Cisco CCNA Certification Test Prep:
Broadcasts, Multicasts, And Unicasts - What's The Difference?
By Chris Bryant, CCIE #12933
Your CCNA test prep introduces you to a lot of new terms, and it can be a little hard to keep them all straight at first! Three of the most important terms you'll see throughout your studies are broadcast, multicast, and unicast. Let's take a look at these three networking terms and how they're used in today's networks.
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:
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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)
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Network layer (Layer 3) broadcast packets have a destination IP address of 255.255.255.255
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:
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EIGRP updates are destined for 224.0.0.10
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OSPF routers listen to 224.0.0.5 for updates
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RIP version 2 uses 224.0.0.9 as a destination IP address
I'm sure you notice a pattern there! The entire range of addresses from 224.0.0.0 - 239.255.255.255 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's health, while multicasting is a vital skill for the CCNP and CCIE exams as well as valuable knowledge to have for working in today's real-world computer networks. To your success,
Chris Bryant
CCIE #12933
chris@thebryantadvantage.com
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