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Ethernet Networking

And I promise to go over all four of the available Ethernet frames in the

upcoming sections.

Ethernet Addressing

Here's where we get into how Ethernet addressing works. It uses the Media
Access Control (MAC) address burned into each and every Ethernet Net-
work Interface Card (NIC). The MAC, or hardware address, is a 48-bit
(6 byte) address written in a hexadecimal format.

Figure 1.11 shows the 48-bit MAC addresses and how the bits are

divided.

F I G U R E 1 . 1 1

Ethernet addressing using MAC addresses

The organizationally unique identifier (OUI) is assigned by the IEEE to an

organization. It's composed of 24 bits, or 3 bytes. The organization, in turn,
assigns a globally administered address (24 bits, or 3 bytes) that is unique
(supposedly, again, no guarantees) to each and every adapter they manufac-
ture. Look closely at the figure... Notice bit 46? Bit 46 must be 0 if it's a
globally assigned bit from the manufacturer, and it has to be a 1 if it's locally
assigned by the network administrator.

Ethernet Frames

The Data Link layer is responsible for combining bits into bytes and bytes
into frames. Frames are used at the Data Link layer to encapsulate packets
handed down from the Network layer for transmission on a type of media
access. There are three types of media access methods: contention (Ethernet),
token passing (Token Ring and FDDI), and polling (IBM Mainframes and
100VG-AnyLAN). The CCNA exam covers primarily Ethernet (contention)
media access--so shall we.

The function of Ethernet stations is to pass data frames between each

other using a group of bits known as a MAC frame format. This provides
error detection from a cyclic redundancy check (CRC). But remember--this

Organizationally

Unique Identifier (OUI)

(Assigned by IEEE)

24 bits

Vendor assigned

I/G

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