Networking Reviews https://www.networkingreviews.com Wed, 24 Sep 2008 06:01:43 +0000 en-US hourly 1 https://wordpress.org/?v=6.9.1 Using Cisco Access Control List to block a subnet https://www.networkingreviews.com/using-cisco-access-control-list-to-block-a-subnet/ https://www.networkingreviews.com/using-cisco-access-control-list-to-block-a-subnet/#respond Wed, 24 Sep 2008 06:01:43 +0000 https://www.networkingreviews.com/?p=251 Question:

I have the following ClassB network 172.31.0.0/16 that I’m trying to create an access list for. I’d like to allow 172.31.240.0/24 but deny all else, so I’m looking the best way to accomplish this with 2 acl lines.

Answer:

The best way to accomplish the above is to explicitly allow the subnet you want and then deny the whole Class B network as following:

access-list 110 permit ip 172.31.240.0 0.0.0.255
access-list 110 deny ip 172.31.0.0 0.0.255.255

Another option with just one ACL entry would be to permit only the subnet 172.31.240.0/24 and thats it. No other ACL entries are needed since there is an implicit deny at the end of the access list statement.

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