The Ethernet LAN - jude-lindale/Wiki GitHub Wiki

  • The networking protocol that is used in most modern computer networks is Ethernet, which is a carrier sense multiple access with collision detection (CSMA/CD) protocol for local area networks.

  • The information in an Ethernet network is exchanged in a frame format.

    • The frame provides a grouping of the information for transmission; this includes the header, data, and trailer.

  • The header

    • The header consists of the preamble, start frame delimiter, destination and source addresses, and length/type field.

  • The data

    • The actual data being transmitted, followed by the padding used to bring the total number of bytes up to the minimum of 46 if the data field is less than 46 bytes.

  • The trailer

    • The last part of the frame is a 4-byte cyclic redundancy check (CRC) value used for error checking. 01fig07

| Preamble |An alternating pattern of 1s and 0s used for synchronization. |
|Start frame delimiter |A binary 8-bit sequence of 1 0 1 0 1 0 1 1 that indicates the start of the frame. |
|Destination MAC address and source |The unique media access control address associated with each computer’s Ethernet network interface card (NIC) or network adapter. |
|MAC address |The associated MAC address, which is 6 bytes (12 hex characters) in length. | |Length/type |An indication of the number of bytes in the data field if this value is less than 1500. (If this number is greater than 1500, it indicates the type of data format—for example, IP and IPX.) | |Data |The variable length of data being transferred from the source to the destination. | |Pad |A field used to bring the total number of bytes up to the minimum of 64 if the data field is less than 64 bytes. | |Frame check sequence |A 4-byte CRC value used for error detection. The CRC is performed on the bits from the destination MAC address through the Pad fields. If an error is detected, the frame is discarded. |

Note

  • The minimum length of the Ethernet frame is 64 bytes from the destination MAC address through the frame check sequence. The maximum Ethernet frame length set by the IEEE 802.3 standard is 1518 bytes: 6 bytes for the destination MAC address, 6 bytes for the source MAC address, 2 bytes for length/type, and 1500 bytes for the data. Ethernet jumbo frames now allow for 9000-byte payload frames with a payload size of 8960 bytes of data.

  • MAC Adress (media access control)

    • The MAC address is 6 bytes, or 48 bits, in length. The address is displayed in 12 hexadecimal digits. The first 6 digits are used to indicate the vendor of the network interface, also called the organizationally unique identifier (OUI), and the last 6 numbers form a unique value for each NIC assigned by the vendor. IEEE is the worldwide source of registered OUIs.
    • Organizationally Unique Identifier (OUI) is the first 3 bytes of the MAC address that identifies the manufacturer of the network hardware -The MAC address, also called the Ethernet, physical, hardware, or adapter address can be obtained from computers operating under Microsoft Windows by typing the ipconfig /all command while in the command mode or at the MS-DOS prompt.

SUMMARY -The MAC address provides the information that ultimately enables the data to reach a destination in a LAN. This is also how computer 1 and the printer communicated directly in the star topology. The switch stored the MAC addresses of all devices connected to its ports and used this information to forward the data from computer 1 directly to the printer. The switch also used the MAC address information to forward the data from computer 5 to computer 6

Company ID-Vendor Serial Number Manufacturer (Company ID)
00-AA-00-B6-7A-57 Intel Corporation (00-AA-00)
00-00-86-15-9E-7A Megahertz Corporation (00-00-86)
00-50-73-6C-32-11 Cisco Systems, Inc. (00-50-73)
00-04-76-B6-9D-06 3COM (00-04-76)
00-0A-27-B7-3E-F8 Apple Computer, Inc. (00-0A-27)