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Chapter 4: Bridges/Switches and LAN Design
LAN Addressing
As a CCNA, you'll be expected to confidently understand and interpret LAN addresses. One
important function of MAC addresses is to identify or address the LAN interface cards on
Ethernet, Token Ring, and FDDI LANs. Frames between a pair of LAN stations use a source
and destination address field to identify each other. These addresses are called
unicast
addresses
, or
individual addresses
, because they identify an individual LAN interface card.
(The term
unicast
was chosen mainly for contrast with the terms
broadcast
,
multicast
, and
group addresses
.)
Having globally unique unicast MAC addresses on all LAN cards is a goal of the IEEE, so the
organization administers a program in which manufacturers encode the MAC address onto the
LAN card, usually in a ROM chip. The first half of the address is a code that identifies the
vendor; this code is sometimes called the
Organizationally Unique Identifier
. The second part
is simply a unique number among cards that the vendor has manufactured. These addresses are
called
burned-in addresses (BIAs)
, sometimes called
Universally Administered Addresses
(UAA)
. The address used by the card can be overridden via configuration; the overriding address
is called a
Locally Administered Address (LAA)
.
Another important function of IEEE MAC addresses is to address more than one LAN card.
Group
addresses (as opposed to unicast addresses) can address more than one device on a LAN.
This function is satisfied by three types of IEEE group MAC addresses:
·
Broadcast addresses
--The most popular type of IEEE MAC address, the broadcast
address, has a value of FFFF.FFFF.FFFF (hexadecimal notation). The broadcast address
implies that all devices on the LAN should process the frame.
·
Multicast addresses
--Used by Ethernet and FDDI, multicast addresses fulfill the
requirement to address a subset of all the devices on a LAN. A station processes a received
frame with a particular multicast destination address only if configured to do so for that
multicast address. An example of multicast addresses is a range of addresses--
0100.5exx.xxxx--where different values are assigned in the last 3 bytes; these MAC
addresses are used in conjunction with Internet Group Multicast Protocol (IGMP) and IP
multicast. IP hosts on an Ethernet that want to receive IP packets to a particular IP
multicast address all use the same Ethernet MAC address, which begins with 0100.5E.
·
Functional addresses
--Valid only on Token Ring, functional addresses identify one or
more interfaces that provide a particular function. For example, c000.0000.0001 is used
by the device on a Token Ring that is currently implementing the Active Monitor function.
A subtle quirk about LAN addressing is that the order of bits in each byte of the addresses is
different between Ethernet and the other LAN types. As Figure 4-4 illustrates, the bytes are
listed in the same order; however, the bit order in each byte is opposite.
ch04.fm Page 138 Monday, March 20, 2000 5:02 PM