Calculating network addresses in tthe shell script

shell-ip-address

#!/bin/sh

# converts IPv4 as "A.B.C.D" to integer
# ip4_to_int 192.168.0.1
# => 3232235521
ip4_to_int() {
  IFS=. read -r i j k l <<EOF
$1
EOF
  echo $(( (i << 24) + (j << 16) + (k << 8) + l ))
}

# converts interger to IPv4 as "A.B.C.D"
#
# int_to_ip4 3232235521
# => 192.168.0.1
int_to_ip4() {
  echo "$(( ($1 >> 24) % 256 )).$(( ($1 >> 16) % 256 )).$(( ($1 >> 8) % 256 )).$(( $1 % 256 ))"
}

# returns the ip part of an CIDR
#
# cidr_ip "172.16.0.10/22"
# => 172.16.0.10
cidr_ip() {
  IFS=/ read -r ip _ <<EOF
$1
EOF
  echo $ip
}

# returns the prefix part of an CIDR
#
# cidr_prefix "172.16.0.10/22"
# => 22
cidr_prefix() {
  IFS=/ read -r _ prefix <<EOF
$1
EOF
  echo $prefix
}

# returns net mask in numberic from prefix size
#
# netmask_of_prefix 8
# => 4278190080
netmask_of_prefix() {
  echo $((4294967295 ^ (1 << (32 - $1)) - 1))
}

# returns default gateway address (network address + 1) from CIDR
cidr_default_gw() {
  ip=$(ip4_to_int $(cidr_ip $1))
  prefix=$(cidr_prefix $1)
  netmask=$(netmask_of_prefix $prefix)
  gw=$((ip & netmask + 1))
  int_to_ip4 $gw
}

# returns default gateway address (broadcast address - 1) from CIDR
cidr_default_gw_2() {
  ip=$(ip4_to_int $(cidr_ip $1))
  prefix=$(cidr_prefix $1)
  netmask=$(netmask_of_prefix $prefix)
  broadcast=$(((4294967295 - netmask) | ip))
  int_to_ip4 $((broadcast - 1))
}

cidr_default_gw 192.168.10.1/24
# => 192.168.10.1
cidr_default_gw 192.168.10.1/16
# => 192.168.0.1
cidr_default_gw 172.17.18.19/20
# => 172.17.16.1

cidr_default_gw_2 192.168.10.1/24
# => 192.168.10.254
cidr_default_gw_2 192.168.10.1/16
# => 192.168.255.254
cidr_default_gw_2 172.17.18.19/20
# => 172.17.31.254

Very nice 🌚

Any ipv6 version of this? Thanks.

This is great, but I think I found a bug in:

cidr_default_gw() {
  ip=$(ip4_to_int $(cidr_ip $1))
  prefix=$(cidr_prefix $1)
  netmask=$(netmask_of_prefix $prefix)
  gw=$((ip & netmask + 1))
  int_to_ip4 $gw
}

The order of operations of $((ip & netmask + 1)) means that netmask + 1 is done first. So if the IP is even, the gateway ends up as a x.y.z.0 instead of x.y.z.1. For odd IPs it works as expected