Hello Sumnima
The wildcard mask is always calculated using binary because that’s what the devices understand, and that’s what the feature uses. However, you can always memorize the valid numbers in decimal. After a while, if you use it enough, you will memorize it quite quickly.
Take a look at the first nine possibilities for wildcard masks along with their binary counterparts::
00000000 00000000 00000000 00000000 0.0.0.0
00000000 00000000 00000000 00000001 0.0.0.1
00000000 00000000 00000000 00000011 0.0.0.3
00000000 00000000 00000000 00000111 0.0.0.7
00000000 00000000 00000000 00001111 0.0.0.15
00000000 00000000 00000000 00011111 0.0.0.31
00000000 00000000 00000000 00111111 0.0.0.63
00000000 00000000 00000000 01111111 0.0.0.127
00000000 00000000 00000000 11111111 0.0.0.255
Do you see a pattern? Take a look at this familiar sequence: 2 4 8 16 32 64 128 256. It’s just numbers doubled each time. Notice that the number that appears in each wildcard mask is simply the values found in this sequence minus 1.
If you can memorize this sequence, and remember to subtract one each time, you can easily determine the dot decimal format of the wildcard mask from the number of bits you want to mask.
How about a more difficult mask like this one?
00000000 00000000 00111111 11111111
You can simply look at each octet and determine the number in decimal for each. The first two octets are zero, the third is 63 and the fourth is 255 for an answer of 0.0.63.255.
I hope this has been helpful!
Laz