From: Rob Webber (rwebber@xxxxxxxxxxxx)
Date: Mon May 07 2001 - 22:00:10 GMT-3
The way prefix lists work are you can specify a network and mask or a
network and a range of masks. Specifying a network and mask is fairly
simple:
ip prefix-list mylist seq 10 permit 172.16.25.0/24
This will allow (match) the exact network 172.16.25.0/24 to pass the list.
However prefix lists can also specify a network with a range of masks. For
example:
ip prefix-list mylist seq 10 permit 172.16.0.0/16 ge 24 le 26
This will take the entire class B network 172.16.0.0 (172.16.0.0/16) and
pass only subnets with a /24, /25 or /26 mask (ge 24 le 26). So the exact
network 172.16.0.0/16 would actually fail the list because it does not have
a mask of /24, /25 or /26.
By default if you only specify "ge" then any subnet with a mask greater than
or equal to the ge value will pass. That is, ge all the way up to /32. For
example:
ip prefix-list mylist seq 10 permit 10.10.10.0/24 ge 28
This list specifies any subnet within the 10.10.10.0/24 range that has a
mask of /28 or greater (255.255.255.240 to 255.255.255.255). Again, the
exact subnet 10.10.10.0/24 would fail because it does not have a mask of /28
or greater.
By default if you only specify "le" then any subnet with a mask less than or
equal to the le value but greater than or equal to the mask specified will
pass. That is, le all the way down to the mask listed. For example:
ip prefix-list mylist seq 10 permit 10.64.0.0/16 le 23
This list specifies any subnet within the 10.64.0.0/16 range that has a
mask between /16 and /23, inclusive (255.255.0.0 to 255.255.254.0). In this
case the exact subnet 10.64.0.0/16 would pass because it has a mask in the
range /16 to /23.
The "permit any any" in a prefix list is:
ip prefix-list mylist seq 200 permit 0.0.0.0/0 le 32
I recommend getting quite familiar with them because they are very powerful
and actually not too bad to use once you get used to them!
Hope this helps - Rob.
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com]On Behalf Of
garry baker
Sent: Monday, May 07, 2001 6:26 PM
To: ccielab@groupstudy.com
Subject: prefix lists
Guys,
I am having problems finding some info on and
understanding prefix lists. Can anyone give a plain
english explanation of prefix lists.
Garry
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