The Ethernet Frame
Note: If the optional 4-byte 802.1Q tag is used, the Ethernet header size will
total 22 bytes, requiring a minimum payload of 42 bytes.
By default, the maximum frame size for Ethernet is 1518 bytes – 18 bytes
of header fields, and 1500 bytes of payload - or 1522 bytes with the 802.1Q
tag. A frame that is larger than the maximum will be discarded as a giant.
With both runts and giants, the receiving host will not notify the sender that
the frame was dropped. Ethernet relies on higher-layer protocols, such as
TCP, to provide retransmission of discarded frames.
Some Ethernet devices support jumbo frames of 9216 bytes, which provide
less overhead due to fewer frames. Jumbo frames must be explicitly enabled
on all devices in the traffic path to prevent the frames from being dropped.
The 32-bit Cycle Redundancy Check (CRC) field is used for errordetection.
A frame with an invalid CRC will be discarded by the receiving
device. This field is a trailer, and not a header, as it follows the payload.
The 96-bit Interframe Gap is a required idle period between frame
transmissions, allowing hosts time to prepare for the next frame
An Ethernet frame contains the following fields:
The preamble is 56 bits of alternating 1s and 0s that synchronizes
communication on an Ethernet network. It is followed by an 8-bit start of
frame delimiter (10101011) that indicates a valid frame is about to begin.
The preamble and the start of frame are not considered part of the actual
frame, or calculated as part of the total frame size.
Ethernet uses the 48-bit MAC address for hardware addressing. The first
24-bits of a MAC address determine the manufacturer of the network
interface, and the last 24-bits uniquely identify the host.
The destination MAC address identifies who is to receive the frame - this
can be a single host (a unicast), a group of hosts (a multicast), or all hosts (a
broadcast). The source MAC address indentifies the host originating the
frame.
The 802.1Q tag is an optional field used to identify which VLAN the frame
belongs to. VLANs are covered in great detail in another guide.
The 16-bit Ethertype/Length field provides a different function depending
on the standard - Ethernet II or 802.3. With Ethernet II, the field identifies
the type of payload in the frame (the Ethertype). However, Ethernet II is
almost entirely deprecated.
With 802.3, the field identifies the length of the payload. The length of a
frame is important – there is both a minimum and maximum frame size.
communication on an Ethernet network. It is followed by an 8-bit start of
frame delimiter (10101011) that indicates a valid frame is about to begin.
The preamble and the start of frame are not considered part of the actual
frame, or calculated as part of the total frame size.
Ethernet uses the 48-bit MAC address for hardware addressing. The first
24-bits of a MAC address determine the manufacturer of the network
interface, and the last 24-bits uniquely identify the host.
The destination MAC address identifies who is to receive the frame - this
can be a single host (a unicast), a group of hosts (a multicast), or all hosts (a
broadcast). The source MAC address indentifies the host originating the
frame.
The 802.1Q tag is an optional field used to identify which VLAN the frame
belongs to. VLANs are covered in great detail in another guide.
The 16-bit Ethertype/Length field provides a different function depending
on the standard - Ethernet II or 802.3. With Ethernet II, the field identifies
the type of payload in the frame (the Ethertype). However, Ethernet II is
almost entirely deprecated.
With 802.3, the field identifies the length of the payload. The length of a
frame is important – there is both a minimum and maximum frame size.
The absolute minimum frame size for Ethernet is 64 bytes (or 512 bits)
including headers. A frame that is smaller than 64 bytes will be discarded as
a runt. The required fields in an Ethernet header add up to 18 bytes – thus,
the frame payload must be a minimum of 46 bytes, to equal the minimum
64-byte frame size. If the payload does not meet this minimum, the payload
is padded with 0 bits until the minimum is met.
including headers. A frame that is smaller than 64 bytes will be discarded as
a runt. The required fields in an Ethernet header add up to 18 bytes – thus,
the frame payload must be a minimum of 46 bytes, to equal the minimum
64-byte frame size. If the payload does not meet this minimum, the payload
is padded with 0 bits until the minimum is met.
Note: If the optional 4-byte 802.1Q tag is used, the Ethernet header size will
total 22 bytes, requiring a minimum payload of 42 bytes.
By default, the maximum frame size for Ethernet is 1518 bytes – 18 bytes
of header fields, and 1500 bytes of payload - or 1522 bytes with the 802.1Q
tag. A frame that is larger than the maximum will be discarded as a giant.
With both runts and giants, the receiving host will not notify the sender that
the frame was dropped. Ethernet relies on higher-layer protocols, such as
TCP, to provide retransmission of discarded frames.
Some Ethernet devices support jumbo frames of 9216 bytes, which provide
less overhead due to fewer frames. Jumbo frames must be explicitly enabled
on all devices in the traffic path to prevent the frames from being dropped.
The 32-bit Cycle Redundancy Check (CRC) field is used for errordetection.
A frame with an invalid CRC will be discarded by the receiving
device. This field is a trailer, and not a header, as it follows the payload.
The 96-bit Interframe Gap is a required idle period between frame
transmissions, allowing hosts time to prepare for the next frame
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