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understanding Prefix-List

Posted on 2013-01-05
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Last Modified: 2013-01-15
I am trying to understand he usage of Prefix-List , but it seems like they are abit complex.
for instance:

1***what is this saying:
ip prefix-list test seq 12 permit 192.168.1.0/16 ge 24 le 24
is it permitting 1902.168.0.0 all the way 192.168.1.0 ?

2***what is this saying:
ip prefix-list test seq 12 permit 192.168.1.0/16 le 32
is it permitting anything that starts with 192.168.X.X all the way 192.18.254.254 ?

3*** what about this, doe s it need "le" or "ge"
0.0.0.0/0

4*** what about these:
0.0.0.0/0 le 32
0.0.0.0/32


Thank you
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Question by:jskfan
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by:lruiz52
lruiz52 earned 664 total points
ID: 38747486
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by:jskfan
ID: 38758305
Can you please check my post above and tell me if I am understanding it the right way or wrong way ?
thanks
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by:harbor235
harbor235 earned 668 total points
ID: 38767069
We are dealing with prefixes or subnets only here, do you understand subnets?

1***what is this saying:
ip prefix-list test seq 12 permit 192.168.1.0/16 ge 24 le 24
is it permitting 1902.168.0.0 all the way 192.168.1.0 ?

is this case of prefix lists 1*** means match all prefixes within the 192.168.1.0/24 networks a) greater an than or equal to 24 bits and b) less than or equal to 24 bits
Fancy way of stating match 192.168.1.0/24  Remember 192.168.1.0/24 is a subnet, all possible networks that can be subnetted by this parent block is in play, so 192.168.1.0/25, 192.168.1.128/25, and so on, get it?

2***what is this saying:
ip prefix-list test seq 12 permit 192.168.1.0/16 le 32
is it permitting anything that starts with 192.168.X.X all the way 192.18.254.254 ?

means match all prefixes within the 192.168.1.0/16 networks less than or equal to 32 bits,
so it means all possible subnets than can be subnetted from 192.168.1.0/16.


3*** what about this, doe s it need "le" or "ge"
0.0.0.0/0

with subnets this is sometimes used to mean all subnets when used with le 32
0.0.0.0/0 is a wildcard.

4*** what about these:
0.0.0.0/0 le 32
0.0.0.0/32

0.0.0.0/32 is an exact match meaning literally network 0.0.0.0 255.255.255.255 you see?

the important point here is that prefix lists deal with parent blocks and possible subnets that can be derived from that parent block.

harbor235 ;}
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Expert Comment

by:pergr
ID: 38767290
It is important to note that the prefix-list is use for matching prefixes (routes) and not individual IP addresses.
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Author Comment

by:jskfan
ID: 38773427
sorry I did not get it
ip prefix-list test seq 12 permit 192.168.1.0/16 ge 24 le 24
so /16 is meaningless since it will pass only subnets with 24 bits ?

I see you mention 25 bits .... I did not get get it...
since the condition says greater or equal 24 and less or equal 24...which means to me just 24
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Accepted Solution

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pergr earned 668 total points
ID: 38773433
The prefix that matches must have the exact subnet mask /24, and the prefix needs to be within 192.168.1.0/16 - which is the same as 192.168.0.0/16.

That would mean all 192.168.x.y/24 .
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Author Comment

by:jskfan
ID: 38773515
I initially understood:
it will take : 192.168.1.0 to 192.168.254.0

Until I saw this, I extracted this from the pdf link given to me above:

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.
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Author Comment

by:jskfan
ID: 38773516
very complex, hard to understand it..
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Expert Comment

by:pergr
ID: 38773519
Correct.
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Author Closing Comment

by:jskfan
ID: 38777365
Thank you Guys!
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