Nmap

Nmap (Network Mapper) is a cross-platform command-line port scanner. It identifies which ports are open on a networked computer, which in turn can help identify what services are running on that computer. This is useful to both sysadmins and hackers alike for discovering potential points of attack in prospective targets. Its ability to scan entire networks is very useful when combined with Whois information about organizations you may be targeting.

Nmap isn't limited to just scanning for open ports, but features such abilities as host discovery, service and operating system detection, version identification, and more.

Nmap is available for Linux, Windows, BSD & MacOS. Download options for all platforms can be found here. Some quick install instructions are below:

$ sudo apt install nmap

$ sudo dnf install nmap

Read the Windows install instructions.

Read the MacOS install instructions.

Basic usage (see which ports are open/filtered/closed):

nmap 192.168.1.150

Identify services running on one computer:

nmap -sV 192.168.1.150

Identify services running on one computer, which does not respond to pings:

nmap -sV -PN 192.168.1.150

Check to see if specific ports are open on one computer:

nmap -p T:21-25,80,443 192.168.1.150

Check to see if specific ports are open on a range of computers:

nmap -p T:21-25,80,443 192.168.1.150-254

There is a GUI version of Nmap called Zenmap, but you honestly do not really need it.

Nmap homepage

Nmap Man page