Scan using TCP SYN scan (default)

This command determines whether the port is listening. Using this command is a technique called half-open scanning. It is called half-open scanning because you don't establish a full TCP connection. Instead, you only send a SYN packet and wait for the response. If you receive a SYN/ACK response, that means the port is listening:

nmap -sS 192.168.1.1