After a while, available IP addresses on the subnet may become hard to come by. Whatever spreadsheet you used to track IP allocations is likely out of date. Here’s a simple script that will scan the specified subnet and give you a list of IPs that don’t ping and have no associated DNS records. This should help you narrow down your choices.

#!/bin/bash
# .. .. .. .. .. .. .. .. .. ...  .. .. .. .. .. .. .. .. .. .. .. .. ...
# :             (              :  : Identify available IP addresses on  :
# :            ( ,)            :  : the subnet                          :
# :           ). ( )           :  :                                     :
# :          (, )' (.          :  :                                     :
# :         \WWWWWWWW/         :  :                                     :
# :          '--..--'          :  :                                     :
# :             }{             :  :                                     :
# :             {}             :  :                                     :
# : |/ _ _ _  \    / _  _|  _  :  :                                     :
# : |\| (_|/_\/\/\/ (_)| |<_\  :  :                                     :
# :          /                 :  :                                     :
# :     www.krazyworks.com     :  :                                     :
# ... .. .. .. .. .. .. .. .. ..  ... .. .. .. .. .. .. .. .. .. .. .. ..
# .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. ..
# Copyright 2015, KrazyWorks LLC                                        :
#                                                                       :
# This program is free software: you can redistribute it and/or modify  :
# it under the terms of the GNU General Public License as published by  :
# the Free Software Foundation, either version 3 of the License, or     :
# (at your option) any later version.                                   :
#                                                                       :
# This program is distributed in the hope that it will be useful,       :
# but WITHOUT ANY WARRANTY; without even the implied warranty of        :
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the         :
# GNU General Public License for more details.                          :
#                                                                       :
# You should have received a copy of the GNU General Public License     :
# along with this program.  If not, see <http://www.gnu.org/licenses/>. :
# .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. ..

if [ ! "" ]
then
	echo "Specify subnet like so: 192.168.1"
	exit 1
fi

subnet=""

configure() {
	tmpfile="/tmp/free_ip_scan.tmp"
}

cleanup() {
	if [ -f "${tmpfile}" ]
	then
		/bin/rm -f "${tmpfile}"
	fi
}

is_alive_ping()
{
	ping -c 4 $1 >/dev/null 2>&1
	[ $? -ne 0 ] && echo $1
}

pingdo() {
	for i in ${subnet}.{10..250}
	do
		is_alive_ping $i & disown
	done | sort -t. -k4 -n > "${tmpfile}"
}

digdo() {
	if [ -r "${tmpfile}" ]
	then
		echo "The following IPs are not pingable and are not in DNS, so they *may* be available:"
		for ip in `cat "${tmpfile}"`
		do
			if [ `dig +short -x "${ip}" 2>/dev/null | wc -l` -eq 0 ]
			then
				echo "${ip} seems available"
			fi
		done | column -t
	else
		echo "Scraped the bottom of the pickle barrel but came up dry"
	fi
}

# RUNTIME

configure
cleanup
pingdo
digdo
cleanup