bash CIDR calculator

shiftee · 12-13-2013, 04:40 AM

Quote:

Originally Posted by David the H.

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

I had to do the following to get it to work:
- [0]=''
+ [0]=0

Test code:

Code:

for i in $(seq 0 255)
do
    MASK="255.255.$i.0"
    BITS=$(mask2cidr $MASK)
    [ $? -eq 0 ] && echo "$MASK   : $BITS"
done

for i in $(seq 1 255)
do
    MASK="255.255.255.$i"
    BITS=$(mask2cidr $MASK)
    [ $? -eq 0 ] && echo "$MASK : $BITS"
done

grail · 12-13-2013, 07:41 AM

Well I guess it is only fair to update the awk:

Code:

#!/usr/bin/awk -f

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

	split(ARGV[1], arr, "[.]")

	for(i = 1; i <= 4;i++){
		total += cidr[arr[i]]
		if(arr[i] < 255) break
	}

	print total
}

I also altered it to take the address as an argument on the command line

Secmas · 02-22-2014, 09:35 AM

This is a really nice post, thanks to all that contributed in this post.

I am doing an script that checks IPTABLES to see if there are IPs that could be joined in one CIDR notation up to the /24 no more than that and wish there could be something that could do this, but in the mean time this code will help me a lot.

Thanks!

nbritton · 10-29-2014, 10:13 AM

For a class project I made a CIDR netmask calculator using base 10 math. I release this code into the public domain.

Code:

#!/bin/bash
# CIDR Netmask Calculator

y=$[2**(32-$1)] # y = 2^(32-x), x = CIDR class

printf "$1\t" # x
printf "$y\t" # y

if ! [ "$[$y/256**0]" -gt "256" ]; then		# y
	echo 255.255.255.$[256-($y/256**0)]	# 256-(y)
	exit
fi
if ! [ "$[$y/256**1]" -gt "256" ]; then		# y/256
	echo 255.255.$[256-($y/256**1)].0	# 256-(y/256)
	exit
fi
if ! [ "$[$y/256**2]" -gt "256" ]; then		# y/256/256
	echo 255.$[256-($y/256**2)].0.0		# 256-(y/256/256)
	exit
fi
if ! [ "$[$y/256**3]" -gt "256" ]; then		# y/256/256/256
	echo $[256-($y/256**3)].0.0.0		# 256-(y/256/256/256)
	exit
fi

Output:

Code:

$ for i in {0..32}; do ./cidr.sh $i; done
0	4294967296	0.0.0.0
1	2147483648	128.0.0.0
2	1073741824	192.0.0.0
3	536870912	224.0.0.0
4	268435456	240.0.0.0
5	134217728	248.0.0.0
6	67108864	252.0.0.0
7	33554432	254.0.0.0
8	16777216	255.0.0.0
9	8388608	255.128.0.0
10	4194304	255.192.0.0
11	2097152	255.224.0.0
12	1048576	255.240.0.0
13	524288	255.248.0.0
14	262144	255.252.0.0
15	131072	255.254.0.0
16	65536	255.255.0.0
17	32768	255.255.128.0
18	16384	255.255.192.0
19	8192	255.255.224.0
20	4096	255.255.240.0
21	2048	255.255.248.0
22	1024	255.255.252.0
23	512	255.255.254.0
24	256	255.255.255.0
25	128	255.255.255.128
26	64	255.255.255.192
27	32	255.255.255.224
28	16	255.255.255.240
29	8	255.255.255.248
30	4	255.255.255.252
31	2	255.255.255.254
32	1	255.255.255.255