From: marvin greenlee (marvin@ccbootcamp.com)
Date: Thu May 26 2005 - 21:02:09 GMT-3
In class A address space, the first bit is 0.
deny 0.0.0.0/1 le 32 will match any routes with the first bit of 0 - (first
octet 0-127).
For Class B address space, the first two bits are 10 (128 to 191).
deny 128.0.0.0/2 le 32 would match any routes with the first two bits as
zero.
deny 128.0.0.0/2 ge 16 le 16 would match routes with a 16 bit mask in this
range.
For class C address space, the first three bits are 110 (192 to 223).
deny 192.0.0.0/3 le 32 would match any routes with the first two bits as
zero.
deny 192.0.0.0/3 ge 24 le 24 would match routes with a 24 bit mask in this
range.
You can't configure a prefix list with the GE value set to the same as the
number of bits that you are matching.
Marvin Greenlee, CCIE#12237, CCSI# 30483
Network Learning Inc
marvin@ccbootcamp.com
www.ccbootcamp.com (Cisco Training)
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
Ralph Sherry
Sent: Thursday, May 26, 2005 4:50 PM
To: ccielab@groupstudy.com
Subject: prefix lists [bcc][faked-from][bayes]
Importance: Low
I have been doing some playing with Prefix lists and find them a bit
confusing. One of the things that I have been playing with in the lab is
trying to block different routes from different ranges of IP address. Here
is one of the examples that I have tried where the goal is to block all
class C address space.
ip prefix-list cisco seq 10 deny 224.0.0.0/3 ge 3
ip prefix-list cisco seq 20 permit 192.0.0.0/2 ge 2
ip prefix-list cisco seq 30 deny 128.0.0.0/1 le 32
The way I read this code would be the first line blocks all IP address space
from 240.0.0.0 and above. The second line would would permit the actual
Class C address space. the last line would take care of the A address
space. Does this scenario make sense?
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