unicast multicast?

[dkblee]

hi!

What's the different between unicast and multicast?

any simple explanation? Thks!

[Kevin Dorrell]

Very briefly, a unicast is sent to a single machine, a multicast is sent to a group of machines, and a broadcast is sent to all machines.

What distinguishes a unicast address? The first bit transmitted on the wire is a zero. In Ethernet, that's the least significant bit of the first octet, e.g. 00:....

So if the first bit transmitted is a 1, then it goes to several machines, e.g. 01:.....

If the second bit is a 1 as well, then it is a broadcast, e.g 03:....., or more commonly FF:FF:FF:FF:FF:FF.

So if it is a multicast, but not a broadcast, how do you know which machines it is addressing? The answer is that the frame is addressing any machine that wants to listen to that (multicast) address.

In the case of IP multicast, there are rules about the correspondence between IP address and multicast address. Here is a document for you to read:

http://www.cisco.com/en/US/tech/tk828/tech_brief09186a00800a4415.html

Happy reading!

Kevin Dorrell

Luxembourg

[Richard Burts]

The beginning of the explanation is on track - the first bit on the wire does distinguish unicast (value of zero) from broadcast/multicast (value of one). Unfortunately it then goes off the track. The second bit on the wire is not a boradcast bit but is the locally administered/globally administered bit. If the NIC is using the manufacturer determined MAC address (globally administered) the second bit should have a value of zero. If the NIC is using a locally administered MAC address (for example if you configure mac-address on the Ethernet interface) the convention is that the second bit should be set to one.

HTH

Rick

[Kevin Dorrell]

OK. So if I understand you right, that means that an address starting 08:00:20 is a manufacturer's MAC unicast address, and if I wanted to set one myself then a suitable choice might start 0a:00:20.

Does that rule extend to multicast addresses: 01:00:5e .. would be a globally assigned multicast address, but 03:00:5e would be one I picked myself?

I notice most globally assigned multicast addresses start 01, so I guess there is list of these somewhere in ICANN, e.g. 01:00:0c is Cisco, 01:00:5e is IP, etc.

So the only special case is ff:ff:ff:ff:ff for broadcast.

Thanks for the info Rick. I'm learning.

Kevin Dorrell

Luxembourg