Some students of Computer Networking especially those appearing in Certification exams such as Cisco Certification Exams or Juniper Certification Exams have problem in calculating wildcard mask. Some know the concept but their method is difficult that might result in wastage of time which may prove to be a problem in certification exams. Also, the wildcard mask is used in extensively in Access Control Lists (ACLs), EIGRP and OSPF, so learning how to calculate wildcard mask is important. We have decided to share a very simple method to calculate the wildcard mask efficiently and hopefully you will not have any ‘how to calculate wildcard mask?’ afterwards.
The method is: first calculate the subnet mask for the network for which you want to find the wildcard mask. Then subtract that subnet mask from 255.255.255.255
An example will explain this method more efficiently. Consider you have a network of 172.16.1.0/24 and you want to calculate the wildcard mask for it. We know that the subnet mask for this network is 255.255.255.0 Now we subtract this subnet mask from 255.255.255.255 as follow:
As you can see, 0.0.0.255 is our required wildcard mask. Using this method we can find any wildcard mask easily.
Take another example, if you have network address of 172.16.0.0/20. We know that the subnet mask for this network address is 255.255.240.0
Using the above mentioned method, wildcard mask is:
Wildcard mask, found nice and easy.
If you any comments or questions, put them in comments section and we would be happy to answer them.