Wildcard Mask Primer
I am going to presume you have read up on IP addressing
and binary math. But for the exam you just want to be able
to quickly and easily work out a wildcard mask and move
onto the next question. Right?
Step 1 – Write out the subnet mask in full. If you have
been given the CIDR value such as 192.168.1.2/27 then use
the easy way to subnet method to write it out in full.
Here is the easy way to subnet:
http://www.howtonetwork.net/public/department72.cfm
If the subnet mask is already written out in full then
that is a bonus. e.g. 192.168.1.2 255.255.255.224
Step 2 – Simply take the number of the subnet mask away
from 255. So in the above example you take 255 away from
255 and get nothing for the first three octets. For the
last octet take 224 away from 255 giving you 31.
255
224-
—
31
Step 3 – There is no step 3! You have the answer already
so stop trying to make it more complicated.
I will write it out again and underneath the subnet mask
is what is left if you take it away from 255.
255.255.255.224
0. 0. 0. 31
Another example. What is the wildcard mask to match subnet
mask 255.255.192.0?
Same again. Just take that number away from 255?
255.255.192.0
0. 0. 63. 255
Write out a few examples for yourself. Just take that number
away from 255 and that is the answer. It will always be an
odd number by the way and always be either:
0
1
3
7
15
31
63
127
Here are some videos on wildcard masks:
http://www.howtonetwork.net/public/department71.cfm
and an article on wildcard masks and access lists:
http://www.howtonetwork.net/public/department49.cfm
Thanks
Paul
www.howtonetwork.net
[you have my permission to copy and post this article
so long as you do not make any changes whatsoever]
Copyright Paul Browning / www.howtonetwork.net 2008
