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Frames in the Ethernet

Summary

Objectives

By the end of this session you should be able to:

  • Identify MAC Addresses on a network
  • Describe the features of an Ethernet Frame

Key Points

  • MAC Addresses are globally unique, fixed IDs assigned to network devices
  • Ethernet is the most common wired LAN technology
    • It sends data by encapsulating chunks of the message into frames

Breakdown

MAC Addresses

Every device that connects to a network has a MAC Address assigned to it during manufacture (sometimes more than one, e.g. if a device has both wired and wireless network connectivity). This is a 6-byte hexadecimal number that is usually separated into bytes by dashes or colons, e.g. 02:34:f8:b8:3b:2a. You should be able to find the MAC Address for your device with a little searching through the settings, or by using the right commands. On Linux you can see it in the output for ip a.

info

MAC stands for Media Access Control. It has nothing to do with the Mac computer products from Apple - that's just a coincidence.

Ethernet Frames

Ethernet networks use MAC addresses to identify where packets are going, and where they're coming from (most modern networks use Ethernet). Ethernet packets are called frames, and they have the following parts:

  • An 8-byte Preamble which identifies the start of the packet. It is always aa-aa-aa-aa-aa-aa-aa-ab
  • A Destination MAC Adress
  • A Source MAC Address
  • The Protocol Type, which is usually IP (08-00)
  • The Data itself, which can be up to 1500 bytes (any more and it gets split into another packet)
  • The CRC Check, a 4-byte value calculated from the rest of the frame. If the CRC Check doesn't match the frame when a computer receives it, the frame is discarded as corrupt.

People don't generally construct Ethernet frames and you may never need to understand this in detail - however, since this is the basis of almost all computer networks it's good to have an awareness of it. Any data sent over a network is broken up into parts, usually no more than 1500 bytes in size, then placed in packets that are assembled according to this protocol.