Doubt about an address. Pls Help

In the answer to this question if the mask to be denied is 255.255.0.0, as shown in the given info: 172.16.0.0/16, In the answer why do they use the inverse mask 0.0.255.255?

18. Which of the following configures interface ethernet 0 to deny inbound packets originated from subnet 172.16.0.0/16?
access-list 1 deny 172.16.0.0 0.0.255.255
access-list 1 permit any
interface ethernet 0
ip access-group 1 in

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Comments

keenon

the 0.0 requires it to check those bits when using the 255 it excludes the bits from being checked.

so if u want to ID 172.16.x.x 0(ID this bit). 0(ID this bit). 255 (no ID). 255 (no ID)

applepie

keenon wrote:

the 0.0 requires it to check those bits when using the 255 it excludes the bits from being checked.

so if u want to ID 172.16.x.x 0(ID this bit). 0(ID this bit). 255 (no ID). 255 (no ID)