Ever since you picked up your first CCNA book, you've heard about
multicasting, gotten a fair idea of what it is, and you've memorized a
couple of reserved multicasting addresses. Now as you prepare to pass
the BCMSN exam and become a CCNP, you've got to take that knowledge to
the next level and gain a true understanding of multicasting. Those of
you with an eye on the CCIE will truly have to become multicasting
experts!
Having said that, we're going to briefly review the basics of
multicasting first, and then future tutorials will look at the
different ways in which multicasting can be configured on Cisco routers
and switches.
What Is Multicasting?
A unicast is data that is sent from one host to another, while a
broadcast is data sent from a host that is destined for "all" host
addresses. By "all", we can mean all hosts on a subnet, or truly all
hosts on a network.
There's a quite a bit of a middle ground there! A multicast is that
middle ground, as a multicast is data that is sent to a logical group
of hosts, called a multicast group. Hosts that are not part of the
multicast group will not receive the data.
Some other basic multicasting facts:
There's no limit on how many multicast groups a single host can belong to.
The sender is usually unaware of what host devices belong to the multicast group.
Multicast traffic is unidirectional. If the members of the multicast
group need to respond, that reply will generally be a unicast.
The range of IP addresses reserved for multicasting is the Class D range, 224.0.0.0 - 239.255.255.255.
That range contains a couple of other reserved address ranges.
224.0.0.0 - 224.0.0.255 is reserved for network protocols only on a
local network segment. Packets in this range will not be forwarded by
routers, so these packets cannot leave the segment.
Just as Class A, Class B, and Class C networks have private address
ranges, so does Class D. The Class D private address range is 239.0.0.0
- 239.255.255.255. Like the other private ranges, these addresses can't
be routed, so they can be reused from one network to another.
The remaining addresses fall between 224.0.1.0 and 238.255.255.255.
That's the "normal" range of multicast addresses. These addresses can
be routed, so they must be unique and should not be duplicated from one
network to the next.
In my next BCMSN / CCNP multicasting tutorial, we'll take a look at the
different ways in which Cisco routers and switches interact to forward
multicast traffic.