One of my old student who is preparing for CCNP asked me on how to write an access-list for permitting/denying even or odd networks. So I am just pasting my email reply to him
Here is a simple tip to write an access-list for even or odd networks.
Lets say we are asked to permit all odd or permit all even for 192.168.1.0/24 ?
We’ll play the game with last octet or I should say the least significant bit of last octet.
-If it is 0, the IP address will be Even
-If it is 1, the IP address will be Odd
192.168.1.00000001 = 192.168.1.1 – odd
192.168.1.00000011 = 192.168.1.3 – odd
192.168.1.00000010 = 192.168.1.2 even
192.168.1.00000100 = 192.168.1.4 even
FOR Even Networks
The IP address will be 192.168.1.0
With the wild card mask as 0.0.0.254
254 = 11111110
Here, 0 means DO CARE of the last bit in IP address (must be ZERO)
Hence ACL will be
access-list 1 permit 192.168.1.0 0.0.0.254
For Odd Networks
The IP address will be 192.168.1.1
With the wild card mask as 0.0.0.254
254 = 11111110
Here, 0 means DO CARE of the last bit in IP address (must be ONE)
Hence ACL will be
access-list 1 permit 192.168.1.1 0.0.0.254
