To better understand why this works:
In the very first octet, the following applies:
class A addresses start with the first bit = 0
class B addresses start with the first two bits = 10
class C addresses start with the first three bits = 110
So the 0.0.0.0/1 means look for a network address of 0.0.0.0, but only pay
attention to the very first bit (and make sure that it is a zero). So
0.0.0.0/1 identifies all class A networks - from 0.0.0.0 to 127.255.255.255.
The ge 8 le 8 says only accept routes with a mask of 255.0.0.0. The
combination of these two identifies all classful class A networks (0.0.0.0/8
to 127.0.0.0/8).
Same with the 128.0.0.0/2 - that means make sure the first two bits are 10,
but then ignore everything else. So this includes all class B addresses -
from 128.0.0.0 to 191.255.255.255.
Rob.
""Rob Webber"" wrote in message
[EMAIL PROTECTED]">news:[EMAIL PROTECTED]...
> I believe this will do what you are looking for. I did a little testing
and
> it seemed to work well:
>
> ip prefix-list classful seq 5 permit 0.0.0.0/1 ge 8 le 8
> ip prefix-list classful seq 10 permit 128.0.0.0/2 ge 16 le 16
> ip prefix-list classful seq 15 permit 192.0.0.0/3 ge 24 le 24
>
> Hope that helps, Rob.
> CCIE 6922
>
> ""William Lijewski"" wrote in message
> [EMAIL PROTECTED]">news:[EMAIL PROTECTED]...
> > Can someone tell me how to create a Prefix-list to only alow classful
> routes
> > for BGP. I know you can do the following with an extended access-list:
> >
> > access-list 100 permit ip 0.0.0.0 127.0.0.0 host 255.0.0.0
> > access-list 100 permit ip 128.0.0.0 63.255.0.0 host 255.255.0.0
> > access-list 100 permit ip 192.0.0.0 31.255.255.0 host 255.255.255.0
> >
> > Is there way to do it? Any good reading material on Prefix-lists?
> >
> > Thanks in advance.
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