Monday, December 22, 2008

Broadcasts, Multicasts, And Unicasts - For CCNA

A broadcast is a data signal that is intended for everyone. A network
broadcast is much like a radio broadcast. A commercial radio
station's tower isn't sending a signal to one particular destination
- it's sending its signal out in all directions in hopes that everyone
who can listen to it will do so. A network broadcast is very similar
in that it's given a destination address that every host on the
network will listen to. Two important broadcast addresses for you
to know for the CCNA certification exams:

Data Link layer (Layer 2) broadcast frames have a destination MAC
address of ff-ff-ff-ff-ff-ff (also expressed as FF-FF-FF-FF-FF-FF,
the case doesn't matter)
Network layer (Layer 3) broadcast packets
have a destination IP address of

By default, switches will forward all broadcast frames out every
port except the one that originally received the broadcast. Also by
default, routers accept broadcasts but do not forward them.

A unicast has a much simpler definition - it's simply data that has
a single destination.

Broadcasts go to everyone, unicasts go to only one -- sounds like
we need a middle ground! Luckily, we do, with multicasts. A multicast
is destined for members of a multicast group. Later on in your CCNP
studies you'll learn much more about multicasting, but there are
several multicast addresses you should know for the CCNA
certification exam:

EIGRP updates are destined for
OSPF routers listen to for updates
RIP version 2 uses as a destination IP address
I'm sure you notice a pattern there! The entire range of addresses
from - is reserved for multicasting. This
is the Class D address range. As with broadcast packets, these
multicast packets are not forwarded by routers.

It's important to know the differences between multicasts,
broadcasts, and unicasts as part of your CCNA test prep,
but this knowledge goes far beyond passing a certification exam.
Knowing how to limit broadcasts helps to improve your network.

Best Regards,
Deepak Arora

No comments: