Skip to main content

Compute an access-list to match even or odd networks

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

Comments

Post a Comment

Popular posts from this blog

LSA types. Type Code

LSA types. Type Code Description 1 Router LSA 2 Network LSA 3 Network Summary LSA 4 ASBR Summary LSA 5 AS External LSA 6 Group Membership LSA 7 NSSA External LSA 8 External Attributes LSA 9 Opaque LSA (link-local scope) 10 Opaque LSA (area-local scope) 11 Opaque LSA (AS scope)
Post a Comment
Read more

OSPF STUB , NSSA AREAs

IP Routing How Does OSPF Generate Default Routes? Document ID: 13692 Contents Introduction Before You Begin Conventions Prerequisites Components Used Types of OSPF Areas Normal Areas Stub and Totally Stub Areas NSSAs Related Information Introduction The way that Open Shortest Path First (OSPF) generates and advertises default routes (0.0.0.0) varies depending on the type of area the default route is being injected into. In this document, we cover normal areas, stub/totally stub areas and not-so-stubby areas (NSSAs). Before You Begin Conventions For more information on document conventions, refer to the Cisco Technical Tips Conventions . Prerequisites There are no specific prerequisites for this document. Components Used This document is not restricted to specific software and hardware versions. Types of OSPF Areas Normal Areas: These areas can either be standard areas or transit (backbone) areas. Standard areas are defined as ...
Post a Comment
Read more

IP prefix-list

IP prefix-list ip prefix-list provides the most powerful prefix based filtering mechanism Here is a quick little tutorial on Prefix-lists for you. A normal access-list CANNOT check the subnet mask of a network. It can only check bits to make sure they match, nothing more. A prefix-list has an advantage over an access-list in that it CAN check BOTH bits and subnet mask - both would have to match for the network to be either permitted or denied. For checking bits a prefix list ALWAYS goes from left to right and CANNOT skip any bits. A basic example would be this: 172.16.8.0/24 If there is only a / after the network (no le or ge) then the number after the / is BOTH bits checked and subnet mask. So in this case it will check the 24 bits from left to right (won't care about the last 8 bits) AND it will make sure that it has a 24 bit mask. BOTH the 24 bits checked and the 24 bit subnet mask must match for the network to be permitted or denied. No we can do a range of subn...
Post a Comment
Read more