Ethernet Frame Decoder & Size Calculator

What Is an Ethernet Frame Decoder & Size Calculator?

An Ethernet Frame Decoder separates raw Layer-2 frame bytes into destination MAC address, source MAC address, EtherType or VLAN information and payload. The size calculator performs the reverse type of analysis by determining header overhead, padding, FCS size and approximate wire occupancy from a payload length.

This is useful when reading packet captures, designing embedded Ethernet protocols, calculating network efficiency or checking how VLAN tagging changes frame length.

Ethernet II Frame Structure

The preamble and Start Frame Delimiter occur before the MAC frame on the wire and are normally not part of packet-capture frame length.

Destination and Source MAC Addresses

The first six bytes form the destination MAC address. The next six bytes form the source MAC address.

EtherType Field

For Ethernet II, the two bytes following the source MAC normally identify the upper-layer protocol unless that position contains a VLAN TPID.

Ethernet Frame Size

A conventional untagged Ethernet MAC frame with FCS is commonly described as:

The 64-byte minimum is measured from the Destination MAC through the FCS; it does not include preamble or inter-frame gap.

Ethernet Minimum Payload

For an ordinary untagged Ethernet II frame, a short payload is padded so that the MAC frame reaches the minimum size.

802.1Q VLAN Frame Size

A single IEEE 802.1Q tag contributes four bytes between the source MAC and the original EtherType. This increases the normal maximum Ethernet frame from 1518 to 1522 bytes. :contentReference[oaicite:1]{index=1}

For small tagged frames, standards-compatible implementations may preserve a 64-byte minimum or pad tagged transmissions to 68 bytes. Therefore this tool does not incorrectly reject every tagged frame below 68 bytes. :contentReference[oaicite:2]{index=2}

Ethernet FCS

The Frame Check Sequence is normally four bytes and protects the transmitted MAC frame against corruption. Many packet-capture interfaces do not provide FCS bytes to the capture application, so a capture can appear four bytes shorter than the physical MAC frame.

Preamble and Start Frame Delimiter

Before an Ethernet MAC frame is transmitted, the physical layer carries seven preamble bytes followed by one Start Frame Delimiter byte.

These eight bytes are useful when calculating wire occupancy but are not normally counted as part of the Ethernet MAC frame size.

Ethernet Inter-Frame Gap

Ethernet also requires idle time between frames equivalent to 96 bit-times, often represented as twelve byte-times when calculating throughput.

The IFG is not a field inside the Ethernet frame, but including it gives a more realistic estimate of physical-link occupancy.

On-Wire Ethernet Size Example

For a standard 1518-byte Ethernet MAC frame:

1500-Byte Payload Example

1500-Byte Payload with One VLAN Tag

This is the familiar standard maximum for a single-tagged frame carrying a 1500-byte payload. :contentReference[oaicite:3]{index=3}

QinQ and Double VLAN Tags

A double-tagged frame contains two four-byte VLAN tags, adding eight bytes relative to the equivalent untagged Ethernet II header.

Actual accepted maximum frame size depends on equipment configuration and network design.

Jumbo Ethernet Frames

Jumbo frames are Ethernet frames carrying payloads larger than the traditional 1500-byte Ethernet MTU. A commonly encountered configuration uses a payload around 9000 bytes.

Jumbo-frame limits are implementation-specific, so the calculator reports the mathematical size rather than claiming that every switch or NIC supports it.

Ethernet Header Overhead

For a large frame, fixed Layer-2 overhead is relatively small. For very short frames, the fixed header, FCS, padding and physical-wire overhead represent a much larger fraction of link usage.

The calculator therefore displays both MAC-frame bytes and physical wire occupancy separately.

Ethernet Frame Decoder FAQs