Network mapper, popularly known as Nmap, is the most widely known port scanner. It finds TCP and UDP open ports with a great success, and it is an important piece of software in the penetration tester's toolkit. Kali Linux comes with Nmap preinstalled. Nmap is regularly updated, and it is maintained by an active group of developers contributing to this open source tool.
By default, Nmap does not send probes to all ports. Nmap checks only the top 1,000 frequently used ports that are specified in the nmap-services file. Each port entry has a corresponding number indicating the likeliness of that port being open. This increases the speed of the scan drastically, as the less important ports are omitted from the scan. Depending on the response by the target, Nmap determines if the port is open, closed, or filtered.