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Cisco CCNA Certification: Broadcasts, Unicasts, and Multicasts
When you begin your CCNA studies, you get hit with a lot of
different networking terms right away that you might not be
familiar with. What makes it a little more confusing is that a
lot of these terms sound a lot alike. Here, we're going to
discuss the differences between broadcasts, multicasts, and
unicasts at both the Data Link (Layer 2) and Network (Layer 3)
layers of the OSI model.
A broadcast is simply a unit of information that every other
device on the segment will receive. A broadcast is indicated by
having every bit of the address set to its highest possible
value. Since a hexadecimal bit's highest value is "f", a
hexadecimal broadcast is ff-ff-ff-ff-ff-ff (or
FF-FF-FF-FF-FF-FF, as the upper case does not affect hex value).
The CCNA exam will demand you be very familiar with hex
conversions, so if you're not comfortable with these
conversions, get comfortable with them before taking the exam!
At layer 3, a broadcast is indicated by setting every bit in the
32-bit binary string to "1", making the dotted decimal value
255.255.255.255. Every host on a segment will receive such a
broadcast. (Keep in mind that switches will forward a broadcast,
but routers do not.) In contrast to a broadcast, a unicast is a
packet or frame with only one destination.
There is a middle ground between broadcasts and unicasts, and
that is a multicast. Where a broadcast will be received by all,
and a unicast is received by only one host,
a multicast will be
received by multiple hosts, all belonging to a "multicast
group". As you climb the Cisco certification pyramid, you'll be
introduced to creating multicast groups and controlling
multicast traffic, but for your CCNA studies you need only keep
certain multicast groups in mind.
Class D addresses are reserved for multicasting this range is
224.0.0.0 - 239.255.255.255. The addresses 224.0.0.0 -
224.255.255.255 are reserved for use by network protocols on a
local network segment, and like broadcasts, routers will not
forward these multicast packets. (Packets with these addresses
are sent with a Time To Live of 1.)
As a CCNA candidate, you should know that OSPF routers use the
address 224.0.0.5 to send hellos, EIGRP routers use 224.0.0.10
to send updates, and RIP version 2 uses 224.0.0.9 to send
routing updates. RIP version 1 and IGRP both broadcast their
updates.
Multicasting gets a bit more complicated as you go from your
CCNA to the CCNP and CCIE, but by simply understanding what
multicasting is, you go a long way toward securing the CCNA.
About the author:
Chris Bryant, CCIE #12933, is the owner of The Bryant Advantage
(http://www.thebryantadvantage.com), home of free CCNA and CCNP tutorials!
For my FREE "How To Pass The CCNA" or "CCNP" ebook, visit the
website and download your copies!
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