bash CIDR calculator

genderbender · 06-03-2008, 11:07 AM

Been working on a code which will take my subnet mask and convert it into CIDR notation, I figured the best way to do it would be to convert each decimal bit to CIDR notation and then just replace the .'s between them with add and show the result. I'm a little stuck though, changing the numbers is proving a little... impossible. e.g 255 = 8 so 255.255.255.255 = 8+8+8+8 = 32, correct CIDR notation for that net mask.

Heres my code so far:

Code:

#!/bin/bash
#none of the below works...
255="8"
254="7"
252="6"
248="5"
240="4"
224="3"
192="2"
128="1"
0="0"
#I want to use the following code to get my netmask, i wrote this when I had no net tho.
#SN=`ifconfig ath0 | awk /'inet addr:'/'{print $4}'`
#Replaced it with this just for now...
echo -n "type subnet mask (255.255.255.255)"
read MASK
#following code will show 255+255+255+255
SN=`echo $MASK | tr '.' '+'`
echo $SN

So I need the math function to add it and a way of splitting the full address into 4 hex bits and thats about it oh and a forward slash at the end, eventual output needs to be /32.

Thanks so much

Hko · 06-03-2008, 01:07 PM

Code:

#!/bin/bash

# Function calculates number of bit in a netmask
#
mask2cidr() {
    nbits=0
    IFS=.
    for dec in $1 ; do
        case $dec in
            255) let nbits+=8;;
            254) let nbits+=7;;
            252) let nbits+=6;;
            248) let nbits+=5;;
            240) let nbits+=4;;
            224) let nbits+=3;;
            192) let nbits+=2;;
            128) let nbits+=1;;
            0);;
            *) echo "Error: $dec is not recognised"; exit 1
        esac
    done
    echo "$nbits"
}

## main ##
MASK=255.255.254.0
numbits=$(mask2cidr $MASK)
echo "/$numbits"
exit 0

genderbender · 06-03-2008, 01:45 PM

Genius, thanks so much and a very elegant way of doing it, I really appreciate it.

matschaffer · 02-05-2009, 01:19 PM

Incase any searchers happen to end up here, I also wrote the reverse (translate cidr to netmask) which I wasn't able to find anywhere.

Code:

cidr2mask() {
  local i mask=""
  local full_octets=$(($1/8))
  local partial_octet=$(($1%8))

  for ((i=0;i<4;i+=1)); do
    if [ $i -lt $full_octets ]; then
      mask+=255
    elif [ $i -eq $full_octets ]; then
      mask+=$((256 - 2**(8-$partial_octet)))
    else
      mask+=0
    fi  
    test $i -lt 3 && mask+=.
  done

  echo $mask
}

jlinkels · 02-05-2009, 06:47 PM

ipcalc does it all for you. (That is ipcalc, the bold spoils the 'i')

jlinkels

matschaffer · 02-06-2009, 08:05 AM

Quote:

Originally Posted by jlinkels

ipcalc does it all for you.

The only ipcalc I could google was an online tool. I (and probably the original poster) needed something easy in the context of a bash script. And it was also a fun little math problem

Thanks,
Mat

r_hartman · 02-04-2011, 09:16 AM

Quote:

Originally Posted by matschaffer

The only ipcalc I could google was an online tool. I (and probably the original poster) needed something easy in the context of a bash script. And it was also a fun little math problem

Thanks,
Mat

ipcalc is part of the RedHat Enterprise Linux distribution, and possibly others.

But this thread inspired me to create a variation on ipcalc, ip4calc.sh, adapted and expanded from the bits of elegant code presented here. It can be downloaded from my website.

Edit: just noticed I can't link urls as this is my first post here.
Website is hac-maarssen.nl, take the Download link, download ip4calc.zip.

Have fun.
René

grail · 02-05-2011, 02:48 AM

Maybe something else to have a look at:

Code:

#!/usr/bin/awk -f

BEGIN {
    bits = 8
    for(x = 255; x >=0; x -= 2^i++)
        cidr[x] = bits--

    printf "Please enter current netmask: "
    getline netmask < "-"

    split(netmask, arr, "[.]")

    for(i = 1; i <= 4;i++)
        total += cidr[arr[i]]

    print total
}

Tinkster · 02-05-2011, 03:09 AM

Quote:

Originally Posted by r_hartman

ipcalc is part of the RedHat Enterprise Linux distribution, and possibly others.

But this thread inspired me to create a variation on ipcalc, ip4calc.sh, adapted and expanded from the bits of elegant code presented here. It can be downloaded from my website.

Edit: just noticed I can't link urls as this is my first post here.
Website is hac-maarssen.nl, take the Download link, download ip4calc.zip.

Have fun.
René

Welcome to LQ,

And out of curiosity: why is your first post unearthing a 2 years gone
corpse of a thread?

Cheers,
Tink

grail · 02-05-2011, 03:16 AM

My bad ... didn't even look at the date

genderbender · 02-05-2011, 04:34 AM

I'd like to delete this, the codes so (embaressingly) bad, there's not even any logic to it. Three years ago I must of been partially retarded.

r_hartman · 02-07-2011, 10:55 AM

@Tinkster
Thanks. I was actually looking for some code to calculate the CIDR network notation in bash, much like the OP, and this was the first hit that actually addressed my question, in many hits that were about on-line calculators. I thought the codes by Hko and MattShaffer were elegant, and from the thread I figured not all Linux/Unix flavors have ipcalc, suggested by jlinkels, shipped with them.

Hence my little exercise and returning my efforts to the community.
I did see the notice stating it was an old thread, but as it was about the only useful hit I got I thought it would be OK to add to it.

Cheers,
René

MPH426 · 05-10-2013, 02:31 PM

I just came across this. It was exactly what I was looking for. I didn't want to rely on ipcalc's availability.

The code posted by Hko works very well. The only problem is, it can give false results.

Once you hit an octet thats below 255 the following octets should be 0. In it's original form one could enter 255.252.224.252 (according the the code = /23 8^O) or any other combination producing false results. Come on, you know somebody will. LOL Adding the break statements fixes that.

Here is my "slight" revision

Code:

#!/bin/bash

# Function calculates number of bit in a netmask
#
mask2cidr() {
    nbits=0
    IFS=.
    for dec in $1 ; do
        case $dec in
            255) let nbits+=8;;
            254) let nbits+=7 ; break ;;
            252) let nbits+=6 ; break ;;
            248) let nbits+=5 ; break ;;
            240) let nbits+=4 ; break ;;
            224) let nbits+=3 ; break ;;
            192) let nbits+=2 ; break ;;
            128) let nbits+=1 ; break ;;
            0);;
            *) echo "Error: $dec is not recognised"; exit 1
        esac
    done
    echo "$nbits"
}

## main ##
# MASK=255.255.254.0
MASK=$1
numbits=$(mask2cidr $MASK)
echo "/$numbits"
exit 0

While I'm at it. Here's a couple of functions to calculate the network and broadcast addresses.

Note: Some versions of the bc caclulator don't come with bitwise functions.
Since I've been using the korn (ksh) shell since the late '80's and it's now public domain, I haven't tried too many others to see if they work.

Code:

#! /bin/ksh
###########################################################################
# netcalc:  Calculate the network address of a given IP address and mask.
###########################################################################
netcalc(){
  echo "$1" | sed -e 's/\./ /g' | read oct1 oct2 oct3 oct4
  echo "$2" | sed -e 's/\./ /g' | read msk1 msk2 msk3 msk4
  ipa=`echo $(($oct1 & $msk1))`
  ipb=`echo $(($oct2 & $msk2))`
  ipc=`echo $(($oct3 & $msk3))`
  ipd=`echo $(($oct4 & $msk4))`
  echo "$ipa.$ipb.$ipc.$ipd"
}
###########################################################################
# bcastcalc: Caclulate the broadcast address of a given IP address and mask.
###########################################################################
bcastcalc(){
  echo "$1" | sed -e 's/\./ /g' | read oct1 oct2 oct3 oct4
  echo "$2" | sed -e 's/\./ /g' | read msk1 msk2 msk3 msk4
  ipa=$(($oct1+(255-($oct1 | $msk1))))
  ipb=$(($oct2+(255-($oct2 | $msk2))))
  ipc=$(($oct3+(255-($oct3 | $msk3))))
  ipd=$(($oct4+(255-($oct4 | $msk4))))
  echo "$ipa.$ipb.$ipc.$ipd"
}

Hope it helps someone.

David the H. · 05-12-2013, 04:04 PM

Now that's the proper way to re-open a thread; making corrections and adding extra info.

Following that, here's a modification of the above function of my own. It's a bit shorter, but I couldn't say whether it's any more efficient.

Code:

#!/bin/bash

mask2cidr() {

    local nbits dec
    local -a octets=( [255]=8 [254]=7 [252]=6 [248]=5 [240]=4
                      [224]=3 [192]=2 [128]=1 [0]=''           )
    
    while read -rd '.' dec; do
        [[ -z ${octets[dec]} ]] && echo "Error: $dec is not recognised" && exit 1
        (( nbits += octets[dec] ))
        (( dec < 255 )) && break
    done <<<"$1."

    echo "/$nbits"

}

mask2cidr "$1"

exit 0

And here are my modified versions of the other functions as well. bash supports bitwise operations too, and only a few modifications are needed to convert from ksh (AFAICT, only the local command and the -a option to read aren't supported by ksh).

Code:

#!/bin/bash

netcalc(){

    local IFS='.' ip i
    local -a oct msk
	
    read -ra oct <<<"$1"
    read -ra msk <<<"$2"

    for i in ${!oct[@]}; do
        ip+=( "$(( oct[i] & msk[i] ))" )
    done
    
    echo "${ip[*]}"
}

bcastcalc(){

    local IFS='.' ip i
    local -a oct msk
	
    read -ra oct <<<"$1"
    read -ra msk <<<"$2"

    for i in ${!oct[@]}; do
        ip+=( "$(( oct[i] + ( 255 - ( oct[i] | msk[i] ) ) ))" )
    done

    echo "${ip[*]}"
}

netcalc "$1" "$2"
bcastcalc "$1" "$2"

One final caveat though. I'm not that experienced with ip stuff, so it's possible that I've made a conceptual mistake somewhere.

MPH426 · 05-13-2013, 11:34 AM

David H.,

Awesome, very cool to see other ways of doing things. I lern't something new today. Just for kicks I converted it to ksh. I didn't know it could set arrays like that. Makes thinks MUCH easier when setting multi dimensional arrays.

You're right about the local not being supported in ksh, but for the read statement, just change -a to -A.

Here's ksh's version. Really not all that much different.

Code:

#!/bin/ksh

mask2cidr() {

  typeset -A octets=([255]=8 [254]=7 [252]=6 [248]=5 [240]=4
                     [224]=3 [192]=2 [128]=1 [0]=0)

  while read -rd '.' dec
  do
    [[ -z ${octets[$dec]} ]] && echo "Error: $dec is not recognised" && exit 1
    (( nbits += ${octets[$dec]} ))
    [[ $dec -lt 255 ]] && break
  done <<<"$1."

  echo "/$nbits"

}

mask2cidr "$1"

exit 0

Code:

#!/bin/ksh

netcalc(){
    IFS='.'
    typeset -A oct msk ip
    i=0

    read -rA oct <<<"$1"
    read -rA msk <<<"$2"

    while [ $i -lt "${#oct[@]}" ]
    do
      ip[$i]=$((oct[$i] & msk[$i]))
      i=$((i+1))
    done

    echo "IP: ${ip[*]}"
}

bcastcalc(){
    IFS='.' 
    typeset -A oct msk
    i=0

    read -rA oct <<<"$1"
    read -rA msk <<<"$2"

    while [ $i -lt "${#oct[@]}" ]
    do
      ip[$i]="$((oct[$i]+(255-(oct[$i] | msk[$i]))))"
      i=$((i+1))
    done

    echo "${ip[*]}"
}

netcalc "$1" "$2"
bcastcalc "$1" "$2"