Q&A for How to Run a Simple Nmap Scan

Nmap don't test the vulnerability, Nmap simply displays it (if any). You can use exploit tools to test it.

You can use that opened port to exploit the target That's why opened ports are vulnerable.

Ports are essentially an address or destination within a 'host' (computer). When you send data on the Internet, a hostname and port number are specified for where the data should go. The hostname, such as 'library.ibm.com', is translated into the 'IP' address: 9.2.216.43 That identifies the specific computer (NIC) on the Internet. The port number is a 16 bit number (from 0 to 65534) which identifies a a service or listener on that computer. For example, port 80 is the usual port for H T T P, for web servers. (This example was for classic Internet, called IPv4, perennially IPv6 network uses longer, 6 byte IP addresses and longer, 4 byte port numbers)

IP means 'Internet Protocol' which is the lowest level of packets, or message blocks, used in the Internet. TCP mean 'Transmission Control Protocol', which is level of packet use built on top of IP packets. Using TCP manages the flow of packets between two computers and insures that no data is lost, duplicated or received out of order between to computers. These days most all applications use TCP/IP. Another protocol is UDP, User Datagram Protocol, which is simpler than TCP but doesn't recover lost packets, duplicated packets or out of order packets.

A program like 'nmap' scans every possible IP address -- within a range -- sending packets (little messages) to various 'ports' (addresses within the target computer) and hopes to get a response. If it gets no response, 'nmap' assumes there is no (useful) device at that address. A sneaky computer might not send any responses at all to computers it doesn't know or that don't have the right password or 'secret handshake' -- thus being hidden from such scanning programs. A home network might only have about 256 possible IP addresses, like: 192.168.0.1 to 192.168.0.254, which might only might take a minute or two.