📖 IT Bible

Dissecting an Ethernet Frame

Data Packet - an all-encompassing term that represents any single set of binary data being sent across a network link

  • represents the concept of one set of data being sent from point A to point B
  • Data packets at the Ethernet level are known as Ethernet Frames…

Ethernet Frames - highly structured collection of info presented in a specific order

  • this way, network interfaces at the physical layer can convert a stream of bits traveling across a link into meaningful data, or vice versa
  • almost all sections are mandatory, and most of them have a fixed size

Preamble - first part of an Ethernet Frame; 8 bytes (64 digits) long and can itself be split into 2 sections

  • the first set of bytes are a series of alternating 1s and 0s, acting as a buffer between frames and can also be used by network interfaces to synchronize internal clocks to regulate speed at which data is sent
  • last byte in the Preamble is the Start Frame Delimiter (SFD)…

Start Frame Delimiter (SFD) - signals to a receiving device that the preamble is over and that the actual frame contents will now follow

Destination MAC Address - hardware address of the intended recipient

Source MAC Address - where the frame originated from

Ether-type Field - 16 bits long and used to describe the protocol of the contents of the frame

VLAN Header - indicates that the frame itself is what’s called a VLAN frame

  • if a VLAN header is present, the EtherType field follows it Virtual VLAN - a technique that lets you have multiple logical LANs operating on the same physical equipment

Payload - in networking terms, is the actual data being transported, which is everything that isn’t a header

Frame Check Sequence - a 4-byte (or 32-bit) number that represents a checksum value for the entire frame

  • Checksum Value is calculated by performing what’s known as a cyclical redundancy check against the frame Cyclical Redundancy Check (CRC) - important concept for data integrity, and is used all over computing, not just network transmissions
  • basically a mathematical transformation that uses polynomial division to create a number that represents a larger set of data
  • anytime you perform a CRC against a set of data, you should end up with the same checksum number

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