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RE: Multicast ACL/Prefix [bcc][faked-from] [mx] posted 03/31/2004
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Ahmed,

If you take a look at 224 and 239 in binary we have:

224 - 1 1 1 0 0 0 0 0
239 - 1 1 1 0 1 1 1 1

For a class D (Multicast) address the first 4 bits are always going to be
the same.  They are always going to be 1110 - these are the bits that we
care about.  The first address that has the first four bits as 1110 is 224.
Looking at 224 again in binary we will apply our wildcard mask to it:

224 - 1 1 1 0 0 0 0 0
wcb - 0 0 0 0 1 1 1 1

The 0's in the wildcard are 'I care bits' and the 1's are 'I don't care
bits'.  We add up the 'I don't care bits' on the wild card and get 15.

So we have 224 with a wildcard of 15 for the first octet - which if we add
224 + 15 gets us the upper range of the Multicast - 239.  We then don't care
about any of the other octets; they can be anything, so the next three
octets will be 0 with a wildcard of 255.

224.0.0.0 15.255.255.255

Even though 224 is really just the first three bits being on we need to
check that 4th bit and make sure it's off.  If that 4th bit happened to be
on that would get us 240, even if the other four bits were off (8,4,2,1),
which is above our upper range of the multicast - 239.  So we need to check
the first four bits and they must be 1110.

The bit checking works for the other classes of addresses as well:

Class A

0   - 0 0 0 0 0 0 0 0
127 - 0 1 1 1 1 1 1 1

For a Class A address the first bit must be 0.

Class B

128 - 1 0 0 0 0 0 0 0
191 - 1 0 0 0 0 0 0 0

For a Class B address the first two bits must be 10.

Class C

192 - 1 1 0 0 0 0 0 0
223 - 1 1 0 1 1 1 1 1

For a Class C address the first three bits must be 110.

Hope that helps some,

Bill Lijewski
CCIE #8642
Network Learning Inc
5 Day R&S CCIE Bootcamp Instructor
bill@xxxxxxxxx

-----Original Message-----
From: Ahmed Mustafa [mailto:ahmed.mustafa@xxxxxxxxxxxxx] 
Sent: Tuesday, March 30, 2004 8:20 PM
To: Bill Lijewski
Subject: Re: Multicast ACL/Prefix [bcc][faked-from] [mx]

Bill,

Why the netmask or subnet mask will be 240.0.0.0 or only 4 bits

Multicast range starts with

11101111 and that makes 224.0.0.0 239.255.255.255.

I just want to understand the concept.

Thanks
----- Original Message -----
From: "Bill Lijewski" <bill@xxxxxxxxx>
To: "'JBell'" <Jennifer_bellucci@xxxxxxxxxxx>; "'GStudy CCIE'"
<ccielab@xxxxxxxxxxxxxx>
Sent: Tuesday, March 30, 2004 12:13 PM
Subject: RE: Multicast ACL/Prefix [bcc][faked-from]


> For an access-list it would be:
>
> 224.0.0.0 15.255.255.255
>
> For a prefix list that would work:
>
> 224.0.0.0/4 le 32
>
> - Bill Lijewski
> CCIE#8642
> Network Learning Inc
> 5 Day R&S CCIE Bootcamp Instructor
> bill@xxxxxxxxx
>
>
> -----Original Message-----
> From: nobody@xxxxxxxxxxxxxx [mailto:nobody@xxxxxxxxxxxxxx] On Behalf Of
> JBell
> Sent: Tuesday, March 30, 2004 11:56 AM
> To: GStudy CCIE
> Subject: Multicast ACL/Prefix [bcc][faked-from]
> Importance: Low
>
> Hi
>
> Can someone tell me how I can filter the whole multicast address range
> using
> minimum lines? In both access-list and prefix list.
> Multicast address range is 224.0.0.0 to 239.255.255.255
>
> Thanks alot
>
> Ms Jennifer Bellucci
>
> Jennifer_bellucci@xxxxxxxxxxx
>
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