Wildcard ACL masks

I need to configure an acl denying access to subnet 172.16.1.0/18 and I need to know the corresponding wildcard mask?

I've read the tech help at http://www.techexams.net/technotes/ccna/accesslists.shtml regarding this. But I'm not following it totally.

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Comments

Sartan

0.0.63.255

I'm a horrible teacher if I wanted to teach wildcard masks

But here ya go.

Some of the newer IOS's support just the regular ol'e subnet mask like we're used to, rather than their weirdo bits.

Webmaster

Quick and dirty 1: /18 is 11111111.11111111.11000000.000000 = 255.255.(128+64).0 = 255.255.192.0 thus to block the network 172.16.1.0 with subnetmask 255.255.192.0 you need to use the remaining bits in the inverse mask:

255.255.255.255
255.255.192.0

-
0.0.63.255

Quick and dirty 2:
32-18 = 14 reading from right to left:

00000000.00000000.00111111.11111111 =
0.0.(32+16+8+4+2+1).(128+64+32+16+8+4+2+1) =
0.0.63.255

If you don't understand subnetting and/or wildcard mask, then don't try to understand them. Write it out, make up networks and subnet them, make 'm access lists from scratch. You will only need to do it a couple of times and it will make a lot more sense.