To switch from an IPV4 netmask to the equivalent CIDR it might occur to do the simplest thing, that is to use a nested "case" block or "if" to obtain the equivalent CIDR so much so that the available netmasks or CIDR are only 32 and that is
| Netmask | Address/CIDR | Adresses |
| 255.255.255.255 | a.b.c.d/32 | 20 |
| 255.255.255.254 | a.b.c.d/31 | 20 |
| 255.255.255.252 | a.b.c.d/30 | 21 |
| 255.255.255.248 | a.b.c.d/29 | 22 |
| 255.255.255.240 | a.b.c.d/28 | 23 |
| 255.255.255.224 | a.b.c.d/27 | 24 |
| 255.255.255.192 | a.b.c.d/26 | 25 |
| 255.255.255.128 | a.b.c.d/25 | 26 |
| 255.255.255.0 | a.b.c.0/24 | 27 |
| 255.255.254.0 | a.b.c.0/23 | 28 |
| 255.255.252.0 | a.b.c.0/22 | 29 |
| 255.255.248.0 | a.b.c.0/21 | 210 |
| 255.255.240.0 | a.b.c.0/20 | 212 |
| 255.255.224.0 | a.b.c.0/19 | 213 |
| 255.255.192.0 | a.b.c.0/18 | 214 |
| 255.255.128.0 | a.b.c.0/17 | 215 |
| 255.255.0.0 | a.b.0.0/16 | 216 |
| 255.254.0.0 | a.b.0.0/15 | 217 |
| 255.252.0.0 | a.b.0.0/14 | 218 |
| 255.248.0.0 | a.b.0.0/13 | 219 |
| 255.240.0.0 | a.b.0.0/12 | 220 |
| 255.224.0.0 | a.b.0.0/11 | 221 |
| 255.192.0.0 | a.b.0.0/10 | 222 |
| 255.128.0.0 | a.b.0.0/9 | 223 |
| 255.0.0.0 | a.0.0.0/8 | 224 |
| 254.0.0.0 | a.0.0.0/7 | 225 |
| 252.0.0.0 | a.0.0.0/6 | 226 |
| 248.0.0.0 | a.0.0.0/5 | 227 |
| 240.0.0.0 | a.0.0.0/4 | 228 |
| 224.0.0.0 | a.0.0.0/3 | 229 |
| 192.0.0.0 | a.0.0.0/2 | 230 |
| 128.0.0.0 | a.0.0.0/1 | 231 |
| 0.0.0.0 | 0.0.0.0/0 | 232 |
If we want instead create a function that converts the netmask to CIDR we need to know how to convert a decimal number to binary first.
fromNetmaskToCidr() {
local netmask=$1
local bin=({0..1}{0..1}{0..1}{0..1}{0..1}{0..1}{0..1}{0..1})
local b=''
local n
local cidr
for n in ${netmask//./ }; do
if [ $n -gt 255 ]; then
echo -e "\t[ERROR] netmask $netmask format error in '.$n'. I will use .255 insted of .$n"
n=255
fi
if [ $n -ne 0 -a $n -ne 128 -a $n -ne 192 -a $n -ne 224 -a $n -ne 240 -a $n -ne 248 -a $n -ne 252 -a $n -ne 254 -a $n -ne 255 ]; then
echo -e "\t[ERROR] netmask $netmask format error in '.$n' (it must be 0,128,192,224,240,248,252,254,255). I will use .255 insted of .$n"
n=255
fi
# $b is the binary of $netmask
b=${b}${bin[$n]}
done
# remove right "0" bits from $b
b=${b/%0*}
cidr=${#b}
echo $cidr
}What we do is convert the 4 octets of the netmask into binary and put them in series in the only variable "b" then remove the 0 bits present on the right of the overall binary number.
Finally, just count the number of bits left to get the CIDR which will then be returned on exit.
