#!/bin/bash

# when provided an interface, this will determine the subnet and the bits for tha tsubnet

if [[ "X" == "X$1" ]]
 then 
    echo "Usage: ${0} <ethx>"
    exit 1
 fi    

eth="$1"
ipaddr=$(ifconfig ${eth} | grep 'inet addr' | awk -F':' '{print $2}' | awk '{print $1}' )
mask=$(ifconfig ${eth} | grep 'inet addr' | awk -F':' '{print $4}')

# echo "${ipaddr} ${mask}"

line=$(echo ${ipaddr} | sed 's/\./ /g')
set ${line}
p1="$1"
p2="$2"
p3="$3"
p4="$4"
# (( ipnum = p4 + 256 * (p3 + 256 * (p2 + 256 * p1)) ))

line=$(echo ${mask} | sed 's/\./ /g')
set ${line}
p1="$1"
p2="$2"
p3="$3"
p4="$4"
# (( masknum = p4 + 256 * (p3 + 256 * (p2 + 256 * p1)) ))

# echo $ipnum $masknum


line=$(echo ${ipaddr} | sed 's/\./ /g')
set ${line}
ip1="$1"
ip2="$2"
ip3="$3"
ip4="$4"

line=$(echo ${mask} | sed 's/\./ /g')
set ${line}
m1="$1"
m2="$2"
m3="$3"
m4="$4"
# echo "mask: $m1 $m2 $m3 $m4"

((n1 = ip1 & m1))
((n2 = ip2 & m2))
((n3 = ip3 & m3))
((n4 = ip4 & m4))
network="$n1.$n2.$n3.$n4"
# echo "network: $n1 $n2 $n3 $n4"

((x1 = 255 - m1))
((x2 = 255 - m2))
((x3 = 255 - m3))
((x4 = 255 - m4))
(( tot = x4 + 256 * (x3 + 256 * (x2 + 256 * x1)) ))
((bits=0))
while ((0 < tot))
 do
    ((bits = bits + 1))
    ((tot >>= 1))
 done 
((msk = 32 - bits))

# echo "subnet: $x1 $x2 $x3 $x4 /$msk"
echo "${network}/${msk}"