Wildcard mask ranges

Stotic

I understand how to get the wildcard mask when asked to find it for a particular address. Subtract it from 255.255.255.255. What confuses me is when they give a list of addresses and say to provide the wildcard mask for these addresses. Can anyone give me the simple explaination that I don't see in books?

NeonNoodle

Sorry for the long lists, but hopefully this will help you. (As you are following along, think about why we can't use the subnet mask 0.0.0.15.)

Here's a group of addresses:
192.168.1.4
192.168.1.5
192.168.1.6
192.168.1.7
192.168.1.8
192.168.1.9
192.168.1.10
192.168.1.11
192.168.1.12
192.168.1.13
192.168.1.14
192.168.1.15
192.168.1.16
192.168.1.17
192.168.1.18
192.168.1.19

Convert the group of addresses to binary.
192.168.1.4 = 11000000.10101000.00000001.00000100
192.168.1.5 = 11000000.10101000.00000001.00000101
192.168.1.6 = 11000000.10101000.00000001.00000110
192.168.1.7 = 11000000.10101000.00000001.00000111
192.168.1.8 = 11000000.10101000.00000001.00001000
192.168.1.9 = 11000000.10101000.00000001.00001001
192.168.1.10 = 11000000.10101000.00000001.00001010
192.168.1.11 = 11000000.10101000.00000001.00001011
192.168.1.12 = 11000000.10101000.00000001.00001100
192.168.1.13 = 11000000.10101000.00000001.00001101
192.168.1.14 = 11000000.10101000.00000001.00001110
192.168.1.15 = 11000000.10101000.00000001.00001111
192.168.1.16 = 11000000.10101000.00000001.00010000
192.168.1.17 = 11000000.10101000.00000001.00010001
192.168.1.18 = 11000000.10101000.00000001.00010010
192.168.1.19 = 11000000.10101000.00000001.00010011

Where a bit varies in the addresses, the corresponding bit in the wildcard mask is assigned a 1. Where the bit doesn't vary in the addresses, the corresponding bit in the wildcard mask is assigned a 0.

We'll have to break up the above group into smaller groups because the number of bits that vary varies (if you want to make sure your wildcard mask only include addresses in the original group, that is)!

Find the number bits in the group that vary. Here there are two:
192.168.1.4 = 11000000.10101000.00000001.00000100
192.168.1.5 = 11000000.10101000.00000001.00000101
192.168.1.6 = 11000000.10101000.00000001.00000110
192.168.1.7 = 11000000.10101000.00000001.00000111
Therefore, the wildcard mask is
0.0.0.3 = 00000000.00000000.00000000.00000011

For this group three bits vary:
192.168.1.8 = 11000000.10101000.00000001.00001000
192.168.1.9 = 11000000.10101000.00000001.00001001
192.168.1.10 = 11000000.10101000.00000001.00001010
192.168.1.11 = 11000000.10101000.00000001.00001011
192.168.1.12 = 11000000.10101000.00000001.00001100
192.168.1.13 = 11000000.10101000.00000001.00001101
192.168.1.14 = 11000000.10101000.00000001.00001110
192.168.1.15 = 11000000.10101000.00000001.00001111
Therefore the wildcard mask is
0.0.0.7 = 00000000.00000000.00000000.00000111

For this group two bits vary:
192.168.1.16 = 11000000.10101000.00000001.00010000
192.168.1.17 = 11000000.10101000.00000001.00010001
192.168.1.18 = 11000000.10101000.00000001.00010010
192.168.1.19 = 11000000.10101000.00000001.00010011
Therefore, the wildcard mask is
0.0.0.3 = 00000000.00000000.00000000.00000011

That's the gist of it. Of course for the group above, we could've just used a wildcard mask of 0.0.0.31, but that's sloppy especially when working with access lists where you really need to be specific. So, try to group the addresses into the largest groups you can that only includes addresses in the list.

Another thing you need to think about when assigning IP addresses is minimizing the amount of groups you have. Had the above group been from 192.168.1.0 to 192.168.1.15, we would have had the same number of addresses, but we would have only needed one wildcard address, 0.0.0.15.

Once you understand how to group them in binary, you can do the computation in dotted decimal by using powers of two. It's a bit.