CCNA Basics: Introduction to Wildcard Mask

Author : Amandeep Kaur

Today we are going to talk about the Wildcard Mask which generally we are using while configuring OSPF. Wild Card Mask is an important concept and everybody should aware of this concept

Sometimes people ask if there is subnet Mask when why we required Wild Card Mask, even i was not aware why it is necessary to use Wild Card mask but let's talk about wild card mask in detail to understand.

What is a wild card mask?  
Wild card masks are used for a variety of different tasks. OSPF area definitions and some access lists use them to define a certain part of the network.  They work much like subnet masks but in reverse.  Let's talk about one of the example in the Subnet C class as below:
  • IP address with Subnet Mask :
  • Network –
  • Subnet mask –
  • Wild Card Mask –

     Explain in details below
Fig 1.1- Wildcard Mask

So what you guys think about the wildcard Mask. The wild card mask is essentially the reverse of the subnet mask. 
So if the binary subnet mask looked like
  • Subnet Mask : 11111111.11111111.11111111.00000000
  • Wild card mask would look like this: 00000000.00000000.00000000.11111111
That’s all fairly easy to understand, however when you start using VLSM it can get a little trickier.  For instance, take this class C subnet.
  • /27
  • Network –  
  • Subnet mask –
  • Wild Card Mask –
The binary would look like this: 11111111.11111111.11111111.11100000
The wild card mask would look like this: 00000000.00000000.00000000.00011111

Now that’s a little more confusing isn’t it?  What might even be harder to understand is if you were simply given this.
  • access-list 1 permit
So if you just subtract the wild card mask from you got the subnet mask.
  • –

Then to convert a mask from standard notation to wild card you simply subtract the subnet mask from  

  •  - =
But for someone who has just be introduced to wild card masks this could be a huge help.If someone would have just told me that I could just subtract as shown above it would have saved me one or two nights of frustration.