Find netmask / network address / broadcast address / supposed gateway from an IP/cidr pair

gistfile1.sh

cidr2mask() {
  local mask=""
  local netaddr=""
  local bcast=""
  local gateway=""
  local full_octets=$(($2/8))
  local partial_octet=$(($2%8))

  ipparts=($(echo $1 | tr "." "\n"))

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

  echo $mask $netaddr $bcast $gateway
}

# Confirm that the IP address is address/mask
addrmask=$1
if [ `expr index "$addrmask" '/'` -eq "0" ]; then
  usage
fi

ip=${addrmask%/*}
cidrmask=${addrmask#*/}

if [ `expr match "$ip" '.*[^0-9.]'` -gt "0" ]; then
  echo "Wrong IP format."
  usage
fi
if [ `expr match "$cidrmask" '.*[^0-9]'` -gt "0" ]; then
  echo "Wrong netmask format."
  usage
fi
echo $ip $cidrmask

tuple=(`cidr2mask $ip $cidrmask`)
netmask=${tuple[0]}
network=${tuple[1]}
bcast=${tuple[2]}
gateway=${tuple[3]}