Linux

 arping is the ultimate tool for finding out if an IP is actively used or not. It uses ARP to detect an IP.

Since it uses ARP it has to be run in the same broadcast domain (VLAN) as the target IP.

 # arping -I eth0 -c 3 208.x.x.x

 # arping -q -c 3 -U -I eth0 192.168.x.x

 arp-scan runs the arping over a range of IPs

 Default timeout is 500 ms it is better to change it to 1000 ms or more to get more accurate results.

 -t 1000

 Sorting IPs and eliminate duplicates

 # arp-scan 199.x.x.x -199.x.x.x | sort -u -k1,1

 # arp-scan -I eno1 199.x.x.x/23

 # arp-scan 199.x.x.x 208.x.x.x 208.x.x.x

 Windows

 arp-ping

 http://www.elifulkerson.com/projects/arp-ping.php

 SoftPerfect Network Scanner

 https://www.softperfect.com/products/networkscanner/

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How to install arp-scan on CentOS ?

(This has been tested on CentOS 7)