ACLs, deny and allow IP address even and odd? 1

[slate13]

How can I deny all even number host addresses and allow all odd IP addresses via a serial connection. Does any one have a short ACL script for this solution.

 

[ChrisAC]

Ermm ... why?

Just curious.

Chris.

**********************
Chris A.C, CCNA, CCSA
**********************

 

[dopehead]

I think if you just change the least significant bit in your mask you would be able to match all Even or Odd host adresses, not very nice to do, and can become quite difficult to manage.



Jan


Network Systems Engineer
CCNA/CQS/CCSP/Infosec

 

[ChrisAC]

Jan,

Give us an example then. Lets say allow all uneven IP's in 172.16.1.0 /24 (.1, .3, .5, .7 etc..) and block all the even ones. How would you achieve that by changing the mask?

I can't think of a way how without putting an acl entry in for each host. No mask defines even or odd!

I'd still like to know the reason why slate13 actually wants to do this?

Chris.

**********************
Chris A.C, CCNA, CCSA
**********************

 

[Triplejolt]

Perhaps his friend is using an even IP address, which doesn't have access to Tek-Tips forum
Hope he's not on a B or A network...

A firm beleiver of "Keep it Simple" philosophy
Cheers
/T

 

[ChrisAC]

**********************
Chris A.C, CCNA, CCSA
**********************

 

[dopehead]

Well, example would then be : 172.16.1.0 /24 change the mask to an inverted acl mask 0.0.0.254 try it, see if i'm wrong. This would make the last bit always have to be '1' which would only give you access from odd host numbers since the last bit has to be a '1'

Jan


Network Systems Engineer
CCNA/CQS/CCSP/Infosec

 

[ChrisAC]

Well, I think that you are correct. I never thought of that.

Have a star!

Chris. *writes it down for future reference*



**********************
Chris A.C, CCNA, CCSA
**********************

 

[ChrisAC]

Well I'll be ... it works!

Did it on a router and had the following acl

access-list 121 permit ip 10.19.72.0 0.0.0.254 any log
access-list 121 deny ip any any log

Sure enough, even numbers were allowed through and odd were blocked.

Apr 19 23:21:22: %SEC-6-IPACCESSLOGP: list 121 permitted tcp 10.19.72.2(0) -> 10.19.72.1(0), 1 packet
Apr 19 23:21:29: %SEC-6-IPACCESSLOGDP: list 121 permitted icmp 10.19.72.2 -> 198.133.219.25 (0/0), 1 packet
Apr 19 23:22:28: %SEC-6-IPACCESSLOGP: list 121 denied udp 10.19.72.3(0) -> 10.19.72.255(0), 1 packet
Apr 19 23:22:30: %SEC-6-IPACCESSLOGP: list 121 denied tcp 10.19.72.3(0) -> 10.19.72.1(0), 1 packet
Apr 19 23:22:35: %SEC-6-IPACCESSLOGP: list 121 denied udp 10.19.72.3(0) -> 192.168.3.113(0), 1 packet
Apr 19 23:23:17: %SEC-6-IPACCESSLOGP: list 121 permitted udp 10.19.72.4(0) -> 10.19.72.255(0), 1 packet
Apr 19 23:23:20: %SEC-6-IPACCESSLOGP: list 121 permitted udp 10.19.72.4(0) -> 192.168.3.113(0), 1 packet
Apr 19 23:23:21: %SEC-6-IPACCESSLOGDP: list 121 permitted icmp 10.19.72.4 -> 198.133.219.25 (0/0), 1 packet



Chris.

**********************
Chris A.C, CCNA, CCSA
**********************