Port, operating system, and service discovery
Kali provides several different tools useful for identifying open ports, operating systems, and installed services on remote hosts. The majority of these functions can be completed using Nmap. Although we will focus on examples using Nmap, the underlying principles apply to the other tools as well.
Port scanning
Port scanning is the process of connecting to TCP and UDP ports to determine what services and applications are running on the target device. In TCP/IP, there are 65,535 ports each for both TCP and UDP on any computer. Some ports are known to be associated with particular services (for instance, TCP 20 and 21 are the usual ports for the File Transfer Protocol (FTP) service).
The first 1,024 are the well-known ports, and most defined services run over ports in this range; accepted services and ports are maintained by IANA (http://www.iana.org/assignments/service-names-port-numbers/service-names-port-numbers.xhtml).
...
