Many engineers (at one time myself too) believe they understand wild card masks. If you ask someone they will most likely tell you that a wild card mask is just the opposite or reverse of a subnet mask. Unfortunately this isn't quite correct. Yes, at first glance a wild card mask "looks" like the reverse and in some ways it really is. But a wild card mask can do so much more. I hope to explain in this article what a wild card is and how we can use it more effectively.
As the name states, a wild card mask matches (wildly with a zero) everything in the network portion of an ip address. As we dive into how this works it will becomes a bit more clear exactly how a wild card mask works and how we can use it.
THE RULES
So there are two basic rules of a wild card mask.
0-bit = match
1-bit = ignore
THE TARGETS
What can wild card masks target?
A single host (or a single IP address)
An entire network (i.e, Class A, B, or C)
An entire subnet
A range of IP addresses
Got it so far? ...Good. But, what does this mean? How does it work?
Keep reading...
Targeting a Single Host
To target a single host with a wild card means that every bit within the IP address of the host must match. The bit that means we are matching is a zero (0-bit). So a wild card mask for a host would be -- 0.0.0.0
Targeting an Entire Network
To target an entire network means that every bit within the NETWORK portion of the IP address must match. All others we can ignore. So for a Class-C network (i.e., 192.168.1.0) the wild card mask would be -- 0.0.0.255
Targeting a Subnet
To target a particular subnet (not classful) we will still need to match on every bit on the NETWORK portion, however now we need to find the proper bit boundary in the last octet used by our network. So for example we need to find the wild card mask for the network 192.168.1.128/25. The easiest way to do this is to simply subtract the subnet mask (255.255.255.128) from 255.255.255.255. 255.255.255.255 minus 255.255.255.128 equals a wild card mask of -- 0.0.0.127
Targeting a Range of IP Addresses
To target a range of IP address is where we begin to see how the wild card mask is different from just a reverse of a subnet mask. The same rules apply 0-match, 1-ignore. If we have a range of 192.168.0.0 through 192.168.1.255 we can match at the 23rd bit. This would allow only the two networks to be targeted. The wild card mask would be 0.0.1.255
Writing this out in binary helps. We know we want to match on the entire 1st and 2nd octets. That part is easy - both are zero's. Its the third octet that is the trick. Lets break that third octect out in binary
192 168.0. ---- binary 00000000
192.168.1. ---- binary 00000001
If you look here the bit we want to match on is the 23rd bit. The subnet mask for a slash 23 is 255.255.254.0. We then subtract it from 255.255.255.255 to get our wild card which is 0.0.1.255
Pretty cool huh?
Target all even Networks or all odd Networks
With what we've learned so far, you might be able to see the possibilities. So now lets take this one last step further. Lets target just all even networks. (definitely a CCIE lab scenario)
Let say we need to route all even networks in the 192.168.0.0 block of space. Where can we match within the third octet that will allow us to do this. Once again we need to break it out in binary to see it more clearly.
192 168.0. ---- binary 00000000
192.168.1. ---- binary 00000001
192.168.2. ---- binary 00000010
192.168.3. ---- binary 00000011
192.168.4. ---- binary 00000100
192.168.5. ---- binary 00000101
192.168.6. ---- binary 00000110
192.168.7. ---- binary 00000111
If look you can see the last bit of the third octet (bit 24) is a zero for every even network. The value for this bit is 1. We can match on this last bit to target the even network by taking the inverse of 1 to get 254. (255-1).
The wild card mask for this would be 0.0.254.255.