Nmap is a popular port scanning tool.It can also be used discover hosts, identify the Operating System, Services running on the ports and lots of other thing.
A simple scan is as follows:
nmap (target address)
This will show us the Ports, their state (open/closed) and the Services running.
- Open Ports: It means that an application on the target host is accepting TCP connection.
- Closed Ports: It means that an application on the target host isn’t listening.
- Filtered Ports: It means that nmap cannot determine whether the the port is open or closed.This may be because of a firewall rule in the target host to block any request packets.