Bash - Convert netmask in CIDR notation?

Question

Example: I have this netmask: 255.255.255.0

Is there, in bash, a command or a simple script to convert my netmask in notation /24?

Simple Python script (not posting as an answer because of that): `import ipaddress,math;print("/" + str(32-math.ceil(math.log2(int(ipaddress.IPv4Address(input()))^2**32-1))))` — L3viathan, May 18 '18 at 14:36

agc · Answer 1 · 2022-12-07T07:43:21.460

7

Function using subnetcalc:

IPprefix_by_netmask() {
    subnetcalc 1.1.1.1 "$1" -n  | sed -n '/^Netw/{s#.*/ #/#p;q}'
}

In pure bash, (i.e. no external utils like sed or bc), convert IP to a long octal string and sum its bits:

IPprefix_by_netmask () { 
   c=0 x=0$( printf '%o' ${1//./ } )
   while [ $x -gt 0 ]; do
       let c+=$((x%2)) 'x>>=1'
   done
   echo /$c ; }

Output of IPprefix_by_netmask 255.255.255.0 (either function):

/24

edited Dec 07 '22 at 07:43

answered May 18 '18 at 22:19

agc

7,973
2
29
50

It's worth noting that the portability of `ipcalc` and `sipcalc` is a little better than that of `subnetcalc` due to better availability, according to [pkgs.org](https://pkgs.org). You may wish to take this into account. – cyqsimon May 11 '22 at 09:22
Among the distros I use and tested: Arch has `ipcalc` and `sipcalc` in its official repo, while `subnetcalc` is only available from the AUR; RHEL 8 and derivatives (with EPEL enabled) similarly have both `ipcalc` and `sipcalc` but not `subnetcalc`. – cyqsimon May 11 '22 at 09:29

Sasha Golikov · Accepted Answer · 2018-12-24T15:49:11.870

6

Example Function for RHEL6/RHEL7:

IPprefix_by_netmask() {
#function returns prefix for given netmask in arg1
 ipcalc -p 1.1.1.1 $1 | sed -n 's/^PREFIX=\(.*\)/\/\1/p'
}

The Result:

$ IPprefix_by_netmask 255.255.255.0
/24

In other Linux distributives ipcalc options may differ.

The same function without ipcalc, tested in Solaris and Linux:

IPprefix_by_netmask() {
    #function returns prefix for given netmask in arg1
    bits=0
    for octet in $(echo $1| sed 's/\./ /g'); do 
         binbits=$(echo "obase=2; ibase=10; ${octet}"| bc | sed 's/0//g') 
         let bits+=${#binbits}
    done
    echo "/${bits}"
}

edited Dec 24 '18 at 15:49

answered May 18 '18 at 15:21

Sasha Golikov

638
5
11

1

The `ipcalc` version of the function outputs nothing on my system, (*Lubuntu v17.10*). Running `ipcalc -p 1.1.1.1 255.255.255.0` returns an *"Unknown option: -p"* error. `ipcalc` version is *0.41-5*. – agc May 18 '18 at 21:56
This works: `ipcalc -n 1.1.1.1 $1 | sed -n '/^Netm/{s#.*= #/#;s/ .*//p;q}'` – agc May 18 '18 at 22:13
or `ipcalc -nb 1.1.1.1 "$1" | sed -n '/Netmask/s/^.*=[ ]/\//p` – David C. Rankin May 18 '18 at 23:14

score 5 · Answer 3 · answered Mar 02 '21 at 12:08

Solution based on awk

While GNU awk is not Bash, it’s installed by default in enough distributions that this may be helpful in the sense of the question:

awk -F. '{
    split($0, octets)
    for (i in octets) {
        mask += 8 - log(2**8 - octets[i])/log(2);
    }
    print "/" mask
}' <<< 255.255.255.240

This prints:

/28

score 1 · Answer 4 · answered Dec 18 '19 at 00:00

Based on Sasha's answer, this script works with dash (tested with Ubuntu 18.04):

IPprefix_by_netmask() {
    #function returns prefix for given netmask in arg1
    bits=0
    for octet in $(echo $1| sed 's/\./ /g'); do 
         binbits=$(echo "obase=2; ibase=10; ${octet}"| bc | sed 's/0//g') 
         bits=$(expr $bits + ${#binbits})
    done
    echo "/${bits}"
}

Bash - Convert netmask in CIDR notation?

4 Answers4

Solution based on awk