I have an Icom DStar repeater that I support. While testing, I changed the default IP address of the Icom DStar controller from the default 172.*.*.* address. The question is how to find out what I changed it to? I have a non-routable class C address on my home network (192.168.1.*). I presumed I set it to something in that subnet. I tried nmap to so a ping scan with this command nmap -sP 192.168.1.* but that did not work. Note I did verify the Icom controller DOES respond to a ping.
I used tcpdump to watch the Ethernet NIC and I saw a bunch of arp requests. One of the source IP addresses 192.168.1.239. I then pinged this one and it replied. While the ping was running, I unplugged the network cable from the front of the Icom controller and the ping stopped replying. So, it looked like I confirmed the address. In the RP2C software, I changed the address to the 239 address, entered the password and it worked!