You can probably find a lot of good sites about Multicast if you start
searching for them on the web.
I thought the Multicast IP to MAC address in the BCMSN book by Karen Webb
was a little confusing, but after I read the LAN switching book by Clark and
Hamilton, I understood exactly what to do.
If you picture an IP address as octet1.octet2.octet3.octet4, the way to
convert it to a MAC address is:
1) Take (octet2, octet3 and octet4)
2) AND octet2 with 127 (or subtract 128 if octet2 >= 128)
3) Convert each octet to hexadecimal values
4) MAC = 01.00.5E.new-octet2.new-octet3.new-octet4
Let's take your example 224.0.165.45
1) 0.165.45
2) 0.165.45
3) 00.A5.2D
4) MAC = 01.00.5E.00.A5.2D
As you can see on this method, 224 uses the 3 first bits in octet1 to
specify it self as a Multicast IP address, which leaves 5 bit left for
addresses. These 5 bits plus the 1 bit of octet2 that is cleared gives you 6
bits or 32 different IP addresses that will end up with the same MAC
address.
Let's take IP address 225.128.165.45 and do the same 4 steps:
1) 128.165.45
2) 0.165.45
3) 00.A5.2D
4) MAC = 01.00.5E.00.A5.2D
This means that you have to think about this before you assign your
Multicast IP addresses to networks where you have more than one group,
because you could end up with two different groups pointing to the same
physical address.
Hth,
Ole
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Ole Drews Jensen
Systems Network Manager
CCNA, MCSE, MCP+I
RWR Enterprises, Inc.
[EMAIL PROTECTED]
http://www.CiscoKing.com
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
NEED A JOB ???
http://www.oledrews.com/job
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-----Original Message-----
From: jim klane [mailto:[EMAIL PROTECTED]]
Sent: Tuesday, February 06, 2001 2:04 PM
To: [EMAIL PROTECTED]
Subject: RE: Possible BCMSN Errata??
do you have a link on thie multicast stuff?
let me know
jim
>From: Ole Drews Jensen
>Reply-To: Ole Drews Jensen
>To: "'Paul Mandella'" , [EMAIL PROTECTED]
>Subject: RE: Possible BCMSN Errata??
>Date: Tue, 6 Feb 2001 13:20:31 -0600
>
>I do not have the book in front of me, but yes, the given binary IP address
>is 224.0.165.45 and the MultiCast MAC will be 01-00-5E-00-09-2D.
>
>Ole
>
>~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> Ole Drews Jensen
> Systems Network Manager
> CCNA, MCSE, MCP+I
> RWR Enterprises, Inc.
> [EMAIL PROTECTED]
> http://www.CiscoKing.com
>~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> NEED A JOB ???
> http://www.oledrews.com/job
>~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>
>
>
>-----Original Message-----
>From: Paul Mandella [mailto:[EMAIL PROTECTED]]
>Sent: Tuesday, February 06, 2001 12:48 PM
>To: [EMAIL PROTECTED]
>Subject: Possible BCMSN Errata??
>
>
>Hello All. While working on examples of converting Multicast IP addresses
to
>Multicast MAC addresses in the Cisco book I came across one that doesn't
>seem to be correct. I ask for opinions on the following to possibly save my
>sanity on this one. Thanks
>
>The IP address in the book is 224.0.9.45
>It is shown as 1110 0000 0000 0000 1010 0101 0010 1101
>
>Seems to me that this IP should read 224.0.165.45 and this would work out
to
>the Multicast MAC being 01-00-5e-00-a5-2d and not the 01-00-5e-00-09-2d
>given in the book.
>
>Did I just lose my mind on this one?? I have not found errata on Cisco
Press
>site that corrects this. Thanks for any input
>
>Paul
>
>
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