Learning by Doing: CISCO Certified Network ... - SCN Research

Objective:
To learn how to create wildcard masks for use with Access Control Lists.
Paper Lab: Wildcard Masks
Background:
People confuse wildcard masks with subnet masks all the time. They are similar after all
because they both are masks but they really are different. A wildcard mask helps an
access control list determine which ip addresses to implement the access control list
commands upon. A nice, neat, simple rule: zero’s denote “exact” match bits…think of
that little razor knife: an “exact-o” knife. Let’s dig in to an example:
1. Write a wildcard mask for a host ip address of 172.16.2.34:
Can you see what they are asking? They want an exact match for a host ip
address here. Let’s convert the ip address to binary:
10101100.00010000.00000010.00100010
Since they want an exact match for all bits then the wildcard mask is filled in with
zero’s (ok…so the bits conversion wasn’t needed but give me a break…you will
see why we added this step in next…)
00000000.00000000.00000000.00000000
Therefore, when we convert this wildcard mask back to decimal we get a wildcard
mask of 0.0.0.0 for our exact host match.
2. Write a wildcard mask for a entire subnet containing the ip address of
172.16.2.34/27
Can you see what they are asking? They want an exact match for the subnet
containing the host ip address here. Let’s convert the ip address to binary:
10101100.00010000.00000010.00100010
Then let’s figure out the network, subnet, and host portions:
10101100.00010000.00000010.00100010
network.network.network.subnet host
Since they want an exact match for all network plus subnet bits then the wildcard
mask is filled in with zero’s in the network and subnet portions and one’s in the
host portion:
00000000.00000000.00000000.00011111
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