Page 67 - Handout Computer Network.

Figure 3-13 shown an IEEE802.3 Ethernet and the OSI Model

The Layer 2 sublayers, LLC and MAC, are active, vital agreements that make technology
compatible and computer communication possible. The MAC sublayer is concerned with the
physical components that will be used to communicate the information. Like the other layers,
the LLC remains relatively independent of the physical equipment that will be used for the
communicative process. The LLC allows multiple Layer 3 protocols, such as IP and IPX, to be
simultaneously supported along with multiple frame types. Figure 3-14 maps a variety of
Ethernet technologies to the lower half of OSI Layer 2, and all of Layer 1. Although there are
other varieties of Ethernet, the ones depicted are the most widely used and are the focus of
this course.
MAC Addressing To allow for local delivery of frames on the Ethernet, there must be
an addressing system, a way of naming the computers and interfaces. Every computer has a
unique way of identifying itself. Each computer on a network has a physical address. No two
physical addresses on a network should ever be alike. Referred to as the Media Access Control
(MAC) address, the physical address is located on the NIC. Other terms for the MAC address
include the hardware address, the NIC address, the Layer 2 address, and the Ethernet address.
Ethernet uses MAC addresses to uniquely identify individual devices. Every device (PC, router,
switch) with an Ethernet interface to the LAN must have a MAC address; otherwise, other
devices cannot communicate with it. A MAC address is 48 bits in length and is expressed as 12
hexadecimal digits. The first six hexadecimal digits, which are administered by the IEEE,
identify the manufacturer or vendor and thus comprise the organizationally unique identifier
(OUI). The remaining six hexadecimal digits comprise the interface serial number, or another
value administered by the specific vendor. MAC addresses sometimes are referred to as
burned-in addresses (BIAs) because they are burned into read-only memory (ROM) and are
copied into random-access memory (RAM) when the NIC initializes. Figure 3-15 illustrates the
MAC address format.

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