data payload and various header and trailer fields, which constitute the overhead.
Based on the provided reference:
- Payload: The data being transported, typically up to 1500 bytes (this is often referred to as the Maximum Transmission Unit or MTU).
- Overhead: The additional bytes for addressing, type identification, error checking, and synchronization. For a standard untagged Ethernet frame, this overhead is 18 bytes.
Breakdown of the Maximum Size
The maximum frame size is calculated by adding the maximum payload size to the standard overhead:
Maximum Frame Size = Maximum Payload + Overhead
Maximum Frame Size = 1500 bytes + 18 bytes
Maximum Frame Size = 1518 bytes
This size refers to the standard Ethernet II or IEEE 802.3 untagged frame.
| Component | Size (bytes) |
|---|---|
| Payload | Up to 1500 |
| Overhead | 18 |
| Total (Max) | 1518 |
The 18 bytes of overhead typically include:
- Destination MAC Address (6 bytes)
- Source MAC Address (6 bytes)
- EtherType/Length (2 bytes)
- Frame Check Sequence (FCS) / CRC (4 bytes)
(Note: The preamble and start-of-frame delimiter (SFD), which are 8 bytes in total, are part of the physical layer transmission but are usually not included when calculating the frame size at the data link layer).
Variations (Beyond Standard)
While 1518 bytes is the standard maximum for untagged frames, it's important to note that frame sizes can vary slightly in specific scenarios:
- VLAN Tagging: Adding a VLAN tag (IEEE 802.1Q) adds 4 bytes to the frame, increasing the maximum size to 1522 bytes.
- MPLS: Adding MPLS labels also increases frame size.
- Jumbo Frames: Many modern networks support "jumbo frames," which are Ethernet frames with a payload size exceeding the standard 1500 bytes, typically up to 9000 bytes. However, these are not part of the original Ethernet standard and require support on all network devices involved.
For the exact standard answer based on the typical untagged frame, 1518 bytes is the correct value derived from the 1500-byte payload and 18-byte overhead.
