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Friday, May 20, 2011

Quick Notes - Prefix-List Matching Class A,B,C

Prefix lists can be used to match a complete address class

Class A = 1.0.0.0 – 126.255.255.255
Class B = 128.0.0.0 – 191.255.255.255
Class C = 192.0.0.0 – 223.255.255.255


The classes can be defined by the starting bits in the first octet


Class A = 0
Class B = 10
Class C = 110



In order to match Class A, B, or C you need to perform the following:



Class A
1.0.0.0/1 le 32 (must match the first bit and the subnet can be less then or equal to 32)
Class B

128.0.0.0/2 le 32 (must match the first two bits and the subnet can be less then or equal to 32)
Class C

192.0.0.0/3 le 32 (must match the first three bits and the subnet can be less then or equal to 32)


Class A = ip prefix-list CLASSA permit 1.0.0.0/1 le 32
Class B = ip prefix-list CLASSB permit 128.0.0.0/2 le 32
Class C = ip prefix-list CLASSAC permit 192.0.0.0/3 le 32


Note: 127.0.0.0 - 127.255.255.255 is reserved for loopbacks.

3 comments:

Anonymous said...

nice stuff just help me clear my doubt

Anonymous said...

CLASS A: 0.0.0.0/1 le 32 is better.

Packets Analyzed said...

Provide more insight and an example of when you have seen a Class A address used between 0.0.0.0 - 0.255.255.255 - I am not suggesting that you could not use "0.0.0.0/1 le 32" but this will include a default route.

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