Free Wildcard Mask Primer

http://www.howtonetwork.net

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:

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-

---

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:

127

Here are some videos on wildcard masks:

and an article on wildcard masks and access lists:

Thanks

Paul

[you have my permission to copy and post this article

so long as you do not make any changes whatsoever]