At some point available IPs on your network may become few and hard to come by. Documentation becomes outdated and does not accurately reflect which IPs are still in use. A good starting point would be to scan a network segment and identify which IPs are not responding to ping. And then you can go from there to figure out which of those can be recycled.

Here’s a simple script that will help you with this task. In this example we’re looking for potentially-available IPs on the 192.168.101 subnet.

#!/bin/bash
is_alive_ping()
{
  ping -c 2 $1 > /dev/null
  [ $? -ne 0 ] && echo Node with IP: $1 is down
}
for i in 192.168.101.{1..254}
do
  is_alive_ping $i & disown
done | sort -t. -k4 -n

A word of caution: just because the IP did not respond to ping, doesn’t mean it is not in use. Always verify.