Basic definitions

exkor5000

hello

can anybody please define the terms Multicast and Unicast?

i tought i got it but now i see that i dont

Thanks
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Danman32

Multicast you send a packet to an IP address that is associated with a group of unicast addresses. Think of it as a distribution group for IP.

Unicast is single host to single host communications. Think of this as sendng an email to your best friend by his email address, and it only goes to him.

Then there's broadcast which is received by all hosts on the subnet.

Unicast - one.\
Multicast - some
Broadcast - all.

Humper

A real life analogy is:

unicast - when you are speaking to ONE other person, say for example on the phone. (ONE TO ONE)

multicast - When you are in a room with many people but you are only speaking to a few of the many people. (ONE TO MANY)

broadcast - If you are in a meeting doing a presentation, you are speaking to everyone in that room and they are all listening.

this may be kinda dumb :P

Danman32

A while back in NYC, some of the suburbs didn't have cable for a LONG time. HBO offered a package where you were given a small microwave antenna/down-converter that you could hook up to your TV via an A-B switch. The microwave signal coming out of Manhattan was not scrambled. All one needed was a microwave antenna/down-converter that was tuned to the right frequency. Many bootleggers sold such a receiver.

Now HBO was not happy about this, and sued. The question became, was the transmission a broadcast, in which anyone would legally have permission to recieve it, or a multicast, in which you would need permission. Being that this was a microwave signal, there was a fine line between the two, as there were laws protecting point-point microwave links to prevent eavesdropping on telephone conversations often relayed by microwave.

exkor5000

great info! much appretiated!

now that I got the LALA part I need to see the DODO part

so could anybody drop me afew real examples of a multicast?
(in term of ips)

thanks
X

exkor5000

answer: (wikipedia seems to have alot of material on those things)

IP Multicast packets always use UDP, not TCP. Because of the connection-oriented nature of TCP, it cannot be used in a one-to-many situation such as Multicasting (or Broadcasting)

The Multicast IP address range is 224.0.0.0 ... 239.255.255.255, or, in CIDR notation, 224.0.0.0/4.

not sure about the 2nd statement because what would prevent me from taking 192.168.1.1 255.194.255.255

I pretty much get the point, should play around with that, looks interesting.

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EdTheLad

exkor5000 wrote:

The Multicast IP address range is 224.0.0.0 ... 239.255.255.255, or, in CIDR notation, 224.0.0.0/4.

not sure about the 2nd statement because what would prevent me from taking 192.168.1.1 255.194.255.255

I pretty much get the point, should play around with that, looks interesting.

X

Multicast uses a specific range of addresses,network devices are configured to understand multicast ip addresses.Multicast packets are essentially flooded
throughout the network where specific hosts are running a protocol IGMP to join the multicast group.The flooding of multicast is controlled using multicast routing protocols and each multicast IP address gets translated to a specific MAC address.The 192.x.x.x address you stated above is a broadcast address that will be processed by all recieving hosts,the difference is only multicast hosts belonging to the multicast group will have the correct MAC address to receive the multicast frames and send to the upper layers,other hosts not belonging to the multicast group will examine the destination MAC in the packet and discard.With a broadcast each frame will have a FF.FF.FF.FF i.e. broadcast MAC address which means very host will receive it process it and throw to the network layer which will check the IP address and possibly discard.So with a broadcast every host must use alot of processes on packets which will only end up getting discarded.Multicast is more efficient.

Danman32

A few real world examples:

The best would be live audio/video streaming, though right now unicast is mostly used at least part way since many routers near the end host are not configured to support IGMP.
Another is Netware logon server discovery over Service Location Protocol (SLP).