How to check public IP address availability

I have a few public IP addresses given by the ISP service provider. My question is how to check which IP addresses are already used up and which are still available to use for new servers? Because I do not know how to check and the previous IT personnel did not leave any information.

…and the previous IT personnel did not leave any information.

I feel your pain on this. Inheriting someone else’s unmapped and undocumented network mess is not pleasant. As explained in this answer you could use nmap like this; of course 192.168.0.1/24 is an example and should be changed to the network range assigned to the system by the ISP:

nmap -sP -PR 192.168.0.1/24

But that would only be beneficial for internal IP addresses since the -PR option is an ARP ping scan. Meaning public IP addresses not on a LAN would not be generating ARP traffic. So in your case, you can just use the plain -sP (ping scan) option on a range like this

nmap -sP 192.168.0.1/24

And that should give you a nice list of all IP addresses in that range that ping you back. But of course, there are some edge cases where the lack of a ping response does not mean that IP address is dead. Some devices will have an IP address assigned to them but it won’t respond to ICMP ping requests. Or there might be dead or dormant devices with IPs assigned but for some reason they are not functioning.

So be careful with data you get from using an nmap ping scan, but it’s better than nothing and a nice place to start your network mapping journey.

If you are more into visual interfaces, I would highly recommend Angry IP Scanner which is a cross platform, Java-based IP address range scanner that does a great job of not just ping scanning your network, but also providing additional data such as hostname (if applicable) and info on open ports as well.

In addition to using nmap and other active scanning tools you should also look at your mac address tables on your firealls, routers, and switches.

These devices will keep a list of the MAC addresses and IP addresses of all devices that have communicated with in a defined time period. Often this time period can be increased for more detailed results.

Use Angry Ip Scanner to scan those Ip ranges and filter those that are marked as red (dead ones).

Angry IP Scanner (or simply ipscan) is an open-source and cross-platform network scanner designed to be fast and simple to use.

It is widely used by network administrators and just curious users around the world, including large and small enterprises, banks, and government agencies.

It runs on Linux, Windows, and Mac OS X, possibly supporting other platforms as well.