How to determine vacant IP addresses

rozilla · 07-29-2008, 07:32 AM

I was looking at the IP of our backup server just now, and realized it didn't conform to the numbering scheme used for the other servers. If was about to ifconfig it to match the scheme. I was going to check the DHCP list to see which IP #s are still available, and that got me wondering. Is there any command I can use to determine if an IP # is free?

acid_kewpie · 07-29-2008, 07:45 AM

yes, you ping it and get no response.

grejon04 · 07-29-2008, 08:27 AM

for something as simple as checking your own network, a ping should do fine. In other cases, you might want something like a port scanner or Nessus, or even just hping set to different common ports, to make sure that something is not just dropping ICMP (ping) packets.

rozilla · 07-29-2008, 08:45 AM

Yes, pinging comes to mind, but one problem I see with this if reliability. What if I'm checking if 192.10.134.3 is free? I ping it, and if it's free I get icmp_seq=1 Destination Host Unreachable, right? The problem is, I would get this exact same error message, in the event of this IP already having been assigned to a machine, and the machine happens to be unplugged for a given reason.
That's why I was hoping for something for definitive than ping.

farslayer · 07-29-2008, 08:57 AM

Use nmap to quickly scan the entire subnet..

nmap -sP 192.10.134.0/24

Host name and IP address info...

Nmap done: 256 IP addresses (78 hosts up) scanned in 14.327 seconds

When dealing with Static IP addresses, it's up to the Network administrator to DOCUMENT the used IP addresses, and keep that documentation up to date. The documentation should be your definitive resource.

immortaltechnique · 07-29-2008, 11:20 AM

have you tried ping ip_address_here? Again you can also try using nmap as earlier advised. That way you get a response then the ip is not free and you can further use nmap to roughly try and pin-point which machine is using the ip.

If not then the machine is off at that moment or the ip is not in use at all.

acid_kewpie · 07-30-2008, 01:27 AM

well of course you can't tell if an IP address is "vacant" compared to "switched off" there is absolutely no *technical* difference whatsoever.