topic ACL- Inverse Mask in Network Security

ACL- Inverse Mask

sadik.bash — Tue, 12 Mar 2019 03:08:46 GMT

Hello,

I have a server with IP address 172.22.94.224/22 and an ACL statement in one of the ASAs as follows "access-list 145 permit ip 172.22.94.224 0.0.0.31 any"

I got confused by the inverse mask address(0.0.0.31) and I would like some clarification.

Much appreciated.

Best, ~sK

Re: ACL- Inverse Mask

Karsten Iwen — Sat, 23 Nov 2013 00:38:37 GMT

if that is really an ACL from an ASA, then it's probably wrong as the ASA doesn't use the inversed wildcardmask. The router-wildcard-mask of 0.0.0.31 would be 255.255.255.224 on the ASA.

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ACL- Inverse Mask

sadik.bash — Sat, 23 Nov 2013 00:49:06 GMT

Sorry, the inv mask is on the 4507 not the ASA. My question is what does this statement "

access-list 145 permit ip 172.22.94.224 0.0.0.31" mean?

Best, ~sK

Re: ACL- Inverse Mask

Karsten Iwen — Sat, 23 Nov 2013 01:06:18 GMT

0.0.0.31 is a mask that has 27 bits set to 0 (match) and five bits set to one (don't match). With that mask in the ACL you allow 32 addresses, in your case from 172.22.94.224 to 172.22.94.255.

Wildcardmasks are very good explained in the Wikipedia-article: http://en.wikipedia.org/wiki/Wildcard_mask

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